Trunking Problems on Cisco switches: How to solve

Certification: Cisco CCNA Routing and Switching - Cisco Certified Network Associate Routing and Switching

The primary purpose of a VLAN is to ensure that there is no physical constraint on the connection of layer 2 switches. These networks hold the capability to span different switches without having any physical constraint. It also adds an additional layer of security by separating the physical networks from the logical. In this process, special links between these switches are used to carry the VLAN. These special links are referred to as Trunks and the process is known as Trunking.  VLAN trunking offers a cheap and reliable way to manage loads of virtual network connections. However, there are various problems regarding trunking which arise on a regular basis. It is essential to eliminate these problems so that there are no issues related to connectivity of data flow over the network. The best way to go about this process is to first identify the problem. Once identified, you then need to follow simple steps and devise ways to solve it in an effective manner. Following are some of the common issues related to trunking and the ways in which you can solve them.

VLAN Leaking

In a VLAN, there are some instances when a switch port starts to behave like a trunk port. This is a crucial problem as the port is then deprived of its original purpose and starts to interfere with the process of trunking. In these conditions, the port starts to accept frames from VLANs from which it is not configured to get the frames. This process is referred to as VLAN Leaking and poses a great deal of problem in the process of trunking. There are various problem related to connectivity which occur due to VLAN leaking.

There are basically two things which you should do in order to solve this issue. You should initially open the “trunk interfaces command” in order to process further. Once done, you are first supposed to check whether all of the VLANs match. If any of them do not match, this means that VLAN trunking has taken place and you should ensure that all of the ports are on the same VLAN. Furthermore, you should also check whether a trunk exists between the switches. It if does not, then you should manually assign trunk links. This is very important because if a trunk does not exist between two switches, then there is no way in which they can communicate over the network.

Mismatch of Trunk mode

You need to know that trunk links are assigned statically to the switch ports. While this is done to avoid many issues, it could lead to a problem as well. If one port is assigned a trunk mode which has not been configured to the neighboring device, the connection will not be established and would lead to a loss of connectivity. Hence, it is essential that two ports are set on the same trunk mode so that they can communicate. The process of configuration of trunk links is done using the “Switchport mode trunk” command. Whenever you find out that there is some problem with the connectivity, you must consider this issue.

In order to troubleshoot this problem, you need to move step by step. This means that you should use the “show interfaces trunk” command to figure out which switch is unable to connect to other. Once done, you then need to find out which port on the switch is configured on a trunk mode separate from others. Finally, you need to assign this port the same trunk mode which has been assigned to other switch ports on the network. This would solve your problem of connectivity and will enable you to bring all of your ports on the same trunk mode. However, if the problem still resorts after you have done this step, you then need to consider the next issue described below.

Incorrect list for VLANs

This is one of the most delicate and crucial issues which you are likely to face with your VLAN trunks. You should be familiar with the fact that it is necessary for a VLAN to be allowed on a trunk and be in its list in order to be transmitted over that trunk. If a VLAN is not listed in the allowed VLANs list for a trunk, then it won’t be able to send any data over it. On the other hand, if a VLAN is accidentally added to the list of a trunk, it will start send and receiving data from it which will lead to serious issues. The unexpected flow of traffic over the trunks definitely points towards this problem which needs to be resolved immediately. There are a number of reasons which could lead to this incorrect list being created, but you should focus more on how to eliminate the issue no matter what is the origin of it.

In order to solve this problem, you have to make use of the “Switchport trunk allowed VLAN” command. This command will prompt you with all the information regarding the list of allowed VLANs on a trunk. You can then check the list to make sure that only allowed VLANs are up on the list and there is no extra or unrecognized VLAN. If there is any extra VLAN on the list, you should immediately delete it to ensure that there is no more unusual flow of data traffic. On the other hand, you should also check whether any of the VLANs is not missing from the list. If any of it is missing, then you know that this is the issue why the connection is not being established properly. Hence, you should then manually add the VLAN in the list to ensure the smooth flow of data over the trunks.

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