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22 Understanding BGP Convergence
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Posted by Petr Lapukhov, 4xCCIE/CCDE in BGP,BGP,CCDE,CCDP 18 Comments Search
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Introduction

BGP (see [0]) is the de-facto protocol used for Inter-AS connectivity nowadays. Even though it is commonly Categories
accepted that BGP protocol design is far from being ideal and there have been attempts to develop a better
Select Category
replacement for BGP, none of them has been successful. To further add to BGP’s widespread adoption, MP-BGP
extension allows BGP transporting almost any kind of control-plane information, e.g. to providing auto-discovery
functions or control-plane interworking for MPLS/BGP VPNs. However, despite BGP’s success, the problems with CCIE Bloggers
the protocol design did not disappear. One of them is slow convergence, which is a serious limiting factor for many
Brian Dennis, CCIEx5 #2210
modern applications. In this publication, we are going to discuss some techniques that could be used to improve
Routing & Sw itching
BGP convergence for Intra-AS deployments. Voice
Security
BGP-Only Convergence Process Service Provider
ISP Dial
Tuning BGP Transport
Brian McGahan, CCIEx4 #8593,
BGP Fast Peering Session Deactivation CCDE #2013::13
BGP and IGP Interaction Design
Data Center
BGP PIC and Multiple-Path Propagation
Routing & Sw itching
Practical Scenario: BGP PIC + BGP NHT Security
Considerations for Implementing BGP PIC Service Provider

Summary Mark Snow , CCIEx4 #14073
Further Reading Data Center
Appendix: Practical Scenario Baseline Configuration Collaboration
Security
Voice
Petr Lapukhov, CCIEx4 #16379,
CCDE #2010::7

BGP-Only Convergence Process Design
Routing & Sw itching
Security
BGP is a path-vector protocol – in other words, a distance-vector protocol featuring complex metric. In absence of Service Provider
any policies, BGP operates like if routes have metric equal to the length of the AS_PATH attribute. BGP routing Voice

polices may override this simple monotonous metric and potentially create divergence conditions in non-trivial BGP
topologies (see [7],[8],[9]). While this may be a serious problem at a large scale, we are not going to discuss these Popular Posts
pathological cases, but rather talk about convergence in general. Like any distance-vector protocol, BGP routing
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process accepts multiple incoming routing updates, and advertises only the best routes to its peers. BGP does not
utilize periodic updates, and thus route invalidation is not based on expiring any sort of soft state information (e.g
prefix-related timers like in RIP). Instead, BGP uses explicit withdrawal section in the triggered UPDATE message
to signal neighbors of the loss of the particular path. In addition to the explicit withdrawals, BGP also support
implicit signaling, where newer information for the same prefix from the same peer replaces the previously learned
information.

Let’s have a look at BGP UPDATE message below. As you can see, the UPDATE message may contain both
withdrawn prefixes and new routing information. While withdrawn prefixes are listed simply as a collection of NLRIs,
new information is grouped around the set of BGP attributes, shared by the group of announced prefixes. In other
words, every BGP UPDATE message contains new information pertaining to a set of path attributes, at least
prefixes sharing the same AS_PATH attribute. Therefore, every new collection of attributes requires a separate
UPDATE message to be sent. This fact is important, as BGP process tries packing as many prefixes per update
message as possible, when replicating routing information.

Look at the sample topology below. Let’s assume that R1′s session to R7 just came up and follow the way that
prefix 20.0.0.0/8 takes to propagate through AS 300. In the course of this discussion we skip the complexities
associated with BGP policy application and thus ignore the existence of BGP Adj-RIB-In space used for processing
the prefixes learned from a peer prior to running the best-path selection process.

Upon session establishment and exchanging the BGP OPEN messages, R1 enters the “BGP Read-Only
Mode”. What this means, is that R1 will not start the BGP Best-Path selection process until it either receives
all prefixes from R7 or reaches the BGP read-only mode timeout. The timeout is defined using the BGP
process command bgp update-delay. The reason to hold the BGP best-path selection process is to
ensure that the peer has supplied us all routing information. This allows minimizing the number of best-path
selection process runs, simplify update generation and ensure better prefix per message packing, thus
improving transportation efficiency.
BGP process determines the end of UPDATE messages flow in either of two ways: receiving BGP
KEEPALIVE message or receiving BGP End-of-RIB message. The last message is normally used for BGP
graceful restart (see [13]), but could also be used to explicitly signalize the end of BGP UPDATE exchange
process. Even if BGP process does not support the End-of-RIB marker, Cisco’s BGP implementation
always sends a KEEPALIVE message when it finishes sending updates to a peer. It is clear that the best-
path selection delay would be longer in case when peers have to exchange larger routing tables, or the
underlying TCP transport and router ingress queue settings make the exchange slower. To address this,
we’ll briefly cover TCP transport optimization later.
When R1′s BGP process leaves read-only mode, it starts the best-path selection running the BGP Router
process. This process walks over new information and compare it with the local BGP RIB contents,
selecting the best-path for every prefix. It takes time proportional to the amount of the new informational
learned. Luckily, the computations are not very CPU-intensive, just like with any distance-vector protocol.
As soon as the best-path process if finished, BGP has to upload all routes to the RIB, before advertising
them to the peers. This is a requirement of distance-vector protocols – having the routing information
active in the RIB before propagating it further. The RIB update will in turn trigger FIB information upload to
the router’s line-cards, if the platform supports distributed forwarding. Both RIB and FIB updates are time-
consuming and take the time proportional to the number of prefixes being updated.
After information has been committed to RIB, R1 needs to replicate the best-paths to every peer that
should receive it. The replication process could be most memory and CPU intensive as BGP process has to
perform a full BGP table walk for every peer and construct the output for the corresponding BGP Adj-RIB-
Out. This may require additional transient memory in the course of the update batch calculation. However,
the update generation process is highly optimized in Cisco’s BGP implementation by means of dynamic
update groups. The essence of the dynamic update groups is that BGP process dynamically finds all
neighbors sharing the same output policies, then elects a peer with the lowest IP address as the group
leader and only generates the updates batch for the group leader. All other members of the same group
receive the same updates. In our case, R1 has to generate two update sets: one for R5 and another for
the pair of RR1 and RR2 route reflectors. The BGP update groups become very effective on route-
reflectors that often have hundred of peers sharing the same policies. You may see the update groups
using the command show ip bgp replication for IPv4 sessions.
R1 starts sending updates to R1 and RR1, RR2. This will take some time, depending on the BGP TCP
transport settings and BGP table size. However, before R1 will ever start sending any updates to any
peer/update group, it checks if Advertisement Interval timer is running for this peer. BGP speaker starts
this timer on per-peer basis every time its done sending the full batch of updates to the peer. If the
subsequent batch is prepared to be sent and the timer is still running, the update will be delayed until the
timer expires. This is a dampening mechanism to prevent unstable peers from flooding the network with
updates. The command to define this timer is neighbor X.X.X.X advertisment-interval XX. The default
values are 30 seconds for eBGP and 5 seconds for iBGP/eiBGP sessions (intra-AS). This timer really starts
playing its role only for “Down-Up” or “Up-Down” convergence, as any rapid flapping changes are delayed
for the amount of advertisement-interval seconds. This becomes especially important for inter-AS route
propagation, where the default advertisement-interval there is 30 seconds.

