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WHITE PAPER

The Technology of IP Routers in the New Public Network
Ericsson IP Infrastructure January 2000

Ericssonl

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TABLE OF CONTENTS
ROUTING BASICS ..................................................................................................................................................................................3 INTRODUCTION .........................................................................................................................................................................................3 ROUTING PROTOCOLS ..............................................................................................................................................................................3 Interior and Exterior Gateway Protocols ..........................................................................................................................................4 IP Multicast .........................................................................................................................................................................................4 SCALING THE INTERNET...........................................................................................................................................................................5 Route Reflectors...................................................................................................................................................................................5 Confederations ....................................................................................................................................................................................6 ROUTING POLICY .....................................................................................................................................................................................6 DESIGNING STABLE INTERNETS...............................................................................................................................................................7 TRAFFIC PATTERNS ARE CHANGING .......................................................................................................................................................8 THE NEW KIDS ON THE BLOCK......................................................................................................................................................8 MULTI PROTOCOL LABEL SWITCHING (MPLS)......................................................................................................................................9 MPLS Forwarding Process ................................................................................................................................................................9 MPLS Scalability.................................................................................................................................................................................9 ROUTING VS. MPLS...............................................................................................................................................................................11 TRAFFIC ENGINEERING...................................................................................................................................................................11 BACKGROUND ........................................................................................................................................................................................11 Evolution of Internet Backbone Traffic Control ..............................................................................................................................11 ISPs Decide Between a Routed Core and an ATM Core............................................................................................ 12 Metric-Based Traffic Control in a Routed Core..............................................................................................................................12 Absence of "Traffic Engineering" across a Routed Core............................................................................................ 12 Advantages and Disadvantages of a Routed Core ...................................................................................................... 12 PVC-Based Traffic Control in an ATM Core ..................................................................................................................................13 Traffic Engineering across an ATM Core................................................................................................................... 13 The "N-Squared" Problem over ATM Cores .............................................................................................................. 14 Advantages and Disadvantages of an ATM Core ....................................................................................................... 14 Traditional Router Cores vs. ATM Cores ........................................................................................................................................15 TRAFFIC ENGINEERING AND MPLS ......................................................................................................................................................15 MPLS Benefits ...................................................................................................................................................................................16 IP LAYER 3 QOS....................................................................................................................................................................................17 BACKGROUND ........................................................................................................................................................................................17 ARCHITECTURE ......................................................................................................................................................................................17 Traffic Conditioning..........................................................................................................................................................................17 Scheduling .........................................................................................................................................................................................18 Congestion Control ...........................................................................................................................................................................18 IP OVER SONET/SDH ..........................................................................................................................................................................19 What About IP Over ATM?........................................................................................................................................ 19 WIRESPEED ROUTERS......................................................................................................................................................................20

e. Routers within the Internet are organized hierarchically. and they use a variety of Interior Gateway Protocols (IGPs) to accomplish the infor mation exchange within the AS. data that needs to cross network boundaries is transmitted through routers. An IP router is a device that forwards IP pa ckets (at Layer 3) by a full exa mination of the IP pa cket header. with a discussion of the use of MPLS for traffic engineering. These routers are called interior routers. It starts with a basic introduction to Internet routing protocols. a router can build a detailed picture of the network topology. ROUTING BASICS INTRODUCTION According to the OSI reference model [see Figure 1 below]. IP routing protocols are dyna mic – routes are calculated at regular intervals by routers and routers communicate with one another by exchanging routing updates. . or AS). By analyzing routing updates from all routers. Someti mes the Network Layer is referred to as the internetworking layer. or the Layer 3. ‘label switching over ATM’. and they use Exterior Gateway Protocols (EGPs) to a ccomplish their function. From this point on. Both play an important role in the new generation of wirespeed giga bit routers and emerging public IP networks.g. However. Nor mally. and continues with a review of MultiProtocol Label Switching (MPLS) technology and a short comparison of ‘traditional routing’ vs. Routers that forward infor mation between ASs are called exterior routers. Some routers are used to forward infor mation through one particular group of networks under the sa me administrative authority and control (called an Autonomous System. and IP routing just ‘routing’. Figure 1: Internetworking Generally. Internetworking is the a bility to link disparate network technologies ( e. determining the opti mal routing path can be very complex. End Node Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Application Presentation Session Transport Network Data Link Physical Router Network Data Link Physical Router Network Data Link Physical IP pa ckets that need to cross network boundaries are transmitted through IP routers. and ATM) into a single extended network .3 This document outlines the underlying technologies for Ericsson's AXI 520 and AXI 540 Internet routers. Ethernet. The forwarding of IP pa ckets is relatively straightforward. an Internet Service Provider is considered as one AS. we will call IP routers just ‘routers’. This informa tion is compiled and stored in a routing ta ble [see Figure 2]. End Node Application Presentation Session Transport Network Data Link Physical The need for both exterior and interior routing protocols is driven by the size of the Internet. at Layer 3) from source to destination. IP (Internet Protocol) is the internetworking. and routing is the process of moving infor mation a cross an internetwork (i. ROUTING PROTOCOLS Routing involves two basic activities: deter mination of opti mal routing paths and forwarding of infor mation pa ckets through an internetwork. Finally. The routing table consists of destination address/next hop pairs. the Network Layer (Layer 3) is concerned with getting pa ckets of user informa tion from the source all the way to the destination.an "Internet". IP routing is the process of moving IP pa ckets a cross the Internet from source to destination. You need both to control the scalability to Internet di mensions. The importance of traffic engineering in the Internet ba ckbone is highlighted. protocol for the global interconnection of networks known as the Internet. Fra me Relay. two newer IP technologies are introduced: IP Layer 3 QoS and IP over SONET/SDH.

