[c-nsp] Router on a stick with multiple bridged interfaces

[Jay Hennigan]

Robert Johnson wrote:
> Hello Cisco experts,
> Here's today's question. I have a simple router on a stick with a
> fastethernet interface and multiple 802.1Q subinterfaces connected to a
> layer 2 switchport in trunk mode. Now say I want to add another switch with
> the same group of VLANs as the first switch, bridged to the first switch.
> Instead of connecting the new switch to a trunk port on the existing switch,
> I'd like to attach it to a router interface. So essentially I want two
> router interfaces that are transparently bridged, with the ability to attach
> routed 802.1Q subinterfaces to both interfaces simultaneously.
> 
> What's the best way to do this?

Turn on IRB in the router.
Configure a bridge group for each VLAN.
Remove the IP configuration from the dot1q subinterfaces.
Add the IP configuration to the BVI for each VLAN.
Assign the subinterfaces to the appropriate bridge groups on both 
physical interfaces.

Consider possible spanning tree issues should someone bridge VLANs the 
two switches accidentally or if you want to intentionally trunk between 
them for interface redundancy.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
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