The process repeats itself on RR1 and RR2, starting with the incoming UPDATE packet reception, best-path
selection and update generation. If for some reason the prefix 20.0.0.0/8 would vanish from AS 100 soon after it
has been advertised, it may take as long as “Number_of_Hops x Advertisement_Interval” to reach to R3 and R4,
as every hop may delay the fast subsequent update. As we can see, the main limiting factors of BGP convergence
are BGP table size, transport-level settings and advertisement delay. The best-path selection time is proportional
to the table size as well as time required for update batching.

Let’s look at a slightly different scenario to demonstrate how BGP multi-path may potentially improve convergence.
Firstly, observing the topology presented on FIG 1, we may notice that AS 300 has two connections to AS 100.
Thus, it may be expected to see two paths to every route from AS 100 on every router in AS 300. But this is not
always possible in situations where any topology other than BGP full mesh is used inside the AS. In our example,
R1 and R2 advertise routing information to the route-reflectors RR1 and RR2. Per the distance-vector behavior,
the reflectors will only re-advertise the best-path to AS 100 prefixes, and since both RRs elect paths consistently,
they will advertise the same path to R3, R4 and R2. Both R3 and R4 will receive the prefix 10.0.0.0/24 from each
of the RRs and use the path via R1. R2 will receive the best path via R1 as well but prefer using its eBGP
connection. On contrary, if R1, R2, R3 and R4 were connected in the full mesh, then every router would have
seen exits via R1 and R2 and be able to use BGP multi-path if configured. Let’s review what happens in the
topology on FIG1 when R1 loses connection to AS 100.

Depending on the failure detection mechanism, be it BGP keepalives or BFD, it will take some time for R1
to realize the connection is no longer valid. We’ll discuss the options for fast failure detection later in this
publication.
After realizing that R5 is gone, R1 deletes all paths via R7. Since RR1 and RR2 never advertised back to
R1 the path via R2, R1 has no alternate paths to AS 100. Realizing this, R1 prepares a batch of UPDATE
messages for RR1, RR2 and R7, containing the withdrawal messages for AS 100 prefixes. As soon as RR1
and RR2 are done receiving and processing the withdrawals, they elect the new best path via R2 and
advertise withdrawals/updates to R1, R2, R3, R4.
R3 and R4 now have the new path via R2, and R2 loses the “backup” path via R1 it knew about from the
RRs. The main workhorses of the re-convergence process in this case are the route-reflectors. The
convergence time is sum of the peering session failure detection, update advertisement and BGP best-
path recalculations in the RRs.

If BGP speakers were able to utilize multiple paths at the same time, then it could be possible to alleviate the
severity of a network failure. Indeed, if load-balancing is in use, then a failure of an exit point will only affect flows
going across this exit point (50% in our case) and only those flows will have to wait for re-convergence time. Even
better, it is theoretically possible to do “fast” re-route in the case where multiple equal-cost (equivalent and thus
loop–less) paths are available in BGP. Such switchover could be performed in the forwarding engine, as soon as
the failure is signaled. However, there are two major problems with the re-route mechanism of this type:

1. As we have seen, the use of route-reflectors (or confederations) has significant effect on redundancy by
hiding alternate paths. Using full-mesh is not an option, so a mechanism needed allowing propagation of
multiple alternate paths in RR/Confederation environment. It is interesting to point out that such mechanism
is already available in BGP/MPLS VPN scenarios, where multiple point of attachments for CE sites could
utilize different RD values to differentiate the same routes advertised from different connection points.
However, a generic solution is required, allowing for advertising multiple alternate paths with IPv4 or any
other address-family.
2. Failure detection and propagation by means of BGP mechanics is slow, and depends on the number of
affected prefixes. Therefore, the more severe is the damage, the slower it is propagated in the BGP. Some
other, non-BGP mechanism needs to be used to report network failures and trigger BGP re-convergence.

In the following sections we are going to review various technologies developed to accelerate BGP convergence,
enabling far better reaction times compared to “pure BGP based” failure detection and repair.

Tuning BGP Transport

Tuning BGP transport mechanism is a very important factor for improving BGP performance in the cases where
purely BGP-based re-convergence process is in use. TCP is the underlying transport used for propagating BGP
UPDATE messages, and optimizing TCP performance directly benefits BGP. If you take the full Internet routing
table, which is above 300k prefixes (Y2010), then simply transporting the prefixes alone will consume over 10
Megabytes, not to count the path attributes and other information. Tuning TCP transport performance includes the
following:

1. Enabling TCP Path MTU discovery for every neighbor, to allow the TCP selecting optimum MSS size. Notice
that this requires that no firewall blocks the ICMP unreachable messages used during the discovery
process
2. Tuning the router’s ingress queue size to allow for successful absorption of large amount of TCP ACK
messages. When a router starts replicating BGP UPDATES to its peers, every peer responds with TCP
ACK message to normally every second segment sent (TCP Delayed ACK). The more peers router has,
the higher will be the pressure on the ingress queue.

Very detailed information on tuning BGP transport could be found in [10] Chapter 3. We, therefore, skip an in-
depth discussion of this topic here.

