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RSPAN trunks

Hi all,

I've got three switches, let's call them A, B, and C.  I've created vlan 99 on each switch and configured vlan 99 as remote-span.  When I add vlan 99 to my trunk ports, at some point, DHCP traffic (and possibly other broadcast traffic) stops working.  When I remove vlan 99 from the trunk ports, DHCP works again.  Adding vlan 99 again does not break things immediately though.

So things look like so:

A (RSPAN source) -> B -> C (RSPAN destination)

The trunk ports carry all sorts of production VLANs on them, as well as the RSPAN VLAN.

Switch A is a 2960

Switch B is a 3560

Switch C is a 2960

All switches are running 12.2(53)SE2

I've now taken down two networks because of this strange behavior.  It seems to all be working fine in my testing, but at some point into production, certain types of traffic stop working.  DHCP is the easiest example for testing and capturing.

Has anybody run into this?  I've searched around and was unable to find anything on this.  Any help would be appreciated.  Thanks!

Chad Peterson
Cisco Employee

I haven't seen that behavior before.  Now when you do this, have you actually setup a monitor session or you just created the RSPAN vlan.  I'm asking because we do see a lot of people setup RPAN vlan then monitor a few vlans themselves....then that creates output drops on their trunk ports where RSPAN vlan is carried due to to much traffic being spanned.

Just something to keep in mind and to look for.

Thanks for the response Chad.  I had the monitor session setup, but I was only monitoring one access interface on one of my switches.  There was just a PC on the port as I was just in test mode.  No real heavy traffic being generated or anything.