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

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

Suppose that Catalyst A is a VTP server. Portions of the configurations of the three Catalyst switches are shown above them. what will happen? . can PC-1 ping PC-2? 2. Can PC-2 and PC-3 ping each other? 3. Where will VLAN1 be pruned? Why? 5.0. Suppose that the trunk between Catalyst B and Catalyst C is configured properly. Catalyst C is a VTP client. If VLAN 15 is created on Catalyst B. Diagram for Scenario 2 [View full size image] 1. PC-2 and PC-3 are assigned to the same IP subnet (using subnet mask 255. Trunking.0. PC-1 and PC-2 both are configured with IP addresses on the same subnet. Will the trunk link between Catalyst B and Catalyst C come up successfully? 4. Notice that each PC connects to a different VLAN number. which switches also will create VLAN 14 using VTP? 6. All switches are in the Bermuda management domain. and Catalyst B is configured for VTP transparent mode. and VTP This scenario is designed to stir your thinking about VLAN and trunking connectivity.Scenario 2: VLANs.0) and the same VLAN. If VLAN 16 is created on Catalyst C. what other switches also will create VLAN 15 through VTP? 7. See the diagram shown in Figure 19-2 and answer the questions that follow. Figure 19-2. If VLAN 14 is created on Catalyst A. You also need to examine switch configurations and apply them to a network diagram. Given the switch configurations shown.

5. VLAN 1 will not be pruned. Both of these protocols are sent over VLAN 1 in ISL encapsulation and over the native VLAN (VLAN 1. VLAN 1 will not be contiguous across the link. Because it is in VTP client mode. On one switch. Because the trunk encapsulation is different on each end of the link. Catalyst B has the link configured as an ISL trunk. respectively. the link is an access VLAN port on VLAN 2. 3. These are physically connected. PC-1 and PC-2 are connected to access VLAN switch ports. DTP and CDP packets will be exchanged between the switches. connecting the two VLANs. Catalyst B is allowed to create. As a side note. no VLAN changes can be performed from the console. each switch will tag VLAN 1 differently. Although VLAN 1 is present on all switches. Catalyst B in transparent mode relays only the VTP information. Again. In this case. 7. . on the other end. and rename VLANs freely. the trunk link on each switch will come up successfully. by default) in dot1Q encapsulation. delete. without interpreting the information. Only Catalyst B creates VLAN 15. and each switch has no knowledge of what VLAN the other has assigned to the link. it is an access VLAN port on VLAN 10. whereas Catalyst C has it configured as an 802. however. no data will pass between them. and these protocols will not pass successfully. but both ends of the link are configured to trunk unconditionally. Because the trunk encapsulations are different. they could not ping each other unless a Layer 3 device were present to route between the Layer 2 VLANs. Therefore. 4.Scenario 2 Answers 1.1Q trunk. These VLANs are significant only to the local switch. it is not pruned because VLAN 1 is ineligible for pruning by definition.Yes. 2. Because it is in transparent mode. Yes. the key is the link between Catalyst B and C. the link between Catalyst A and B is the key. Only Catalyst C creates VLAN 14 in response to VTP advertisements. No. data can pass across the link freely. However. no VLAN activity will be advertised to other neighboring switches. This is because DTP packets will be exchanged. even though the trunk will not work end to end because of the encapsulation mismatch. Normally. 6. Catalyst C will not allow any VLANs to be created unless they are learned from a VTP server in the bermuda domain. VLAN 2 and VLAN 10. if these were assigned to different VLANs. Therefore.