BGP Fast Peering Session Deactivation

When using BGP-only convergence mechanic, detecting a link failure is normally based on BGP KEEPALIVE
timers, which are 60/180 seconds by default. It could be noted that TCP keepalives could be used for the same
purpose, but since BGP already has similar mechanics these are not of any big help. It is possible to tune the BGP
keepalive timers to be as low as 1/3 seconds, but the risk of peering session flapping become significant with such
settings. Such instability is dangerous since there is no built-in session dampening mechanism in BGP session
establishment process. Therefore, some other mechanism should be preferred – either BFD or fast BGP peering
session deactivation. The last option is on by default for eBGP sessions, and tracks the outgoing interface
associated with the BGP session. As soon as the interface (or the next-hop for multihop eBGP) is reported as
down, the BGP session is deactivated. Interface flapping could be effectively dampened using IP Event Dampening
in Cisco IOS (see [14]) and hence is less dangerous than BGP peering session flapping. The command to disable
fast peering session deactivation is no bgp fast-external-fallover. Notice that this feature is by default off for
iBGP sessions, as those are supposed to be routed and restored using the underlying IGP mechanics.

Using BFD is the best option on multipoint interfaces, such as Ethernet, that do not support fast link down
detection e.g. by means of Ethernet OAM. BFD is especially attractive in the platforms that implement it in the
hardware. The command to activate BFD fallover is neighbor fall-over bfd. In the following sections, we’ll
discuss the use of IGP for fast reporting of link failures.

BGP and IGP Interaction

BGP prefixes typically rely on recursive next-hop resolution. That is, next-hops associated with BGP prefixes are
normally not directly connected, but rather resolved via IGP. The core of BGP and IGP interaction used to be
implemented in the BGP Scanner process. This process runs periodically and among other work performs full
BGP table walk and validates the BGP next-hop values. The validation consists of resolving the next-hop
recursively through the router’s RIB and possibly changing the forwarding information in response to IGP events.
For example, if R1 crashes on FIG1, it will take 180 seconds for the RRs to notice the failure based on BGP
KEEPALIVE message. However, the IGP will probably converge faster and report R1′s address as unreachable.
This event will be detected during the BGP Scanner process run and all paths via R1 will be invalidated by all
BGP speakers in AS 100. The default BGP Scanner run-time is 60 seconds, and could be changed using the
command bgp scan-time. Notice that setting this value too low may result in extra burden on router’s CPU if you
have large BGP tables, since the scanner process has to perform full table walk every time it executes.

The periodic behavior of BGP Scanner is still too slow to effectively respond to IGP events. IGP protocols could be
tuned to react to a network change within hundreds of milliseconds (see [6]) and it would be desirable to make
BGP aware of such changes as quickly as possible. This could be done with the help of BGP Next-Hop Tracking
(NHT) feature. The idea is to make the BGP process register the next-hop values with the RIB “watcher” process
and require a “call-back” every time information about the prefix corresponding to the next-hop changes. Typically,
the number of registered next-hop values equals the number of exits from the local AS, or the number of PEs in
MPLS/BGP VPN environment, so next-hop tracking does not impose heavy memory/CPU requirements. There are
normally two types of events: IGP prefix becoming unreachable and IGP prefix metric change. The first event is
more important and reported faster than metric change. Overall, IGP delays report of an event for the duration of
bgp nextop trigger delay XX interval which is 5 seconds by default. This allows for more consecutive events to
be processed and received from IGP and effectively implements event aggregation. This delay is helpful in various
“fate sharing” scenarios where a facility failure affects multiple links in the network, and BGP needs to ensure that
all IGP nodes have reported this failure and IGP has fully converged. Normally, you should set the NHT delay to be
slightly above the time it takes the IGP to fully converge upon a change in the network. In a fast-tuned IGP
network, you can set this delay to as low as 0 seconds, so that every IGP event is reported immediately, though
this requires careful underlying IGP tuning to avoid oscillations. See [6] for more information on tuning the IGP
protocol settings, but in short, you need to tune the SPF delay value in IGP to be conservative enough to capture
all changes that could be caused by a failure in the network. Setting SPF delay too low may result is excessive
BGP next-hop recalculations and massive best-path process runs.

As a reaction to an IGP next-hop change, the BGP process has to start BGP Router sub-process for re-
calculating the best paths. This will affect every prefix that has the next-hop changed as a result of IGP event, and
could take significant amount of time, based on number of prefixes associated with this nexthop. For example, if an
AS has two connections to the Internet and receives full BGP tables over both connections, then a single exit
failure will force full-table walk for over 300k prefixes. After this happens, BGP has to upload the new forwarding
information to RIB/FIB, with the overall delay being proportional to the table size. To put it in other words, BGP
convergence is non-deterministic in response to an IGP event, e.g. there is no well-defined finite time for the
process to complete. However, if the IGP change did not result in any effects to BGP next-hop, e.g. if IGP was able
to repair the path upon link failure and the path has the same cost, then BGP is not needed to be informed at all
and convergence is handled at IGP level.

The last, less visible contributor to faster convergence is Hierarchical FIB. Look at the figure below – it shows
how FIB could be organized as either “flat” or “hierarchical”. In the “flat” case, BGP prefixes have their forwarding
information directly associated – e.g. the outgoing interface, MAC rewrite, MPLS label information and so on. In
such case, any change to a BGP next-hop may require updating a lot of prefixes sharing the same next-hop, which
is a time consuming process. If the next-hop value remains the same, and only the output interface changes, the
FIB update process still needs walking over all BGP prefixes and reprogramming the forwarding information. In
case of “hierarchical” FIB, any IGP change that does not affect BGP prefixes, e.g. output interface change, only
requires walking over the IGP prefixes, which are not as numerous as BGP. Therefore, hierarchical FIB
organization significantly reduces FIB update latency in the cases where only IGP information needs to be
changed. The use of hierarchical FIB is automatic and does not require any special commands. All major
networking equipment vendors support this feature.

The last thing to discuss in relation to BGP NHT is IGP route summarization. Summarization hides detailed
information and may conceal changes occurring in the network. In such case, BGP process will not be notified of
the IGP event and will have to detect failure and re-converge using BGP-only mechanics. Look at the figure below
– because of summarization, R1 will not be notified or R2′s failure and the BGP process at R1 will have to wait till
BGP session times out. Aside from avoiding summarization for the prefixes used for iBGP peering, an alternate
solution could be using multi-hop BFD [15]. Additionally, there is some work in progress to allow the separation of
routing and reachability information natively in IGP protocols.

