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

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?
3.
Catalyst B has been given the command no switchport no negotiate for interface

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

You also need to examine switch configurations and apply them to a network diagram. can PC-1 ping PC-2? 2. If VLAN 14 is created on Catalyst A. Suppose that Catalyst A is a VTP server. All switches are in the Bermuda management domain. Trunking. Catalyst C is a VTP client. Will the trunk link between Catalyst B and Catalyst C come up successfully? 4.0. Notice that each PC connects to a different VLAN number. which switches also will create VLAN 14 using VTP? 6. PC-2 and PC-3 are assigned to the same IP subnet (using subnet mask 255. and VTP This scenario is designed to stir your thinking about VLAN and trunking connectivity. If VLAN 16 is created on Catalyst C. Can PC-2 and PC-3 ping each other? 3. Where will VLAN1 be pruned? Why? 5. If VLAN 15 is created on Catalyst B. what other switches also will create VLAN 15 through VTP? 7. Figure 19-2. and Catalyst B is configured for VTP transparent mode. Suppose that the trunk between Catalyst B and Catalyst C is configured properly. See the diagram shown in Figure 19-2 and answer the questions that follow. what will happen? .0) and the same VLAN. Portions of the configurations of the three Catalyst switches are shown above them.Scenario 2: VLANs.0. Given the switch configurations shown. PC-1 and PC-2 both are configured with IP addresses on the same subnet. Diagram for Scenario 2 [View full size image] 1.

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