You are on page 1of 4

Scenario 1: Trunking and DTP

This scenario is built around a network of switches connected by trunking links. You need
to think about how DTP operates and how trunks are negotiated (or not) between
switches. Consider the network shown in Figure 19-1 and answer the questions that
follow. Assume that all switches shown support DTP.

Figure 19-1. Diagram for Scenario 1

What is the mode of the link between Catalyst A and Catalyst B?
2.Suppose that the network administrator types these commands for interface
Gigabit Ethernet 0/1 on Catalyst B:

Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport no negotiate

What will the link mode be now?
Catalyst B has been given the command no switchport no negotiate for interface

all switch supervisor CPUs will receive and process the broadcasts because each switch has an IP address for management assigned to VLAN 1. (For this reason. All hosts on VLAN 1 (PC-1. The two PC devices are connected to different VLANs. User- generated broadcasts can overload the switch supervisor to the extent that it no longer can keep track of its control or "overhead" protocols. 5. In addition. The switches are passively waiting for the other to initiate trunking. Where would the effects of this storm be experienced in this network? Consider both devices and links. and so forth. Can PC-2 ping PC-4? 6. The link is still an access link. No. such as VTP. while Catalyst C actively tries to negotiate trunking. 4. Catalyst A expects trunking on the link. Trunking finally has been established. 3. All trunk links between switches will transport the broadcast frames. with no trunking established. Catalyst A is waiting to be asked to trunk. no traffic will cross between them. PC-2. Will PC-4 receive the broadcasts? Scenario 1 Answers 1. 6. it is recommended to reserve VLAN 1 for control protocol traffic only. Trunking is still not established. Instead. transporting VLANs 1 through 1005. Trunking. Both switches A and B will use DTP. What is the link mode now? 4. What is the mode of the link between Catalyst A and Catalyst C? 5. and B will effectively ask A to bring up a trunk link. Suppose that PC-1 begins to generate a broadcast storm. and PC-3) will experience the broadcast storm. Catalyst B will never try to negotiate trunking because its DTP packets have been silenced. because both switches are set to auto mode. CDP.) . Assume that all links between Catalyst switches are in trunking mode. 2. Gigabit Ethernet 0/1. all user traffic should be kept off VLAN 1. Without a router or Layer 3 device connecting the VLANs. and Catalyst B is set to no negotiate.

If VLAN 14 is created on Catalyst A. Figure 19-2. Catalyst C is a VTP client. Portions of the configurations of the three Catalyst switches are shown above them. what other switches also will create VLAN 15 through VTP? 7. and Catalyst B is configured for VTP transparent mode. PC-2 and PC-3 are assigned to the same IP subnet (using subnet mask 255. All switches are in the Bermuda management domain. If VLAN 15 is created on Catalyst B. Suppose that Catalyst A is a VTP server. what will happen? . which switches also will create VLAN 14 using VTP? 6. If VLAN 16 is created on Catalyst C. and VTP This scenario is designed to stir your thinking about VLAN and trunking connectivity. You also need to examine switch configurations and apply them to a network diagram. PC-1 and PC-2 both are configured with IP addresses on the same subnet.0.0. Trunking. Suppose that the trunk between Catalyst B and Catalyst C is configured properly. Where will VLAN1 be pruned? Why? 5.0) and the same VLAN. Diagram for Scenario 2 [View full size image] 1. See the diagram shown in Figure 19-2 and answer the questions that follow. Will the trunk link between Catalyst B and Catalyst C come up successfully? 4. can PC-1 ping PC-2? 2. Given the switch configurations shown. Notice that each PC connects to a different VLAN number. Can PC-2 and PC-3 ping each other? 3.Scenario 2: VLANs.

Because the trunk encapsulations are different. Because it is in VTP client mode. they could not ping each other unless a Layer 3 device were present to route between the Layer 2 VLANs. VLAN 2 and VLAN 10. Catalyst C will not allow any VLANs to be created unless they are learned from a VTP server in the bermuda domain. 6. 5. Yes. Because it is in transparent mode.Yes. 3. These VLANs are significant only to the local switch. no VLAN activity will be advertised to other neighboring switches. As a side note. and these protocols will not pass successfully. even though the trunk will not work end to end because of the encapsulation mismatch. Only Catalyst B creates VLAN 15. by default) in dot1Q encapsulation. it is an access VLAN port on VLAN 10. respectively. However. Catalyst B has the link configured as an ISL trunk. each switch will tag VLAN 1 differently. On one switch. In this case. no VLAN changes can be performed from the console. connecting the two VLANs. but both ends of the link are configured to trunk unconditionally. whereas Catalyst C has it configured as an 802. however. Catalyst B is allowed to create. VLAN 1 will not be contiguous across the link. Catalyst B in transparent mode relays only the VTP information. the link is an access VLAN port on VLAN 2. . DTP and CDP packets will be exchanged between the switches. VLAN 1 will not be pruned. 4. delete. no data will pass between them. Normally. on the other end. without interpreting the information. data can pass across the link freely. Both of these protocols are sent over VLAN 1 in ISL encapsulation and over the native VLAN (VLAN 1. Again. and each switch has no knowledge of what VLAN the other has assigned to the link.Scenario 2 Answers 1. Because the trunk encapsulation is different on each end of the link. Only Catalyst C creates VLAN 14 in response to VTP advertisements. This is because DTP packets will be exchanged. 7. Therefore. if these were assigned to different VLANs. These are physically connected. the key is the link between Catalyst B and C. the trunk link on each switch will come up successfully. the link between Catalyst A and B is the key. Therefore.1Q trunk. No. PC-1 and PC-2 are connected to access VLAN switch ports. 2. and rename VLANs freely. Although VLAN 1 is present on all switches. it is not pruned because VLAN 1 is ineligible for pruning by definition.