You can see now how NHT may allow BGP to react quickly to the events inside its own AS, provided that
underlying IGP is properly tuned for fast convergence. This fast convergence process effectively covers core link
and node failures, as well as edge link and node failures, provided that all these could be detected by IGP. You
may want to look at [1] for detailed convergence breakdowns. Pay special attention that edge link failure requires
special handling. If your edge BGP speaker is changing the next-hop value to self for the routes received from
another autonomous system, than IGP will only be able to detect failures for paths going to the BGP speaker’s own
IP address. However, if the edge link fails, the convergence will follow along the BGP path, using BGP withdrawal
message propagation through the AS. The best approach in this case is to leave the eBGP next-hop IP address
unmodified and advertise the edge link into IGP using the passive interface feature or redistribution. This will allow
the IGP to respond to link down condition by quickly propagating the new LSA and synchronously trigger BGP re-
convergence on all BGP speakers in the system by informing them of the failed next-hop. In topologies with large
BGP tables this takes significantly less time compared to BGP-based convergence process. And lastly, despite all
benefits that BGP NHT may provide for recovering from Intra-AS failures, the Inter-AS convergence is still purely
BGP driven, based on BGP’s distance-vector behavior.

BGP PIC and Multiple-Path Propagation

Even though BGP NHT enables fast reaction to IGP events, the convergence time is still not deterministic, because
it depends on the number of prefixes BGP needs to be processed for best-path selection. Previously, we
discussed how having multiple equal-cost BGP paths could be used for redundancy and fast failover at the
forwarding engine level, without involving any BGP best-path selection. What if the paths are unequal – is it
possible to use them for backup? In fact, since BGP treats the local AS as a single hop, all BGP speakers select
the same path consistently, and changing from one path to another synchronously among all speakers should not
create any permanent routing loops. Thus, even in scenarios where equal-cost BGP multi-path is not possible, the
secondary paths may still be used for fast failover, provided that a signaling mechanism to detect the primary path
failure exists. We already know that BGP NHT could be used to detect a failure and propagate this information
quickly to all BGP speakers, triggering local switchover. This switchover does not require any BGP table walks and
best-path re-election, but simply is a matter of changing the forwarding information – provided that hierarchical FIB
is in use. Therefore, this process does not depend on the number of BGP prefixes, and thus known as Prefix
Independent Convergence (PIC) process. You may think of this process as a BGP equivalent to IGP-based Fast
Re-Route, though in IGP failure deception is local to the router and in BGP failure detection is local to the AS. BGP
PIC could be used any time there are multiple paths to the destination prefix, such on R1 in the example below,
where target prefix is reachable via multiple paths:

We have already stated the problem with multiple paths – only one best path is advertised by BGP speakers and
the BGP speaker will only accept one path for a given prefix from a given peer. If a BGP speaker receives multiple
paths for the same prefix within the same session it simply uses the newest advertisement. A special extension to
BGP known as “Add Paths” (see [3] and [16]) allows BGP speaker to propagate and accept multiple paths for
the same prefix. The “Add Paths” capability allows peering BGP speakers to negotiate whether they support
advertising/receiving multiple paths per prefix and actually advertise such paths. A special 4-byte path-identifier is
added to NLRIs to differentiate multiple paths for the same prefix sent across a peering session. Notice that BGP
still considers all paths as comparable from the viewpoint of best-path selection process – all paths are stored in
the BGP RIB and only one is selected as the best-path. The additional NLRI identifier is only used when prefixes
are sent across a peering session to prevent implicit withdrawals by the receiving peer. These identifiers are
generated locally and independently for every peering session that supports such capability.

in addition to propagating backup paths, the “Add Paths” capability could be used for other purposes, e.g.
overcoming BGP divergence problems described in [9]. Alternatively, if backup paths are required but “Add Path”
feature is not implemented, one of your options could be using full-mesh of BGP speakers, such as on the figure
below. In this case, multiple exit point information is preserved and allows for implementing BGP PIC functionality.

Pay attention to the fact that BGP PIC is possible even without the “Add Paths” capability in RR scenarios,
provided that RRs propagate the alternate paths to the edge nodes. This may require IGP metric manipulation to
ensure different exit points are selected by the RRs or using other techniques, such as different RD values for
multi-homed site attachment points.

Practical Scenario: BGP PIC + BGP NHT

In this hands-on scenario we are going to illustrate the use of IGP tuning, BGP NHT configuration and BGP PIC
and demonstrate how they work together. First, look at the topology diagram: R9 is advertising a prefix, and R5,
R6 receive this prefix via the RRs. In normal BGP environment, provided that the RRs elect the same path, R5 and
R6 would have just one path for R9′s prefix. However, we tune the scenario, disabling the connections between R1
and R4 and R2 and R3, so R3 has better cost to exit via R1 and R4 has better cost via R2. This will make the RRs
elect different best-paths and propagate them to their clients.

The following is the key piece of configuration for enabling the fast backup path failover to be applied to every
router in AS 100. As you can see, the SPF/LSA throttling timers are tuned very aggressively to allow for fastest
reaction to IGP events. BGP nexthop trigger delay is set to 0 seconds, thus fully relying on IGP to aggregate
underlying events. In any production environment, you should NOT use these values and pick up your own,
matching your IGP scale and convergence rate.

router ospf 100
timers throttle spf 1 100 5000
timers throttle lsa all 0 100 5000
timers lsa arrival 50
!
router bgp 100
bgp nexthop trigger delay 0
bgp additional-paths install
no bgp recursion host

The command bgp additional-paths install when executed in non BGP-multipath environment allows for
installing backup paths in additional to the best one elected by BGP. This, of course, requires that the additional
paths have been advertised by the BGP Route Reflectors. At the moment of writing, Cisco IOS does not support
the “Add Paths” capability, so you need to make sure BGP RRs elect different best-paths in order for the edge
routers to be able to use additional paths. The command no bgp recursion host requires special explanation on
its own. By default, when a BGP prefix loses next-hop, the CEF process will attempt to look-up the next longest-
matching prefix for the next-hop to provide fallback. When additional repair paths are present, this functionality is
not required and will, in fact, slower the convergence. This is why it’s automatically disabled when you type the
command bgp additional-paths install and thus typing it with the “no” prefix is not really required.

Now that we have our scenario set up, we are going to demonstrate the fact that at least in current implementation,
Cisco IOS BGP process does not exchange/detects the capabilities for “Add Path” feature. Here is a debugging
output from a peering session establishment process, which shows that no “Add Path Capability” (code 69, per the
RFC draft) is being exchanged during session establishment.