1. mainly for use in the Internet. a version has been created to support both CLNP and IP networks. Although IS-IS was created to 1 Figure 3: Interior and Exterior Gateway Protocols From a purely technical standpoint. BGP-4 is the de facto standard for exterior routing in the Internet. OSPF was developed in the late 1980s by the Internet Engineering Task Force (IETF). IP Multicast Today.0 ." Note that routing ta bles can also contain other para meters. .12 54. they cannot by themselves provide a global connectivity solution for the whole Internet. Distant Vector Multicast Routing Protocol (DVMRP) is an early multicast protocol used for building the multicast ba ckbone a cross the Internet.Interior Router E -.32. such as infor mation a bout the desira bility of a path.0.4 Destination Address 127.42. .42. Both OSPF ( Open Shortest Path First) and IS-IS (Inter mediate System-to-Inter mediate System) are called Link State protocols. . cost. . . The entire route is not known at that start of the journey. another for m of traffic is required.156. in ea ch router. Autonomous System I I OSPF E I Autonomous System E IS-IS BGP BGP Autonomous System E BGP I I I OSPF I Figure 2: Routing Table I -. standardized recently by the IETF.42. . IP routing is described as "connectionless1. . Instead. the next destination is calculated by matching the destination address within the IP pa cket with the routing table in the router.12 54. .e. . Currently. it’s in the end-stations (hosts).0 34. Although the Link State protocols have good routing scalability.10. typeof-service and authentication.0 142. IS-IS is quite similar to OSPF. and involves the transmission of a single IP pa cket to multiple hosts. etc) of a path.Interior Gateway Protocol. . 54. and both support routing hierarchies. the metric (i. One of the major cornerstones of the Internet is to keep the network as simple as possible. someti mes referred to as the Dijkstra algorithm. EGP was introduced to control the expa nsion of routing and to provide a more structured view of the Internet.56.32. Interior and Exterior Gateway Protocols As stated above.Exterior Router IGP -. called the Border Gateway Protocol (BGP) [see Figure 3]. it’s in the network. ASs run Interior Gateway Protocols such as OSPF and IS-IS within their boundaries. and so on. OSPF or IS-IS EGP -.. The new for m of traffic is called multicast traffic.32. . and interconnect via an Exterior Gateway Protocol.Exterior Gateway Protocol. IS-IS was developed by International Or ganization for Standardization (ISO). or unicast traffic (i. traffic from one single network device to another). This version is usually referred to as Integrated IS-IS. BGP IP routing specifies that IP packets travel through the Internet one hop at the time.32.113. route in ISO Connectionless Network Protocol (CLNP) networks. (re)ordering of packets etc.e. . Multicast traffic requires a new class of routing protocols – multicast protocols.42. It has Compared to ATM switching which is connection-oriented.12 54.42.32. But as application developers work to deliver the sa me data (such as audio and video required for conferencing) to multiple devices connected to a network.32.0 176. the Internet is a collection of ASs. In the connection-oriented case.10 .10. Protocol Independent Multicast (PIM) is a more sophisticated protocol. most Internet applications use “point-topoint”. The argument between connection-oriented and connection-less really has to do with where to put the complexity of retransmission of packets. Both are based on the shortest path first algorithm.10 54. Next Hop 54. in the connection-less case. delay.10 .5.42.

BGP is called Internal BGP (IBGP). Then. This is not. When used between ASs. One way to reduce the IBGP mesh is to configure a route reflector. The route reflector reflects (redistributes) routing infor mation to ea ch client peer and to all nonclient peers. Figure 4 below illustrates a simple IBGP configuration with three IBGP speakers (Routers A. BGP routers are arranged in clusters. called Border Gateway Protocol (BGP). B. minimizing the number of update messa ges sent within the AS. In this process. BGP is called External BGP (EBGP). They are only logically fully meshed. PIM is opti mized for many data strea ms but ea ch data strea m goes to a relatively small number of hosts. Note. such as OSPF and IS-IS. this requirement does not scale when there are many IBGP routers. the whole picture. PIM is optimized for traffic intended for almost all hosts. along with any number of client peers. Migrating from a non-route reflector to a route reflector design is easy since only the route . In route reflection. it must advertise it to both Router B and C. none of the client peers sends routes to other client peers. BGP supports two types of exchanges of routing infor mation: exchanges between different ASs and exchanges within a single AS.5 two modes of behavior: dense mode and sparse mode. within their boundaries and interconnecting via an Exterior Gateway Protocol. BGP deployments are configured such that all BGP routers within a single network must be fully meshed so that any external routing infor mation will be redistributed to all other routers within that network. Because the route reflector redistributes routes within the cluster. while in sparse mode. and C). if the route ca me from a non-client peer. Route Reflectors As previously mentioned. BGP peers outside the cluster are called non-client peers. If the route ca me from a client peer. In dense mode. B and C do When the route reflector receives a route. Each cluster consists of at least one router that acts as a route reflector. The route reflector concept is growing in popularity for large networks because it enables scalability without a lot of overhead. When used within a single AS. the BGP routers within the cluster do not have to be fully meshed. that Routers A. Autonomous System RD Route Reflector Non-Clients RA RE RC RB Clients Cluster Physical Connection IBGP Figure 5: Route Reflector with Client & Non-Client Peers IBGP RA IBGP EBGP Figure 4: External BGP and Internal BGP Router B and C do not re-advertise the IBGP learned route to other IBGP speakers because the routers do not pass routes learned from internal neighbors on to other internal neighbors. however. SCALING THE INTERNET not have to be directly connected to ea ch other. it selects the best path. Autonomous System IBGP RC RB Route reflection provides one mea ns of decreasing BGP control traffic. the route reflector sends the route to all client peers within the cluster. However. thus preventing a routing infor mation loop. the route reflector sends the route to all non-client peers and to all client peers except the originator. When Router A receives a route from an external neighbor. the Internet is a collection of ASs running Interior Gateway Protocols.

These routes are used to forward traffic through the router.6 reflectors need to be modified to behave as route reflectors. Autonomous System Routing policy allows you to control the routing infor mation that is transferred between the routing protocols and the routing table. the routing software calculates the best routes to ea ch destination. Routing policy also allows you to set the infor mation associated with a route as it is being imported or exported by the routing table. From the collected routing infor mation. You do not want a routing protocol to receive from the routing table all the a ctive routes learned by that protocol. ! BGP Import OSPF Import Export IS-IS Import Export Export ! Routing Table Figure 7: Importing and Exporting Routes ! . Confederations are based on the concept that an AS can be broken into multiple sub-ASs. and may be advertised to neighbors via one or more routing protocols. or the BGP community. all others would be running as usual. you do not want the routing ta ble to learn a bout certain routes. Network Neighbors Routes Import Policy Export Policy Network Neighbors Routes Sub AS #1 EBGP RD RA IBGP Sub AS #2 IBGP IBGP RB IBGP RC RE Protocol Physical Connection BGP peers Routing Table Protocol Network Neighbors Network Neighbors Figure 6: Confederations Packets In Forwarding Table Packets Out ROUTING POLICY All routing protocols store their routing infor mation in a common routing ta ble. Confederations Confederations are another way to deal with the explosion of an IBGP mesh within an AS. such as the preference value. By applying routing policy to routes being exported from the routing ta ble you control the routes that a protocol advertises to its neighbors [see Figure 8]. Routing policy allows you to control (filter) which routes the routing protocol imports into the routing ta ble and which routes a routing protocol can export from the routing ta ble. This is someti mes called route redistribution. That is. An IBGP full mesh is used within the sub-ASs. and EBGP is used between the sub-ASs. You can filter the routing infor mation so that only some of it is transferred. Figure 8: Importing and Exporting Routing Policies Routing policies are defined in the following circumstances: ! You do not want a routing protocol to transfer all its routes into the routing table. You want to set the infor mation associated with a route. and you can set properties associated with the routes. as well as between the confederation itself and the outside ASs [see Figure 6 below]. You want a routing protocol to receive active routes learned from another routing protocol. By applying routing policy to routes being imported to the routing ta ble you control the routes that the routing protocol process uses in order to deter mine active routes. autonomous system path.

