Bridging a Cisco Trunk

Peter Wood peter at
Wed Jan 11 10:28:55 PST 2006


 > 	I have two cisco switches, configured to put ports 2-6 on each of
 > them into vlan 100. Then I have port 1 on both set to trunk between 
the two
 > switches. If I have a device on port 2 on switch1 it can ping a device on
 > port 2 on switch2.

I do this quite often, and it works very well on 6.0 for me. You haven't 
mentioned what version your using, but I will assume you have if_bridge. 
If you don't and you're gonna use this machine alot for bridging, I'd 
recommend moving to 6.0.

So presumably, you have two interfaces, plugged into the trunk port on 
each cisco. For arguements sake, we'll say you have an fxp0 and fxp1.

So first step is you need to make sure these two interfaces are "up", 
very important, if they arn't, then it wont work. It's easy to forget if 
you arn't assigning IP's to them.

Remove "polling" if you don't have it compiled into the kernel, but 
again if you're gonna be bridging packets alot, get it compiled in. It 
helps.... alot.

ifconfig_fxp0="up polling"
ifconfig_fxp1="up polling"

Now create the vlans (and the bridge for later on).

cloned_interfaces="vlan0 vlan1 bridge0"
ifconfig_vlan0="vlan 100 vlandev fxp0 up"
ifconfig_vlan1="vlan 100 vlandev fxp1 up"

In the above please note the "up"s, if they arn't up then it wont 
bridge. Now setup the bridge, again noticing the "up".

ifconfig_bridge0="addm vlan0 addm vlan1 up"

It should now be working, watch the kernel console and the cisco's logs 
to see if there are any mismatches or bridging loops.

It also seems that you have to put the "up" at the end of these 
commands, it took an hour of debugging last night after I had put the up 
at the start of the ifconfig_vlan lines.

Give it a go, send a reply to both me and the list if you are still stuck,

Peter Wood :: <peter at>

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