R5#debug ip bgp 10.0.3.3
BGP debugging is on for neighbor 10.0.3.3 for address family: IPv4 Unicast
R5#clear ip bgp 10.0.3.3

BGP: 10.0.3.3 active rcv OPEN, version 4, holdtime 180 seconds
BGP: 10.0.3.3 active rcv OPEN w/ OPTION parameter len: 29
BGP: 10.0.3.3 active rcvd OPEN w/ optional parameter type 2 (Capability) len 6
BGP: 10.0.3.3 active OPEN has CAPABILITY code: 1, length 4
BGP: 10.0.3.3 active OPEN has MP_EXT CAP for afi/safi: 1/1
BGP: 10.0.3.3 active rcvd OPEN w/ optional parameter type 2 (Capability) len 2
BGP: 10.0.3.3 active OPEN has CAPABILITY code: 128, length 0
BGP: 10.0.3.3 active OPEN has ROUTE-REFRESH capability(old) for all address-families
BGP: 10.0.3.3 active rcvd OPEN w/ optional parameter type 2 (Capability) len 2
BGP: 10.0.3.3 active OPEN has CAPABILITY code: 2, length 0
BGP: 10.0.3.3 active OPEN has ROUTE-REFRESH capability(new) for all address-families
BGP: 10.0.3.3 active rcvd OPEN w/ optional parameter type 2 (Capability) len 3
BGP: 10.0.3.3 active OPEN has CAPABILITY code: 131, length 1
BGP: 10.0.3.3 active OPEN has MULTISESSION capability, without grouping
BGP: 10.0.3.3 active rcvd OPEN w/ optional parameter type 2 (Capability) len 6
BGP: 10.0.3.3 active OPEN has CAPABILITY code: 65, length 4
BGP: 10.0.3.3 active OPEN has 4-byte ASN CAP for: 100
BGP: nbr global 10.0.3.3 neighbor does not have IPv4 MDT topology activated
BGP: 10.0.3.3 active rcvd OPEN w/ remote AS 100, 4-byte remote AS 100
BGP: 10.0.3.3 active went from OpenSent to OpenConfirm
BGP: 10.0.3.3 active went from OpenConfirm to Established

This means that we need to rely on the BGP RRs to advertise multiple different paths in order for the edge nodes
to leverage the backup path capability.

R5#debug ip bgp updates
BGP updates debugging is on for address family: IPv4 Unicast
R5#debug ip bgp addpath
BGP additional-path related events debugging is on
R5#clear ip bgp 10.0.3.3

BGP(0): 10.0.3.3 rcvd UPDATE w/ attr: nexthop 20.0.17.7, origin i, localpref 100, metric 0, originator
10.0.1.1, clusterlist 10.0.3.3, merged path 200, AS_PATH
BGP(0): 10.0.3.3 rcvd 20.0.99.0/24
BGP(0): 10.0.3.3 rcvd NEW PATH UPDATE (bp/be - Deny)w/ prefix: 20.0.99.0/24, label 1048577, bp=N, be=N
BGP(0): 10.0.3.3 rcvd UPDATE w/ prefix: 20.0.99.0/24, - DO BESTPATH
BGP(0): Calculating bestpath for 20.0.99.0/24

Here you can see that the RR with IP address 10.0.3.3 sends us an update that has better information than the
one we currently know. However, before you enable the bgp additional-paths install there would be just one
path installed for the prefix:

R5#show ip route repair-paths 20.0.99.0
Routing entry for 20.0.99.0/24
Known via "bgp 100", distance 200, metric 0
Tag 200, type internal
Last update from 20.0.17.7 00:02:31 ago
Routing Descriptor Blocks:
* 20.0.17.7, from 10.0.3.3, 00:02:31 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 200
MPLS label: none

But as soon as the bgp additional-paths install option has been enabled, the output of the same command
looks different:

R5#show ip route repair-paths 20.0.99.0
Routing entry for 20.0.99.0/24
Known via "bgp 100", distance 200, metric 0
Tag 200, type internal
Last update from 20.0.17.7 00:00:03 ago
Routing Descriptor Blocks:
* 20.0.17.7, from 10.0.3.3, 00:00:03 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 200
MPLS label: none
[RPR]20.0.28.8, from 10.0.4.4, 00:00:03 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 200
MPLS label: none

You may also see the second path in the BGP table with the “b” (backup) flag:

R5#show ip bgp 20.0.99.0
BGP routing table entry for 20.0.99.0/24, version 39
Paths: (2 available, best #1, table default)
Additional-path
Not advertised to any peer
200
20.0.17.7 (metric 192) from 10.0.3.3 (10.0.3.3)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 10.0.1.1, Cluster list: 10.0.3.3
200
20.0.28.8 (metric 192) from 10.0.4.4 (10.0.4.4)
Origin IGP, metric 0, localpref 100, valid, internal, backup/repair
Originator: 10.0.2.2, Cluster list: 10.0.4.4

And if you check the CEF entry for this prefix, you will notice there are multiple next-hops and output interfaces
that could be used for primary/backup paths:

R5#show ip cef 20.0.99.0 detail
20.0.99.0/24, epoch 0, flags rib only nolabel, rib defined all labels
recursive via 20.0.17.7
recursive via 20.0.17.0/24
nexthop 10.0.35.3 Serial1/0
recursive via 20.0.28.8, repair
recursive via 20.0.28.0/24
nexthop 10.0.35.3 Serial1/0
nexthop 10.0.45.4 Serial1/2

Notice that in oder to use the PIC functionality, BGP multi-path should be turned off – otherwise, equal-cost paths
will be used for load-sharing, not for primary/backup behavior. You may opt to using equal-cost multipath if allowed
by the network topology, as it offers better resource utilization and CEF switching layer allows for fast path failover
in case of equal-cost load-balancing. Now for debugging the fast failover process. We want to shut down R1′s
connection to R7 and see fast backup path switchover at R5. There are few caveats here, because we have very
simplified topology. Firstly, we only have one prefix advertised into BGP on R9. Propagating this prefix through
BGP is almost instant, since BGP best-path selection is done quickly and advertisement delay does not apply to a
single event. Thus, if we shutdown R1′s connection to R7, which is used as primary path, then R1 will detect the
link failure and shutdown the session. Immediately after this BGP process will flood an UPDATE with prefix removal
and this message would reach R5 and R6 even before OSPF finishes SPF computations. The reason being, of
course, single prefix propagated via BGP and no advertisement-interval used to delay to a single event.

