On Tue, 21 Aug 2001, Sasha wrote:
> He is doing transparent bridging between pvc's, hence
> routing issues are irrelevant.
> The problem here is that a packet that comes into a physical
> interface is not transmitted back through the same physical
> interface (although on another pvc), and bridging will not work
> (you may call it "split horizon", but I wouldn't).
YES! You got it correct! One must remember that when bridging on a
router, its just like a real bridge/switch. Interfaces are like ports on
a bridge, and a packet entering a port will never go back out that same
port. Using sub interfaces fixes it.
Now why does it work when you put them on 2 different layer3 networks?
Because this forces packets to tag the BVI, and thus get routed (BVI
must be setup with secondary addressing). And routing can go out the
port, tag the BVI, and go back down the port, but bridging will not work!
> (This limitation is intended to avoid bridging loops, I think, because
> STP will treat physical interface as a single bridge port.)
>
> The common solution is the use of p2p subinterfaces.
Yes
>
> The config may be modified in one of two ways:
>
> * put pvc's on separate point-to-point subinterfaces:
>
> int ser4/0.200 point-to-point
> frame interface-dlci 200
> bridge-group 1
> int ser4/0.224 point-to-point
> frame interface-dlci 224
> bridge-group 1
> ...
>
> * use point-to-multipoint interface:
>
> int ser4/0.200 multipoint
> frame map bridge 200 broadcast
> bridge-group 1
> int ser4/0.224 multi
> frame map bridge 224 broadcast
> bridge-group 1
Yes, or use different layer 3 networks, you can put numerous secondaries
on the BVI, although ugly looking. Great job, I know this is not an
obvious problem when first looked at.
Brian
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