the average rate of updates is relatively high (100-200 updates a second). categorizes routes as either well behaved or ill behaved. the route is suppressed.7 FLAP SOURCE In Figure 9 below. excessive levels of route flap are extremely dangerous. There are two methods of reducing da ma ging route flap: route a ggregation and flap da mpening by holding off introduction of routing updates of unsta ble routes. Because the Internet is so large. ea ch time a route flaps. Route aggregation is the more powerful tool. so the routing updates generated by topological changes of local significance do not rea ch ba ckbone networks. network upgrades. The other mechanism. However. as it limits visibility of details of topology. a large ISP provides Internet access to a second-tier ISP that advertises several hundred routes into the large ISP. Corp Customer Network A Network B Network C Network D Tens/Hundreds of Routes Figure 10: Route Flaps Rolling Over All Backbone Networks Dial-In Users Small ISP Policy Filters Large ISP The Internet Corp Customer Figure 9: Policy Filtering The control provided by the routing policies is strategic to backbone networks because it is the funda mental tool that controls how the networks are used. Routing policies determine the paths selected across the Internet and can play a role in the path selected across the Service Provider's network. every transition between operational and non-operational states of network equipment affecting connectivity of some globally visible network generates waves of update messa ges rolling over all ba ckbone networks. because every new update requires matching against routing policies. Handling of routing updates is very computationally intensive. insta bilities of IGPs and human error. Some level of route fla p is unavoida ble. Effectively. software problems. . Factors that affect route insta bilities on the Internet include hardware failures. and routers ( particularly older software-based routers) ‘die’ under the burden of doing route updating. which could be several thousands of rules for a border router of a large ISP. Under route flap da mpening. The large ISP configures policy rules so that it accepts only the routes that it expects to receive from the smaller ISP. DESIGNING STABLE INTERNETS One major problem that has ca used poor Internet service is so-called ‘route fla pping’. Whenever the penalty rea ches a predefined threshold. The large ISP updates the list of routes that it will accept from the smaller ISP by modifying its routing policy filtering rules. link failures. route flap da mpening. When the smaller ISP gets a new customer. The strea m of routing updates received by all backbone routers causes route flapping. since routing updates carry vital infor mation about changes in network topology. for exa mple. it infor ms the large ISP that it will be forwarding an additional set of routes. it is given a penalty. The recent history of a route is used as a basis for esti mating future sta bility.

while only 20% of the traffic will flow between networks. Assume there are four to five major ba ckbone providers in the US. Internet traffic is doubling every 6 to 7 months. and is even more pronounced for small regional and local ISPs. At the sa me ti me. Changes in the local loop require changes in the ba ckbone.g.it's today's generation of software-based routers. Internet ba ckbones grow much faster than conventional routing technology does.8 It must be noted that the route flap phenomenon is not unique to IP. That means that 75-80% of the traffic will leave. Gbps/MHz Internet Bandwidth Router CPU Speed time Figure 12: Internet Bandwidth vs. Today's networks also have little ability to deliver Quality of Service ( QoS) to real-time services such as voice. The reason for that is that progress of conventional routing technology depends on the progress in speed of microprocessors. doubling every two years.e. . We are seeing a two orders-of-ma gnitude improvement in residential access rates (28. they ea ch carry 20-25% of the total backbone traffic). any networking technology that incorporates dyna mic adaptive routing ( e. Cable Modems. TRAFFIC PATTERNS ARE CHANGING In the past. The Internet is suffering from severe growing pains. Today's IP networks use conventional routing protocols to mana ge the network topology. and it is growing VERY fast. We will see a dra matic increase of voice and video in addition to best-effort data traffic on the Internet in coming years. Conventional Router Speed 20% 80% Backbone 80% Workgroup Figure 11: Traffic Pattern Change 20% Data communication and telecommunication is merging. network architects have commonly used the rule that 80% of the traffic will stay local in the network. and 20-25% will stay in their networks. ISPs building new public IP networks need more than just speed. PNNI) will have to deal with route flap.e. This is true for ea ch one of the ba ckbone provides in this exa mple. Since route flap is caused by propa gation of infor mation about topological changes. Traffic patterns have now shifted.8K to 2Mbps) through xDSL. ISPs are deploying a new generation of wirespeed routers to a ccommodate customer demands. and in business access rates (1. and can be difficult to opti mize. IP alone doesn’t solve these problems . and routing is one of the major bottlenecks. THE NEW KIDS ON THE BLOCK The Internet is growing. revising the 80/20 rule to 20/80. i. These changes in the local loop put even more pressure on the Internet ba ckbone.5Mbps to 155Mbps) through metropolitan area fiber infrastructure (fiber-to-the-business).but let's exa mine some new developments in the IETF that address these issues and bring. Fixed Wireless (LMDS) and FTTC. But it is not routing that is slow . with 20% intranetwork traffic and 80% inter-network traffic [see Figure 11 below]. Consider an exa mple. which follows Moore’s Law. However. And ea ch have ea ch have roughly the sa me number of customers/subscribers (i.