It may seems like that disabling BGP fast external fallover on R1 could help us to take BGP out of the equation.
However, we still have BGP NHT enabled in R1 – as soon as we shut down the link, the RIB process would report
to BGP of the next-hop failure and UPDATE message will be sent right away. Thus, we would also need to disable
NTH in R1, using the command no bgp nexthop trigger enable. If we think further, we’ll notice that we also
need to enable NHT in R3 and R4, just so that they cannot to generate their own UPDATEs to R5 ahead of OSPF
notification. Therefore, prior to running experiment we disable BGP NHT in R1, R3, R4 and disable fast external
fallover in R1. This will allow the event from R1 propagate via OSPF ahead of BGP UPDATE message and trigger
fast switchover on R5. The below is the output of the debugging commands enabled on R5 after we shut down
R1′s connection to R7.

R5#debug ip ospf spf
OSPF spf events debugging is on
OSPF spf intra events debugging is on
OSPF spf inter events debugging is on
OSPF spf external events debugging is on

R5#debug ip bgp addpath
BGP additional-path related events debugging is on

R5 receive the LSA at 26.223 then BGP starts the path switchover at 26.295 – It took 72ms to run SPF, update
RIB and inform BGP of the event and then change the paths.

14:00:26.223: OSPF: Detect change in topology Base with MTID-0, in LSA type 1, LSID 10.0.1.1 from
10.0.1.1 area 0
14:00:26.223: OSPF: Schedule SPF in area 0, topology Base with MTID 0
Change in LS ID 10.0.1.1, LSA type R, spf-type Full
….
14:00:26.295: BGP(0): Calculating bestpath for 20.0.99.0/24, New bestpath is 20.0.28.8 :path_count:-
2/0, best-path =20.0.28.8, bestpath runtime :- 4 ms(or 3847 usec) for net 20.0.99.0
14:00:26.299: BGP(0): Calculating backuppath::Backup-Path for 20.0.99.0/24:BUMP-VERSION-BACKUP-
DELETE:, backup path runtime :- 0 ms (or 193 usec)

14:00:32.439: BGP(0): 10.0.3.3 rcvd UPDATE w/ prefix: 20.0.99.0/24, - DO BESTPATH
14:00:32.443: BGP(0): Calculating bestpath for 20.0.99.0/24, bestpath is 20.0.28.8 :path_count:-
2/0, best-path =20.0.28.8, bestpath runtime :- 0 ms(or 222 usec) for net 20.0.99.0
14:00:32.443: BGP(0): Calculating backuppath::Backup-Path for 20.0.99.0/24, backup path runtime :- 0
ms (or 133 usec)

In the debugging output above, you can see that the BGP process in R5 switched to backup path even before it
received the UPDATE message from R3, signaling the change of the best-path in the RR. Notice that the update
does not have any path identifiers in the NLRI, as the RR has only a single best-path. Let’s see how much time it
actually took to run SPF, as compared to overall detection/failover process:

R5#show ip ospf statistics

OSPF Router with ID (10.0.5.5) (Process ID 100)

Area 0: SPF algorithm executed 15 times

Summary OSPF SPF statistic

SPF calculation time
Delta T Intra D-Intra Summ D-Summ Ext D-Ext Total Reason
00:28:00 44 0 0 4 0 4 56 R
…..

As you can see, the total SPF runtime was 56ms. Therefore, the remaining 20ms were spent on updating RIB and
triggering the next-hop change event. Of course, all these numbers have only relative meaning, as we are using
Dynamips for this simulation, but you may use similar methodology when validating real-world designs.

Considerations for Implementing BGP Add Paths

Even though the Add Paths feature is not yet implemented, it is worth considering the drawbacks of this approach.
One drawback is that the amount information needed to be sent and stored is now multiplied by the number of
additional paths. Previously, the most stressed routers in BGP AS were route reflectors, that had to carry the
largest BGP tables. With the Add-Path functionality, every non-RR speaker now receives all information that RR
stores in its BGP table. This puts extra requirement on the edge speakers and should be accounted when
planning to use this feature. Furthermore, additional paths will utilize extra memory on the forwarding engines, as
now PIC-enabled prefixes have multiple alternate paths. However, since the number of prefixes remains the same,
TCAM fast lookup memory resources will not be wasted, and thus only dynamic RAM is being affected the most.
You may read more about scalability/performance trade-offs in [17]

Summary

Achieving fast BGP convergence is not easy, because BGP is a complicated routing protocol running overlay on
top of an IGP process. We found out that tuning purely BGP-based convergence requires the following general
steps:

Tuning BGP TCP Transport and router ingress queues to achieve faster routing information propagation.
Proper organization of outbound policies for achieving optimum update group construction.
Tuning BGP Advertisement Interval if needed to respond to fast “Down->Up” conditions
Activating BGP fast external fallover and possible BFD for fast external peering session deactivation.

As we noticed previously, pure-BGP based convergence is the only thing available for Inter-AS scenarios.
However, for fastest convergence inside a single AS, understanding and tuning BGP and IGP interaction can make
BGP converge almost as fast as the underlying IGP. This allows for fast recovery in response to intra-AS link and
node failures, as well as to edge link failures. Optimizing BGP and IGP interaction requires the following:

Tuning the underlying IGP for fast convergence. It is possible to tune the IGP even for large network to
converge under one second.
Enabling BGP Next-Hop Tracking process for all BGP speakers and tuning the BGP NHT delay in
accordance with IGP response time.
Applying IGP summarization carefully to avoid hiding BGP NHT information.
Leveraging IGP for propagation of external peering link failures, in addition to relying on BGP peering
session deactivation.
Using the Add-Path Functionality in critical BGP speakers (e.g. RRs) to allow for propagation of redundant
paths if supported by implementation.
Use BGP PIC or fast backup switchover in the environments that allow for multiple paths to be propagated
– e.g. multihomed MPLS VPN sites using different RD values.

We’ve also briefly covered some caveats resulting from the future use of “Add-Path” functionality, such as
excessive usage of memory resources on router-processor and line-cards and extra toll on BGP best-path
process due to the growth of alternate paths. There are few things that were left out of the scope of this paper. We
didn’t not concentrate on the detailed mechanics of BGP fast peering session deactivation e.g. for multihop
sessions and we did not cover the MP-BGP specific features. Some MP-BGP extensions such as the additional
import scan interval and edge control plane interworking have their effects on end-to-end convergence, but this is
a topic for another discussion.

