How to Troubleshoot VLANs and Trunks? Troubleshooting VLANs VLANs are a fairly straightforward feature which rarely requires troubleshooting. A few of the problems that we see are mostly configuration errors. Common ones include: 1. Inter-VLAN routing not working—Check that the link between the switches and the routers is set up correctly and the relevant VLANs are allowed and not pruned. The “show interface trunk” command will provide the required information. Also check that the router’s sub-interfaces are configured with correct encapsulation and VLAN and that the sub-interface’s IP address is the default gateway for the hosts. 2. VLANs cannot be created—Check if the VTP mode on the switch is set to client. VLANs cannot be created if the VTP mode is client. Another important factor is number of VLANs allowed on the switch. The “show vtp status” command will provide both the information required. 3. Hosts within the same VLAN cannot reach each other—It is important that hosts in a VLAN have an IP address belonging to the same subnet. If the subnet is different, then they will not be able to reach each other. Another factor to consider is if the hosts are connected to the same switch or not. If they are not connected to the same switch, then ensure that the trunk link(s) between the switches is/are working correctly and that the VLAN is not excluded from the allowed list/not pruned. The “show interface trunk” command will show needed information regarding the trunk link. Troubleshooting trunks Apart from a hardware/cable fault, most of the trunking problems originate from configuration errors. Common problems are discussed below: 1. Trunk will not come up—First, check if the interface status is up/up using the “show ip interface brief” or “show interface ” command. The second thing to check is the mode configured on the switchport. This can be done using the “show interface switchport” command. This command will show an output similar to the one given below: SwitchA#show interface fa1/1 switchport Name: Fa1/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: Disabled Access Mode VLAN: 0 ((Inactive)) Important points to note are the “Administrative mode”, “Negotiation of Trunking”, and “Administrative Trunking Encapsulation” lines. They will tell you the mode, DTP status, and the trunking protocol on the port. Remember that ports set on Auto/Auto mode will not trunk (covered previously). A trunking protocol mismatch will not allow the trunk to come up. Also remember that the default trunking protocol is dot1q on the 2950 and usually ISL on other models of a Cisco switch. 2. Trunk does not carry traffic from relevant VLANs—Trunks carry the traffic for all VLANs, by default. Only two things can cause this problem: Allowed list and Pruning. The “show interface trunk” command will show which VLANs are allowed across the trunk and which VLANs are pruned. Troubleshooting VTP VTP problems are discussed below: 1. VTP client does not receive or apply information from the server—First thing to check if whether the trunk link is configured and active between the VTP server and the client. This includes trunk link between any switches between the VTP server and client if the client in consideration is not directly connected. Secondly ensure that the VTP domain and password are correct. Another important factor is the revision number. If the VTP client is an old switch with pre-existing configuration, then it might have a higher revision than the one being advertised by the server. In such situations change the domain of the client to something else and then revert it back to the correct domain. This will reset the revision number on the client. “show vtp status” command helps in verifying the VTP configuration. You can see plenty of examples of the show commands in the theory and labs section. 2. New VTP client caused a change of VLAN database in the entire network—This can happen only happen if the client was brought from lab or another network (using the same domain name) and had a higher revision number. This can be verified using the “show vtp status” command. 3. VTP pruning is not working correctly—If there is a VTP transparent switch in between the VTP server and VTP client, then VTP pruning will not work. Another reason why VTP pruning will appear not to be working correctly is configuration of allowed VLANs on the trunk links. Some VLANs might have been removed manually. This can be verified using “show interface trunk” command.
Posted on: Sun, 07 Dec 2014 14:03:11 +0000
Trending Topics
Recently Viewed Topics
© 2015