as well as over PPP fra mes on point-topoint lines. MPLS Forwarding Process The forwarding process of each LSR is based on the concept of “label swa pping”. Therefore if one or more labels can be encoded directly into the fields that are accessed by these lega cy switches. the LSR exa mines the label and uses it as an index into its forwarding table [see Figure 14 below]. such as ATM switches. allowing a packet to be forwarded from one Label Switching Router (LSR) to another LSR across an ISP network [see Figure 13]. MPLS Scalability MPLS scalability is provided by two of the principles of routing. We will refer to such devices as "ATMLSRs". Alternatively. which are similar to PVCs in ATM and Fra me Relay networks. be used as LSRs. . .by using the Virtual Circuit field in the ATM/Fra me Relay pa ckets to carry the MPLS label. An LSR is a router that supports MPLS. .39/16 LSP path Figure 13: Label Switched Paths (LSPs) An LSR that receives an IP packet can choose to forward it along an LSP by wra pping an MPLS header around the pa cket and forward it to another LSR. The first is that forwarding follows an inverted tree rooted at a destination.9 MULTI PROTOCOL LABEL SWITCHING (MPLS) MPLS is an opti mized switching technology for IP networks. IP’s shortest path IP prefixes and are link-local. The basic idea of MPLS is to prepend IP pa ckets with a Layer 2 routing la bel at the edge of an "MPLS cloud". The labeled pa cket will be forwarded along the LSP by ea ch LSR until it rea ches the end of the LSP. Each entry in the forwarding table contains an inbound label that is ma pped to a set of forwarding infor mation a pplied to all pa ckets that carry the same inbound la bel. . Traffic to 138. Inbound Interface 2 2 Inbound Label 25 256 Outbound Interface 5 3 Outbound Label 185 735 . with suitable software upgrades. The la bels are bound to . and can run within ATM or Fra me Relay networks. ATM switches use the input port and the incoming VPI/VCI value as the index into a "cross-connect" table. It should be noted that MPLS forwarding procedures are similar to those of traditional Layer 2 "label swa pping" devices. . . Figure 14: MPLS Forwarding Table 138. and to perform all forwarding within the cloud based only on the label value.for exa mple ATM or Fra me Relay switching . at which time the MPLS header will be removed and the pa cket will be forwarded based on Layer 3 infor mation like the IP destination address. . An LSP is created by concatenation of one or more la bel switched hops. from which they obtain an output port and an outgoing VPI/VCI value. The second is that the concept of aggregation or hierarchies. . it may use a label "shim" inserted in PPP or Ethernet pa ckets. .39/16 MPLS may make direct use of underlying Layer 2 forwarding . then the lega cy switches can. The key feature provided by MPLS is the a bility to provide label-switched paths (LSPs). The important point here is that the path of the LSP is not limited to what the IGP would choose as the shortest path to rea ch the destination IP address. . . When a packet containing a label arrives at an LSR.

39/16 Traffic to 138. For an LSR.e.39/16 Traffic to 138. the merge operation is less straightforward. if cells from several upstrea m links are transmitted onto the sa me downstrea m VPI/VCI. the data packets are encapsulated into an ATM Adaptation Layer. and the AAL5 PDU is segmented into ATM cells with a VPI/VCI value and the cells are transmitted in sequence. it is impossible to reassemble the pa ckets later. in large enterprise ba ckbone or ISP networks.41/16 IP AA Pac L5 ke PD t U Traffic to 208. point-to-point LSPs between all ingress nodes to all the egress nodes in the MPLS network.39/16 Traffic to 127. 138. the result ma y be the interleaving2 of cells from various pa ckets within a single VC. Hence. the merge operation is straightforward: both incoming LSPs will perfor m a standard label switching operation.39/16 Traffic to 138.39/16 Traffic to 138.14/16 208. this will not scale well. In those cases. if one attempts to perfor m merging. 138. 2 In ATM.41/16 127. but will result in the sa me outbound la bel. 138. i. However.e. Two or more LSPs can be aggregated if they share a portion of their path.10 LSPs with a single exit point sharing a common internal path can be merged to for m a multipoint-topoint tree. and result in corruption of the original PDUs by mis-sequencing the cells of each PDU. Note that for an ATM-LSR.39/16 Merging therefore requires capabilities (i. When this happens. effectively bundling the LSPs into a single tunnel.39/16 Traffic to 138. In ATM. say AAL5.39/16 138. where the ATM-LSR switch does not support merging. For small networks the full mesh connection approa ch may suffice and not pose any scalability problems. The la bel sta ck mechanism allows LSP tunneling to nest to any depth. .39/16 Traffic to 138. then cells from one PDU can get interleaved with cells from another PDU on the outgoing VPI/VCI.14/16 Figure 17: Traffic Aggregation in an MPLS Network ATM-LSR Figure 16: Cell Interleaving in an ATM Switch LSPs can be aggregated with other LSPs by adding a new la bel to the sta ck of la bels for ea ch LSP. These aggregated LSPs can be ter minated at any point. a full mesh connection a pproa ch has to be deployed. resulting in de-aggregation of traffic.39/16 Figure 15: Traffic Merging Traffic to The second feature MPLS uses to scale networks is aggregation or hierarchies. multipoint-to-point connectivity) which are not always available in ATM forwarding hardware.

and as the demands of customers become greater. Metric-based control was adequate because Internet ba ckbones were much smaller than today in ter ms of the number of routers. This ma pping of traffic onto a physical topology is called traffic engineering. forwarding perfor mance is no longer an issue. In the past. recent advances in silicon technology allow ASIC-based route lookup engines to run as fast as ATM VPI/VCI lookup engines.41/16 127. Traffic to 127. and the a mount of traffic.11 Traffic to 138. The cost angle is harder to figures out because vendors’ list prices are not necessarily a direct indication of cost. However. as the circuits supporting IP grow faster. and the cost of buffering. the task of ma pping traffic onto that topology must be tackled. Is this accurate? Recall that IP forwarding is ba sed on a longest-ma tch lookup. the ma pping simply took pla ce as a byproduct of routing configurations. Label switching only reduces the cost of the IP address lookup and forwarding decision. It will be hard to justify label switching from a performance or cost standpoint alone.39/16 second. the ma pping of traffic onto physical topologies needs to be a pproa ched in a funda mentally different way.that label switching would offer improved perfor mance and do so at a lower cost that conventional approa ches. . the sa me type of lookup as ATM). The weaknesses in this haphazard ma pping were often resolved by simply over-provisioning bandwidth. the impa ct on the cost of the whole system remains modest. As ISP networks grow larger.41/16 Traffic to 208. such as the cost of the switching fabric itself. The offered load must be supported in a controlled way and in an efficient manner. Instead. Because routers now can forward pa ckets at line rate/wirespeed (just like an ATM switch can forward cells). ma pping traffic onto a physical topology was not approa ched in a particularly scientific way. The true benefit of MPLS is the increased trafficengineering capabilities that it offers to conventional routing. It has historically been fa ct that ATM switches could perfor m faster fixed-length lookups in hardware and that routers could perfor m longest-ma tch lookups in software. Even if this could be driven to zero. ROUTING VS. This is because there are many other costs in building a device that can forward pa ckets at many giga bit per TRAFFIC ENGINEERING BACKGROUND Once the physical network topology exists.39/16 138. the number of links.5-2 Mbps) and T3/E3 (3245 Mbps) links. MPLS One of the claims made for label switching is that it offers advantages over more conventional routing techniques -. Evolution of Internet Backbone Traffic Control In the early 1990s when ISPs networks were constructed over T1/E1 (1.14/16 208. traffic control was achieved by ma nipulating routing metrics. while MPLS is based on an exa ct match lookup (i. Arguments about price and performance in label switching versus conventional routing are not very relevant.14/16 Figure 18: Aggregated LSP in an MPLS Network One useful application of this technique is in Virtual Private Networks.e.