Further Reading

[0]RFC4271: Border Gateway Protocol
[1]Advanced BGP Convergence Techniques
[2]Graph Overlays on Path Vector: A Possible Next Step in BGP
[3]BGP Add Paths Capability
[4]BGP Convergence in much less than a second
[5]BGP PIC Configuration Guide
[6]OSPF Fast Convergence
[7]An Analysis of BGP Convergence Properties
[8]RFC4451: BGP MULTI_EXIT_DISC (MED) Considerations
[9]RFC3345: Border Gateway Protocol (BGP) Persistent Route Oscillation Condition
[10]BGP Design and Implementation by Randy Zhang
[11]RFC 4274: BGP Protocol Analysis
[12]Day in the Life of a BGP Update in Cisco IOS
[13]RFC 4724: Graceful Restart for BGP
[14]Optimizing IP Event Dampening
[15]RFC 5883: Multihop BFD
[16]BGP Add Path Overview
[17]BGP Add Paths Scaling/Performance Tradeoffs

Appendix: Practical Scenario Baseline Configuration

The below are the initial configurations for the Dynamips topology used to validate BGP PIC behavior.

====R1:====
hostname R1
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
ip address 20.0.17.1 255.255.255.0
no shut
!
interface Serial 1/2
no shut
ip address 10.0.12.1 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.13.1 255.255.255.0
!
interface Serial 1/3
ip address 10.0.14.1 255.255.255.0
!
interface Loopback0
ip address 10.0.1.1 255.255.255.255
!
router ospf 100
router-id 10.0.1.1
network 0.0.0.0 0.0.0.0 area 0
passive-interface Serial 1/0
!
router bgp 100
neighbor 10.0.3.3 remote-as 100
neighbor 10.0.3.3 update-source Loopback0
neighbor 10.0.4.4 remote-as 100
neighbor 10.0.4.4 update-source Loopback0
neighbor 20.0.17.7 remote-as 200

====R2:====
hostname R2
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
ip address 20.0.28.2 255.255.255.0
no shut
!
interface Serial 1/2
no shut
ip address 10.0.12.2 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.24.2 255.255.255.0
!
interface Serial 1/3
no shut
ip address 10.0.23.2 255.255.255.0
!
interface Loopback0
ip address 10.0.2.2 255.255.255.255
!
router ospf 100
router-id 10.0.2.2
network 0.0.0.0 0.0.0.0 area 0
passive-interface Serial 1/0
!
router bgp 100
neighbor 10.0.3.3 remote-as 100
neighbor 10.0.3.3 update-source Loopback0
neighbor 10.0.4.4 remote-as 100
neighbor 10.0.4.4 update-source Loopback0
neighbor 20.0.28.8 remote-as 200

====R3:====
hostname R3
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 10.0.13.3 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.35.3 255.255.255.0
!
interface Serial 1/2
no shut
ip address 10.0.34.3 255.255.255.0
!
interface Serial 1/3
no shut
ip address 10.0.23.3 255.255.255.0
!
interface Serial 1/4
no shut
ip address 10.0.36.3 255.255.255.0
!
router ospf 100
router-id 10.0.3.3
network 0.0.0.0 0.0.0.0 area 0
!
interface Loopback0
ip address 10.0.3.3 255.255.255.255
!
router bgp 100
neighbor IBGP peer-group
neighbor IBGP remote-as 100
neighbor IBGP update-source Loopback0
neighbor IBGP route-reflector-client
neighbor 10.0.1.1 peer-group IBGP
neighbor 10.0.2.2 peer-group IBGP
neighbor 10.0.5.5 peer-group IBGP
neighbor 10.0.6.6 peer-group IBGP
neighbor 10.0.4.4 remote-as 100
neighbor 10.0.4.4 update-source Loopback0

====R4:====
hostname R4
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 10.0.24.4 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.46.4 255.255.255.0
!
interface Serial 1/2
no shut
ip address 10.0.34.4 255.255.255.0
!
interface Serial 1/3
no shut
ip address 10.0.14.4 255.255.255.0
!
interface Serial 1/4
no shut
ip address 10.0.45.4 255.255.255.0
!
router ospf 100
router-id 10.0.4.4
network 0.0.0.0 0.0.0.0 area 0
!
interface Loopback0
ip address 10.0.4.4 255.255.255.255
!
router bgp 100
neighbor IBGP peer-group
neighbor IBGP remote-as 100
neighbor IBGP update-source Loopback0
neighbor IBGP route-reflector-client
neighbor 10.0.1.1 peer-group IBGP
neighbor 10.0.2.2 peer-group IBGP
neighbor 10.0.5.5 peer-group IBGP
neighbor 10.0.6.6 peer-group IBGP
neighbor 10.0.3.3 remote-as 100
neighbor 10.0.3.3 update-source Loopback0

====R5:====
hostname R5
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 10.0.35.5 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.56.5 255.255.255.0
!
interface Serial 1/2
no shut
ip address 10.0.45.5 255.255.255.0
!
router ospf 100
router-id 10.0.5.5
network 0.0.0.0 0.0.0.0 area 0
!
interface Loopback0
ip address 10.0.5.5 255.255.255.0
!
router bgp 100
neighbor 10.0.3.3 remote-as 100
neighbor 10.0.3.3 update-source Loopback0
neighbor 10.0.4.4 remote-as 100
neighbor 10.0.4.4 update-source Loopback0

====R6:====
hostname R6
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 10.0.46.6 255.255.255.0
!
interface Serial 1/1
no shut
ip address 10.0.56.6 255.255.255.0
!
interface Serial 1/2
no shut
ip address 10.0.36.6 255.255.255.0
!
router ospf 100
router-id 10.0.6.6
network 0.0.0.0 0.0.0.0 area 0
!
interface Loopback0
ip address 10.0.6.6 255.255.255.0
!
router bgp 100
neighbor 10.0.3.3 remote-as 100
neighbor 10.0.3.3 update-source Loopback0
neighbor 10.0.4.4 remote-as 100
neighbor 10.0.4.4 update-source Loopback0

====R7:====
hostname R7
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 20.0.17.7 255.255.255.0
!
interface Serial 1/1
no shut
ip address 20.0.78.7 255.255.255.0
!
interface Serial 1/2
no shut
ip address 20.0.79.7 255.255.255.0
!
interface Loopback0
ip address 20.0.7.7 255.255.255.0
!
router ospf 1
router-id 20.0.7.7
network 0.0.0.0 0.0.0.0 area 0
passive-interface Serial 1/0
!
router bgp 200
neighbor 20.0.17.1 remote-as 100
neighbor 20.0.9.9 remote-as 200
neighbor 20.0.9.9 update-source Loopback0
neighbor 20.0.8.8 remote-as 200
neighbor 20.0.8.8 update-source Loopback0

