Sunday, August 13, 2017

Cisco 6800 Instant Access (IA) switch trunk ports

The 6800 Instant access switch allows you to extend the core switch into access closets throughout the campus. I wrote a blog on configuring the 6880-x to work with the 6800IA here.

From the 6800IA Cisco Catalyst Instant Access FAQ:

Q. What is Cisco Catalyst ® Instant Access?
A. Instant Access is a solution that uses Cisco IOS ® Software to connect Cisco ® Catalyst 6800ia access switches to Cisco Catalyst 6500 or 6800 Series core switches. Once connected, the entire configuration works as a single extended switch with a single management domain. The solution is intended to simplify your campus network operations and management.

What does that mean? It means you can connect the 6800IA to a 6880 or 6500 series core switch and manage it from the core switch. But the 6800IA isn't a standalone switch, it's a Fabric Extender (FEX) and has some limitations that a standalone switch doesn't.

Again, from the FAQ:
Q. Why is the default configuration of Instant Access client host port configuration “switch trunk allowed vlan 1” and not “all”?

A. Each Instant Access host port can be configured in access or trunk mode (default is dynamic). If in trunk mode, there is a constraint as to how many VLANs can be trunked on each port.
Note: No more than 1,000 VLANs can be associated with a single FEX ID, divided by the number of Instant Access trunk ports.

To make sure that this constraint is followed, implementation requires specifying explicitly which VLANs will be trunked. We recommend no more than 20 VLANs per Instant Access trunk port 
(up to the total of 1000 per FEX), to limit the amount of BPDU processing.

On a standalone Cisco switch, by default, a trunk port passes all VLANs. The 6800IA by default only passes VLAN1!  Here is an example of a trunk port configured to work with an access point that needs vlans 1, 4, 201, 202, 203 and 204. If you don't explicitly allow a vlan it isn't passed (other than vlan1 of course).

It is very easy to forget this if you are replacing older switches with IAs and basically copying the configs! Obviously, if the port is only passing vlan1 and you need 4, 201, 202, 203 and 204 the SSIDs won't work correctly.

interface GigabitEthernet101/1/0/1
 description < Access Point >
 switchport trunk allowed vlan 1,4,201,202,203,204
 switchport mode trunk
 logging event trunk-status

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