This eliminates the "nsquared" problem associated with ATM networks. At the ti me. Absence of "Traffic Engineering" across a Routed Core If the only method of traffic control is to manipulate IGP metrics. DC is the SF-to-Chica go-to-DC route [see Figure 19 a bove]. . DC POP.C. is manifest in the complexity of adding new edge nodes and the stress that a full-mesh topology places on routing protocols. Assume that the "shortest path" selected by the routing protocol for traffic between San Francisco and Washington. However. the physical topology and the logical topology are identical. the load on ISP networks was increasing to the point where they si mply had to go faster than T3. DC traffic. metric-ba sed traffic control beca me more and more complicated. Reliance on IGP metrics creates paths that become traffic magnets. Metric-Based Traffic Control in a Routed Core As discussed earlier. they were not yet available on router platfor ms. The result is congestion and poor perfor mance that does not exploit the economies of the bandwidth provisioned across the entire WAN. as the ma gnitude and intrica cy of carrier networks increased. In the next several sections. Network administrators could still adjust link metrics to avoid congestion. routing metric manipulation provided a basic tool for traffic control in the early 1990s. even if the solution meant a serious overhaul to the core of their network? Figure 19: IGP Shortest Path between SF and DC " As we will see. but it beca me mor e difficult to ensure that a metric adjustment in one part of an enor mous network didn't create a new problem in another part of the network. Chicago Salt Lake Denver San Francisco Atlanta Phoenix Dallas New Orleans IGP shortest path route S. situations like this will occur often. it is possible for some of a network's links to be lightly used and other links heavily congested. to the point where it was simply not a viable option. Each ISP chartered their future course based on answers to a few questions: " beca use all trunks cost money even if they are underutilized. Advantages and Disadvantages of a Routed Core There are a number of benefits gained by remaining with a routed core when compared to migrating to an ATM-based core: " In a routed core. to D. Meanwhile.12 ISPs Decide Between a Routed Core and an ATM Core Around 1994 or 1995. However. The result of this is that the traffic from San Francisco to Washington.C. DC. OC-3/STM-1 interfaces (155 Mbps) were available only on ATM switches. New York Detroit Washington D. This "n-squared" problem. This state is not cost-effective for the ISP There are several potential paths between the San Francisco POP and the Washington. the ISPs that chose to migrate to an ATM core thrived and continued to experience growth.F. Did the ISP have enough faith in the ability of their traditional router vendor(s) to rapidly deliver OC3/STM-1 and OC-12/STM-4 interfaces for their products? Did the ISP consider the lack of bandwidth a significant threat to their business that required immediate resolution. the ISPs that remained with a traditional router core had grea ter challenges to grow because of the late delivery and poor perfor mance of OC-3/STM-1 SONET/SDH interfa ces for routers. The ISPs had to make a decision whether they were going to continue with a routed core or migrate to an ATM core. Also assume that there is a large a mount of traffic going from San Francisco to Chica go and also a large a mount of traffic going from Chica go to Washington. discussed in the next section. we'll discuss the advanta ges and disadvantages of ea ch of these choices. if the network is only ca pa ble of selecting links based on the IGP metrics. DC has to compete a gainst both the San Francisco-to-Chica go and Chica go-to-Washington.

not the secondary PVCs]. point-to-point circuits between two routers. Oslo Router Stockholm Router " PVC-Based Traffic Control in an ATM Core When IP runs over an ATM network. This may be satisfactory in a sparsely connected network. The routers do not have direct a ccess to infor mation concerning the physical topology of the underlying network. [Note that only the primary PVCs are illustrated. but also participates in a full-mesh of ba ckup PVCs with every other router. Figure 21 Below depicts the logical topology for an ISP network with an ATM core. 124 Mbps is available for data while 31Mbps is consumed by the ATM overhead. backup Permanent Virtual Circuits (PVCs) must be designed and installed in the switches before a failure occurs. it is extremely difficult to design secondary PVCs that can provide the sa me degree of resiliency as IP routing inherently provides. Physical Topology Logical Topology S S S S R S R R R " PVC #1 PVC #2 PVC #3 Figure 20: Physical vs. but in a richlyconnected network it is necessary to control the paths that traffic takes in order to load the links relatively equally. routers circle the edge of the ATM cloud. these features are imma ture. 1. this means that on a 155-Mbps OC-3 link. the traffic load is not equally distributed across the network's links. As ISP networks become more richly connected. For some ISPs. Metric-based traffic control does not offer an adequate solution for traffic engineering. are more resilient in failure modes. Figure 20 below illustrates how the physical topology a cross the ATM core differs from the logical topology across the ATM core. the paths that the PVCs take through the network are typically calculated by an off-line configuration utility. Because failures have the potential of occurring at any point in a network. while other links remain underutilized. However. ea ch router not only participates in a full mesh of PVCs with the other routers. Logical Topology Despite these advanta ges. London Traffic Engineering Across an ATM Core For an ISP with an ATM core. In an ATM circuit-based network. and an ISP frequently has to resort to full off-line path calculation. by virtue of their connectionless operation. if you consider a 2. R Routed cores. Each router communicates with every other router by a set of PVCs configured a cross the ATM physical topology.488-Gbps OC-48 link. they have knowledge only of the individual PVCs that appear to them as si mple Router ATM Core Router Frankf Router Paris Figure 21: Full Mesh of PVCs in an ATM Network . After the PVC mesh has been calculated. Some ATM switch vendors offer proprietary techniques for routing PVCs on-line while taking some traffic engineering concerns into a ccount. traditional routed cores have a few significant disadvanta ges: " In a routed core. causing inefficient use of network resources. The lack of a cell tax in a routed core means that the available bandwidth is used much more efficiently. Some of the links can become congested. However.990 Gbps is available for data and 498 Mbps is required for the ATM overhead (almost a full OCR 12!).13 " There is no "cell tax" in a routed core. If you assume 20% overhead for ATM when factoring in framing and realistic distribution of packets sizes. the supporting configurations are downloaded into the routers and the ATM switches to i mplement the full-mesh logical topology. it is difficult to ensure that a metric adjustment in one part of the network doesn't cause problems in another part of the network.