====R8:====
hostname R8
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 20.0.28.8 255.255.255.0
!
interface Serial 1/1
no shut
ip address 20.0.78.8 255.255.255.0
!
interface Serial 1/2
no shut
ip address 20.0.89.8 255.255.255.0
!
interface Loopback0
ip address 20.0.8.8 255.255.255.0
!
router ospf 1
router-id 20.0.8.8
network 0.0.0.0 0.0.0.0 area 0
passive-interface Serial 1/0
!
router bgp 200
neighbor 20.0.28.2 remote-as 100
neighbor 20.0.9.9 remote-as 200
neighbor 20.0.9.9 update-source Loopback0
neighbor 20.0.7.7 remote-as 200
neighbor 20.0.7.7 update-source Loopback0

====R9:====
hostname R9
!
ip tcp synwait-time 5
no ip domain-lookup
no service timestamps
!
line con 0
logging synch
exec-timeout 0 0
privilege level 15
!
ip routing
!
interface Serial 1/0
no shut
ip address 20.0.79.9 255.255.255.0
!
interface Serial 1/1
no shut
ip address 20.0.89.9 255.255.255.0
!
interface Loopback0
ip address 20.0.9.9 255.255.255.0
!
interface Loopback100
ip address 20.0.99.99 255.255.255.0
!
router ospf 1
router-id 20.0.9.9
network 0.0.0.0 0.0.0.0 area 0
!
router bgp 200
neighbor 20.0.8.8 remote-as 200
neighbor 20.0.8.8 update-source Loopback0
neighbor 20.0.7.7 remote-as 200
neighbor 20.0.7.7 update-source Loopback0
network 20.0.99.0 mask 255.255.255.0

Tags: bgp convergence, bgp nht, bgp pic, CCDE, CCDP, ccie, next-hop tracking

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About Petr Lapukhov, 4xCCIE/CCDE:
Petr Lapukhov's career in IT begain in 1988 w ith a focus on computer programming, and progressed into netw orking
w ith his first exposure to Novell NetWare in 1991. Initially involved w ith Kazan State University's campus netw ork
support and UNIX system administration, he w ent through the path of becoming a netw orking consultant, taking part in
many netw ork deployment projects. Petr currently has over 12 years of experience w orking in the Cisco netw orking
field, and is the only person in the w orld to have obtained four CCIEs in under tw o years, passing each on his first
attempt. Petr is an exceptional case in that he has been w orking w ith all of the technologies covered in his four CCIE
tracks (R&S, Security, SP, and Voice) on a daily basis for many years. When not actively teaching classes, developing
self-paced products, studying for the CCDE Practical & the CCIE Storage Lab Exam, and completing his PhD in Applied
Mathematics.
Find all posts by Petr Lapukhov, 4xCCIE/CCDE | Visit Website

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18 Responses to “Understanding BGP Convergence”

November 22, 2010 at 8:26 am
Maher El Zein, CCIE No. 21032

Really Amazing … Thank you Petr.

Reply

November 22, 2010 at 4:55 pm
john

Thank you for the great post; how long does it take you to put these posts together? You are very detailed and yet make it
understandable…thanks (again)…and we appreciate your effort!

Reply

November 23, 2010 at 4:12 am
Amit

Wow, Cisco must be acknowledged for bringing the “Add-Path” feature to the IOS already while it is still in the draft phase.

Of course, thank u Petr for such a detailed post.

Reply

November 23, 2010 at 4:45 am
Amit

BTW, how did you test it on dynamips? IOS 15.0(1)M4 doesn’t support the Add-Path feature on 7200 routers.

Reply

November 23, 2010 at 9:13 am
Petr Lapukhov, 4xCCIE/CCDE

@Amit,

I was using the 12.2(33)SRE train for testing The Add Paths capability is not in 15.x train yet as far as I know.

Reply

November 23, 2010 at 6:19 pm
Joe

Great post, what system on you using to run dynamips?

Reply

November 23, 2010 at 8:06 pm
Petr Lapukhov, 4xCCIE/CCDE

@Everyone,

My apologies for creating some confusion here. It appears at present moment Cisco IOS 12.2(33)SRE DOES NOT support the “Add
Paths” feature, but only the fast BGP PIC. My scenario backup behavior was based on the fact that R3 and R4 (the RRs) were
advertising different best-paths, for their closest exit points respectively. You still need to use MPLS VPN RDs to allow for multiple-
path propagation in MPLS VPN environment.

Reply

November 24, 2010 at 5:58 pm
Dan Kirkland, 3xCCIE

Petr,

Another great post!

(FYI, looks like there’s a small typo in the link for reference #16.)

Reply

December 19, 2010 at 6:06 pm
Mustard Bayramov

greate post. btw add-path is support in 4.0 XR

Reply

January 27, 2011 at 12:54 pm
Convergência, no BGP « Infraestrutura da Internet

[...] Lapukhov, Understanding BGP Convergence. Disponível em: http://blog.ine.com/2010/11/22/understanding-bgp-convergence/.
Acessado em: [...]

Reply

May 20, 2011 at 10:47 am
Faraz

So just to clarify, BGP PIC is a feature thats allows a router to install backup routes for a prefix for fast re-route capability.

Add Path is a feature that allows a peer to send/accept multiple paths for the same prefix.

Reply

August 1, 2011 at 2:44 am
yognshunz

thanks for your great job.

Reply

January 17, 2012 at 11:32 pm
Abdul Kaleem

Thanks good document

Reply

February 4, 2012 at 3:08 am
saurabh

Very detailed n informative post..thanks Petr

Reply

April 1, 2012 at 11:15 am
Mohit Bhalla

how did a ibgp able to understand or what kind of logical operation occur to stop sending the routes it learned from one ebgp to
other ibgps

Reply

July 24, 2012 at 11:10 am
fornarina

Incredible !!. What a great paper !!. You are the best. Thanks Petr.

Reply

September 19, 2013 at 7:00 am
Fulvio Allegretti

Great post, thanks. (Fig 6 is not completely right, the routers in as 200 should be R7 and R8, not 5 and 6 again)

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February 7, 2014 at 11:36 am
bgp – advertising multiple paths | njetwork.si

[...] Server, IDR Working Group draft Internet Exchange Route Server Operations, GROW Working Group draft Understanding BGP
Convergence, Posted by Petr Lapukhov Share this:TwitterFacebookLike this:Like [...]

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