Verify which vlans are allowed on a trunk

Troubleshooting Unknown Protocol Drops? Created by KGrev on AM. I have many sites that provide this same service but this one is causing them issues.

I'm not noticin Created by Akarfars on AM. Hello,I have the following table. This table was constructed based on IP I have a problem with assigning the default getaway and serial interfaces, Created by from88 on AM. Created by sebastien3 on AM. The SPA card has a defect, I have to replace it! Should the ASR be turned off? Ask a Question. Find more resources. Blogs Networking Blogs Networking News.

Project Gallery. New Community Member Guide. Also it looks like 99 is your native vlan, which essentially means it sends the frames in an almost untagged fashion but not quite Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Which VLANs are allowed on this trunk?

Ask Question. Asked 7 years, 7 months ago. Active 5 years, 2 months ago. Viewed 24k times. Improve this question. Axel Kennedal Axel Kennedal 4 4 gold badges 9 9 silver badges 16 16 bronze badges. If the VLAN to which the port belongs is deleted, the port becomes inactive. All ports belonging to the VLAN that was deleted are unable to communicate with the rest of the network.

A common task of a network administrator is to troubleshoot trunk link formation or links incorrectly behaving as trunk links. Sometimes a switch port may behave like a trunk port even if it is not configured as a trunk port. Figure displays a flowchart of general trunk troubleshooting guidelines. Figure Trunk Troubleshooting Flowchart. To troubleshoot issues when a trunk is not forming or when VLAN leaking is occurring, proceed as follows:. To display the status of the trunk, determine the native VLAN used on that trunk link and verify trunk establishment using the show interfaces trunk command.

Connectivity issues occur in the network if a native VLAN mismatch exists. Data traffic for VLANs, other than the two native VLANs configured, successfully propagates across the trunk link, but data associated with either of the native VLANs does not successfully propagate across the trunk link. The previous output indicates that there is an active trunk despite the native VLAN mismatch.

One end of the trunk is configured as trunk mode "off" and the other as trunk mode "on. When configuring VLANs and trunks on a switched infrastructure, the following types of configuration errors are the most common:.

If an issue with a trunk is discovered and if the cause is unknown, start troubleshooting by examining the trunks for a native VLAN mismatch. If that is not the cause, check for trunk mode mismatches, and finally check for the allowed VLAN list on the trunk. The next two sections examine how to fix the common problems with trunks.

Trunk links are normally configured statically with the switchport mode trunk command. Cisco Catalyst switch trunk ports use DTP to negotiate the state of the link. When a port on a trunk link is configured with a trunk mode that is incompatible with the neighboring trunk port, a trunk link fails to form between the two switches.

In Figure , PC4 cannot connect to the internal web server. The topology indicates a valid configuration. Why is there a problem? Figure Trunk Scenario Topology. Check the status of the trunk ports on switch S1 using the show interfaces trunk command. An examination of the trunks on switch S3 reveals that there are no active trunk ports.

This explains why the trunk is down as shown in the output. After the configuration change, the output of the show interfaces command indicates that the port on switch S1 is now in trunking mode. For traffic from a VLAN to be transmitted across a trunk, it must be allowed on the trunk.

To do so, use the switchport trunk allowed vlan vlan-id command. The documentation has been updated to show that the VLANs allowed on the trunk are 10, 20, and In this scenario, PC5 cannot connect to the student email server.

Check the trunk ports on switch S3 using the show interfaces trunk command as shown in the output that follows. Someone updated the documentation but forgot to reconfigure the ports on the S1 switch, as shown in the output. The show interfaces trunk command is an excellent tool for revealing common trunking problems. You can classify wired traffic based not only on the incoming physical port and channel configuration but also on the VLAN associated with the port and channel. By default, physical ports on the controller are trusted and are typically connected to internal networks while untrusted ports connect to third-party APs, public areas, or other networks to which access controls can be applied.

When you define a physical port as untrusted, traffic passing through that port needs to go through a predefined access control list policy. For example, this setup is useful if your company provides wired user guest access and you want guest user traffic to pass through an ACL to connect to a captive portal.

You can set a range of VLANs as trusted or untrusted in trunk mode. If the traffic is classified as untrusted then traffic must pass through the selected session access control list and firewall policies.

Table 1: Classifying Trusted and Untrusted Traffic.