alternate paths are calculated on demand whenever a link or peer fails. This eliminates the traffic magnet effect of least-cost routing. the ISP is required to increase the number of PVCs from 20 to 30. 5 Border Routers 6 Border Routers The "n-squared" problem also manifests itself in the stress that a full-mesh of PVCs pla ces on routing protocols. there are a number of advanta ges to deploying an ATM-based core in an ISP network: " An ATM-based core fully supports traffic engineering via PVC configuration. Routing with a full-mesh of PVCs works over smaller networks. Each PVC is provisioned so that it is a ble to accommodate the anticipated load. the per-PVC statistics provided by the switches facilitate the monitoring of traffic patterns. For exa mple. by virtue of their connectionoriented operation. In a routed core. when growing the network from five to six routers. This full-mesh of PVCs is the source of the "nsquared" problem. the ISP has all the information it needs to remedy the situation. Figure 22 below illustrates the "n-squared" problem – by the requirement to increase the number of PVCs when a new router is added to the network. They constantly monitor traffic on the PVCs. with large networks there are significant scaling problems. The "N-Squared" Problem over ATM Cores One of ATM's limitations is that it requires a fullmesh overlay of PVCs to provide Layer 3 connectivity.14 ATM PVCs provide a tool for supporting precision traffic engineering. As discussed earlier. The ATM cell tax can consume a significant amount of bandwidth. which creates over-utilized and underutilized links. The task of adding a new router becomes even more challenging when the number of routers increases from 50 to 51. The "n-squared" effect makes it laborious to add new routers to the network and pla ces excessive stress on routing protocols. permitting the ISP to precisely distribute traffic across all of their links so that the trunks are evenly used. The new PVCs need to be positioned a cross the physical topology so that they have minimal impact on the existing PVCs. If a given PVC begins to experience congestion. In an ATMbased core. the IGP metric for the pri mary PVC is set such that it is more preferred than the ba ckup PVC. backup paths have to be calculated in " " 5(4) = 20 6(5) = 30 Figure 22: The "n-squared" Problem . This requires the addition of 100 new PVCs. The elimination of the cell tax in a routed core means that bandwidth is used as efficiently as possible and not wasted on unnecessary overhead. This guarantees that the ba ckup PVC is used only when the pri mary PVC is not available. " " Yet despite the significant advantage of supporting traffic engineering. the cell tax on an OC-48 can be as much as a full OC-12. they are merged into the IP network by running IGP across ea ch PVC. ATM-based cores still have a number of substantial limitations: " The full-mesh of ATM PVCs exhibits the "nsquared" problem. ea ch router has too many adjacencies with logical neighbors. In an ATM-based core. the underlying physical path supporting a particular PVC can be modified to accommodate shifting traffic loads a cross the physical links. As the network's traffic matrix evolves over time. Between any two routers. After the PVCs are installed in the switched infrastructure. allowing lower costs and better service to its customers. Advantages and Disadvantages of an ATM Core Despite the "n-squared" problem. Traffic engineering makes the ISP more competitive within its market. Network designers provision each PVC to support specific traffic-engineering objectives. However. are less resilient in failure modes. ATM-based cores. The ISP deter mines the path for the PVCs based on mea sured traffic patterns so that traffic flows are distributed across different physical links. The flooding of LSPs becomes inefficient. The addition of a single new router requires ten additional PVCs. and the Dijkstra calculation becomes inefficient because of the large number of logical links.

By running an IP network over an ATM network. In the context of traffic engineering. ISPs are not willing to give up the control that ATM provides. the cell tax. ISPs have become very comfortable with the level of control that ATM-based cores provide when compared to traditional routed cores. operate. Despite the numerous limitation of ATM (i. " ATM-based cores require the management of two different networks: an ATM infrastructure and a logical IP overlay. any emerging traffic engineering solution must combine the advanta ges of routed cores and ATM-based cores while eliminating their disadvantages. routing happens on the routers and traffic engineering happens on the ATM switches). . ISPs that decided to pursue an ATM-based core quickly discovered that the ATM core provided them with two critical features: the ability to perform traffic engineering to equalize the traffic loads across the network. The following table provides a summary of the benefits and limitations of traditional routed cores and ATM-based cores. and the cost of mana ging two separate networks).e. The task of managing any given network has a specific a mount of associated cost. Traffic engineering is accomplished with MPLS by esta blishing LSPs between ingress points and egress points [see Figure 23 on the following pa ge]. the "n-squared" problem. As a result. and debug. ISPs were simply looking to obtain more bandwidth so they could handle increasing a mounts of network traffic. Routing and traffic engineering occur on different sets of boxes (that is. A few of the benefits offered by MPLS include: • Ability to precisely control the use of valuable resources during periods of rapid growth • Stability under congestion and failure modes • Providing ISPs the foundation for value-added services TRAFFIC ENGINEERING AND MPLS Traffic enters and exits a backbone network from the network's border routers. and perPVC statistics to count traffic. there are two configuration processes to design. the border routers are called the ingress and egress points to and from the network. ISPs understand that they require trafficengineering ca pa bilities similar to ATM in order to successfully run their networks. ATM Cores Back in 1994. Multiprotocol Label Switching (MPLS) provides the solution to support traffic engineering in large service provider networks. Today.15 advance and then installed in the switches to provide an immediate backup capability. while eliminating the disadvantages. “n-squared” problem Less resilient in failure modes Requires mana gement of two separate networks " ATM Cores Traffic Engineering supported through PVC configuration Per-PVC statistics monitor traffic patterns and provide feedba ck to traffic engineering Traditional Router Cores vs. Traditional Router Cores Advantages Physical topology ma tches logical topology No “n-squared” problem No cell tax Resilient in failure modes Disadvantages Underutilized links Overutilized links Engineering by routing metrics is complex ATM cell tax Full mesh of ATM PVCs. an ISP doubles its overhead because it needs to manage two separate networks. It combines the advantages of router-based and ATM-based cores. As we enter the a ge of the optical Internet..

