CDP question

wuh · ‎11-05-2008

We have a router 2600 connected to a 3560 switch. On router side the link was configured as IP interface, on switch side was configured as trunk, 1q, native vlan 7XX.

Problem: switch can see the router in show CDP nei command, but the router can't. The input traffic in the router is 0.

After i changed the interface from trunk mode to access mode in the switch, it solved the problem.

Question: why is it? what blocked the traffic?

Thanks,

Han

Amit Singh · ‎11-05-2008

Han,

Did you see the switch logs. It might be reporting the native vlan mismatch error. This is not a recommended config as per the CDP goes, due to which it might have been reporting the CDP native vlan mismatch and wasnt generating the CDP info for the neighbor.

wuh · ‎11-05-2008

Amit:

I didn't see mismatch error in the switch. You know what, we have exactly same pair on other location with exact same configurations on both ends, except the router sides are configured sub interface for "router in a stick". and they don't have the problem find each other with CDP.

My coworker told me it is because the sub interface is configured as 1q trunk in router side, it'll send the cdp traffic, whereas the problematic one only has one side trunked. the other side won't send.

Is it right?

thanks,

Han

Giuseppe Larosa · ‎11-05-2008

Hello Han,

when using multiple Vlan subifs on the router you can see multiple CDP neighbor one for each Vlan/subif.

Despite CDP, it is not a good thing to have a trunk port where only a L3 interface is on the router.

Hope to help

Giuseppe

wuh · ‎11-05-2008

Giuseppe:

thanks, when you say it's not a good thing to have a trunk port where only a L3 interface on the router, can you explain a little?

thanks,

han