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

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

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

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