An MPLS core converges the Layer 2 and Layer 3 networks required in an ATM-based core. Now ima gine that the ISP wants to establish an LSP between a router in New York and a router in San Francisco. any number of LSPs can be specified as backups for the primary LSP. normal IP routing is used to determine the path of the LSP.e. " " " " " . The head-end LSR then uses the Resource Reservation Setup Protocol (RSVP) as a dyna mic signaling protocol to install forwarding state in each LSR. If this ha ppens. Let's conclude by exa mining how well MPLS meets this challenge. LSP between Ingress LSR and Egress LSR Figure 23: LSP between Ingress LSR and Egress LSR Recall that the essence of traffic engineering is ma pping traffic onto a physical topology. Calculate the full path for the LSP off-line and statically configure the head-end LSR with the full path. If a circuit on which the primary LSP is routed fails. For exa mple. imagine an ISP has a topology that includes two east-west paths across the country: one in the north through Chicago and one in the south through Dallas. it was suggested that any emerging solution providing traffic engineering across the optical Internet must combine the advantages of ATM and routed cores while eliminating the disadvantages. operation. The partial path that is specified can include any combination of strict and loose routes. This eliminates the "nsquared" problem associated with ATM networks. and it does not reserve bandwidth or provide any assurance of minimal delay or jitter (i. In an MPLS core. This configuration doesn't provide any value in terms of traffic engineering. " Calculate the full path for the LSP off-line and statically configure all LSRs in the LSP with the necessary forwarding state. and permits routing and traffic engineering to occur on the sa me platform. The head-end LSR uses RSVP to " An MPLS core fully supports traffic engineering via LSP configuration. The ISP could configure the partial path for the LSP to include a single loose-routed hop of an LSR in Dallas and the result is that the LSP will be routed along the southern path. the head-end LSR can call on RSVP to create forwarding state for one of the ba ckup LSPs. configuration. In an MPLS core. The management of a single network reduces costs. This simplifies the design. Ericsson's implementation of MPLS in its Internet routers supports a number of different ways to route an LSP: " In all these cases. or important traffic goes over the most reliable links. " LSR Egress LSR LSR LSR LSR Configure the head-end LSR with just the identification of the tail end LSR.16 LSR LSR Ingress LSR LSR install the forwarding state along the LSP just as above. MPLS Benefits Previously. no quality-of-service guarantees). This is analogous to how some ISPs are currently using ATM. the per-LSP statistics reported by the LSRs provide exactly the type of information required for configuring new traffic engineering paths and also for deploying new physical topologies. Calculate a partial path for the LSP off-line and statically configure the head-end LSR with a subset of the LSRs in the path. In this case. Note that RSVP is being used only to install the forwarding state. though the configuration is easy and it might be useful in situations where services like Virtual Private Networks (VPNs) are needed. the physical topology and the logical topology are identical. the head-end LSR will notice because it will stop hearing RSVP messa ges from the remote end. and debugging of the entire network. This permits the ISP to precisely distribute traffic across all of their links so the trunks are evenly used. This means that the crux of the issue is deter mining the path for LSPs. The lack of a cell tax in an MPLS core means that the provisioned bandwidth is used much more efficiently than in an ATM core.

network providers have tended to provide all of their customers with the sa me type of perfor mance (a best-effort service). with network providers offering better perfor mance to customers who are willing to pay more." ARCHITECTURE The IETF Differentiated Services architecture is based on a simple model where traffic entering a network is conditioned at the edges of the network. and proba bly a power of two. MPLS is strategically significant for traffic engineering because it can provide most of the functionality available from the overlay model in an integrated and scalable manner and at lower cost than competing alternatives. in the immortal words of Mike O'Dell3. the network provider needs to be a ble to categorize traffic entering its network into multiple categories or classes of service. . Traditionally.17 " MPLS support for a dyna mic protocol. outlining the characteristics and behavior of network connectivity offered by the provider to the customer. simplifies the deployment of traffic engineered LSPs across the network. there is a need to offer different types or grades of perfor mance to customers. and shaper [see Figure 24 below]. meter. marker. However. since MPLS is supported over ATM and point-to-point IP. The architecture achieves scalability by implementing complex conditioning functions only at network edge nodes. and by applying per-hop behaviors to aggregates of traffic which have been a ppropriately marked using the DS field in the IP header. The pa ckets that are out-ofprofile may be either marked or shaped according to the rules specified in the SLA. In order to provide the notion of differentiated services. and then a meter measures ma tched pa ckets against the profile as defined in the SLA. with increasing competition a mong network providers. Future MPLS support for constraint-based routing achieves the sa me control as more-manual traffic engineering but with less human intervention because the network itself participates in LSP calculation. The number of classes Marker Packets Classifier Meter Traffic Conditioner Shaper Figure 24: Traffic Conditioner A pa cket strea m nor mally passes to a classifier first. and assigned to different behavior aggregates or class of service. 3 Mike O'Dell is responsible for overall network architecture and technical strategic direction for UUNET. 4 A traffic strea m is an administratively significant set of one or more applicationto-application flows. Traffic Conditioning A traffic conditioner may contain the following elements: classifier. The pa ckets within the profile may leave the traffic conditioner or may be marked by the marker. Both IP-over-ATM networks and pure IP router networks can evolve into IP/MPLS networks. The SLA would typically also include a billing scenario as well as the perfor mance aspects expected with the associated contra ct. such as constraints on the type and a mount of data that can be sent on the network. MPLS provides the foundation for ISPs to offer valueadded services. be "more than three. " " " IP LAYER 3 QOS BACKGROUND A network service is a contract between a network provider and its customer. A Service Level Agreement (SLA) may specify different aspects of network behavior. of service must. less than nine. The classifier and the meter select the pa ckets within a traffic stream4 a nd measure the strea m against a traffic profile. such as RSVP. and the perfor mance aspects of communication. The marker and shaper perfor m control actions on the pa ckets depending on whether the traffic strea m is within its associated profile.

An enhancement to RED. potentially. Rather then drop all arriving pa ckets. WRED uses this precedence to deter mine how the core treats different types of traffic. The weight represents how much the queue is serviced by the scheduler. Instead of waiting for the queues to fill and then drop all incoming pa ckets (tail drop). and that traffic gets predicta ble service.18 Scheduling A per-hop behavior is defined as “the forwarding behavior a packet receives at a given network node”. . one for pa ckets marked In-profile and one for pa ckets queues 75 50 10 2 weighted scheduling packets in packets out Figure 26: Weighted Fair Queuing (WFQ) Congestion Control A congestion-handling scheme is also needed to ensure that higher value traffic receives preferential treatment during congestion situations and as lower value traffic is discarded earlier and. is the consideration of In/Out of profile marking of pa ckets – called Random Early Detection In/ Out (RIO). RED statistically drops more pa ckets from large users than small. RIO is in principle two RED algorithms operating at the sa me ti me. Therefore. WF Q ensures that queues do not starve of ba ndwidth. and then serves these queues in a “non first-in/first-out” fashion. Standard traffic may be dropped more frequently than premium traffic during periods of congestion. congestion control intelligence monitors the average queue size and performs early discard on randomly selected pa ckets. Per-hop behaviors are implemented in nodes by queue ma nagement and pa cket scheduling mechanisms [see Figure 25 below]. WRED provides separate thresholds and weights for different IP precedence. more aggressively. RED is only a solution for congested nodes/links and not a solution for overloaded networks. WRED is usually used in the core routers of a network. rather than the edge. Drop probability congestion handling 100% Figure 25: Scheduling of Packets Weighted Fair Queuing ( WFQ) provides bandwidth allocation and delay bounds to traffic by segregating the traffic into a number of queues. queue selection based on DS field 0% Queue length min max Figure 27: Random Early Detection (RED) In addition. Of course. Weighted RED (WRED) generally drops pa ckets selectively based on IP precedence ( or DiffServ label value). traffic sources that generate the most traffic are more likely to be slowed down than traffic sources that generate little traffic. pa ckets are dropped with a low. risk instead of waiting until the queue is full. the other queues will be given a share of the total available ca pa city according to their respective weights. compared to the other queues. Edge routers assign IP precedence to pa ckets as they enter the network. If a queue is empty. where an assigned weight per queue is considered [see Figure 26]. but increasing proba ble. queues scheduling packets in packets out Random Early Detection (RED) is a congestion avoidance mechanism. allowing you to provide different qualities of service for different traffic.

In an IP-centric network. simplicity."PPP over SONET and SDH Circuits". The cell structure of ATM responsible for these characteristics is seen only as a "cell tax" consuming 20-30% of the WANs bandwidth without generating any value. With proper setting of parameters. RFC 1548 . The IETF has standardized on the Point-to-Point Protocol (PPP) to perfor m this function. and a cyclic redundancy check is added at the end. and simplicity. ma pped directly to the SONET/SDH layer using "Packet over SONET" or POS. and multicasting. IP pa ckets are . What About IP Over ATM? ISPs delivering pure data services are concerned with getting every possible drop of perfor mance out of their WAN infrastructure. one control byte and two for protocol bytes). ISPs don't benefit from ATM's excellent multiplexing and jitter mana gement ca pa bilities. and drops much more aggressively than the In algorithm. According to the OSI reference model. The IP pa ckets are serially pla ced in the SONET/SDH fra me. higher perfor mance. These trends have brought about the birth of a new IP-centric network model.19 marked Out-of-profile. IP OVER SONET/SDH The demand for bandwidth is skyrocketing in dataservice networks. the SONET/SDH protocol is the physical layer (Layer 1) and IP is a network layer protocol (Layer 3). RFC 1549. With no real-ti me traffic load in their networks (i. full-service network that takes advantage of IP Layer 3 QoS and such inherent IP characteristics as scalability. A data link layer (Layer 2) is required to interfa ce the SONET/SDH protocol with IP.e. no base of circuit-switched telephony subscribers).Point-to-Point Protocol (PPP) 0% Queue length min OUT max min OUT IN max IN Figure 28: Random Early Detection In/Out (RIO) The for mat of IP pa ckets in the SONET/SDH synchronous payload envelope is simple.. The IP-centric network infrastructure is an IP-based. With this dema nd comes the need for efficient bandwidth utilization. One byte is dedicated to the pa cket fla g at the beginning of the string and four bytes are dedicated to the PPP header (one address byte. the Out of profile traffic can be controlled before the queue grows to the point that any In traffic is dropped [see Figure 28 below]. The Out algorithm starts dropping at a much shorter avera ge queue length."PPP in HDLC Framing". The applica ble Request for Comments are: " " " Drop probability 100% RFC 1619 .

carriers with a significant base of traditional circuit-switched services will still benefit from the unique ca pabilities of ATM. However. These platfor ms. and scalability.ericsson. to tens of millions of pa ckets per second. using shared memory or crossbar switching fabrics combined with routing in ASICs. Inc. Mbps 80 60 40 20 0 46 110 238 494 1006 1500 IP over SONET IP over ATM (AAL5SNAP) OC-3 Payload Residential Customers Access POP Aggregation Router xDSL/Cable Modem Backbone POP High Speed Local Access xDSL /CableModems Bytes per IP Packet OC-3 xDSL/Cable Modem Figure 29: IP over SONET vs. Ericsson's AXI 520 IP Core Router and AXI 540 Edge Aggregation Router are both exa mples of this new generation of wirespeed routers. Business Customers Core Router High-Capacity Access Router Figure 30: The Wirespeed Router as the Cornerstone of the Backbone POP WIRESPEED ROUTERS Traditionally. Data com Networks 77 South Bedford Street Burlington. and MPLS for traffic engineering. IP over ATM SONET OC-3 160 140 120 100 support IP QoS.com/datacom . T3/E3. together with Ericsson's advanced IP network ma na gement solutions. MA 01803 www. OC-12/STM-4 and OC-48/STM-16 directly over SONET/SDH or over ATM. Ericsson Inc. They support a wide range of WAN interfaces including T1/E1. No longer does routing need to be avoided due to the performance degradation it introduces. IP over ATM (source: Cisco Systems.20 IP over SONET OC-3 vs. The new generation of routers perfor ms IP routing in specialized hardware. OC3/STM-1. Packet over SONET in combination with giga bit routers leverages existing SONET/SDH technology and delivers simple and efficient bandwidth use. These ASIC-based wirespeed routers can forward IP at the sa me performance levels as Layer 2 switches. They also This new class of wirespeed routers changes many assumptions that organizations have been using to design their networks. Layer 3 QoS. with low latency.) Cable/xDSL Access Mux OC-12 Variable Rate (up to OC-3) OC-12 OC-48 IP Backbone Consideration of IP over SONET/SDH when a business model is IP-centric is essential. routing has been perfor med by software running on one or more microprocessors contained within a routing product. This new breed of routers can route pa ckets at multigiga bit speeds. deliver a highly effective and integration solution for carrier-class IP networks [see Figure 30 below]. IP Multicast. These so called ‘software based routers’ were relatively slow and expensive. and can justify the "cell tax" for IP data services (at least until IP QoS technology is sufficiently mature to provide perfor mance equivalent to ATM).