[c-nsp] Bridging Serial Interfaces

[Brett Frankenberger]

On Fri, Nov 12, 2010 at 05:40:32PM -0500, Todd Shipway wrote:
> I've got a rather basic question, or at least I hope it is.
> 
> A customer is trying to migrate from a point-to-point setup to a
> point-to-multipoint setup.  I'm trying to help them with this by
> supplying serial and multilink ppp interfaces.  They want these
> interfaces to be bridged.  I've got the interfaces up and running and
> when configured for routing, they work fine.  But they have strange
> default routing policies and need the bridging to be in place.  I
> setup the bridge-group on each of the interfaces and set the protocol
> to ieee.
> 
> interface Serial9/0/0/27:0
>  no ip address
>  no cdp enable
>  bridge-group 2
> !
> interface Serial9/0/0/28:0
>  no ip address
>  no cdp enable
>  bridge-group 2
> bridge 2 protocol ieee
> I show the bridge group to be up and running as expected:
> Bridge Group 2 is running the IEEE compatible Spanning Tree protocol
>    Port 106 (Serial9/0/0/27:0) of bridge group 2 is forwarding
>    Port 107 (Serial9/0/0/28:0) of bridge group 2 is forwarding
>  
> The routers at the remote end of the serial links are configured as below....
> 
> Router1:
> Interface serial 0
> Ip address 172.16.0.1 255.255.255.252
> Router2:
> Interface serial 0
> Ip address 172.16.0.2 255.255.255.252
> Both router1 and router2 are using the default route below:
> Ip route 0.0.0.0 0.0.0.0 serial 0
> 
> The issue is that no traffic will pass over the bridge.  Router1 is
> unable to ping router2 and vice versa.  Any ideas as to what I'm
> missing with this?

You've configured your router to bridge Ethernet frames between the two
serial interfaces.  (Ethernet over HDLC framing)

You've configured the Router1 and Router2 to send IP frames over the
serial interface.  (IP over HDLC framing)

That won't work for reasons that should be obvious.  Some options:

(a) Connect the two T1s at the router in the middle with an L2VPN Layer
2 local-swtching connection, if that's supported on your platform.

(b) Configure Router1 and Router2 with integrated routing and bridging. 
This would put the IP addresses on a BVI interface that would then be
bridged to the serial interface.

(c) Configure the serial interfaces on Router1 and Router2 as
half-bridges.  This results in the same functionality as Integrated
Routing and Bridging -- you get IP over Ethernet over Serial, rather
than IP over Serial -- but it's simpler to configure.  AFAIK, it's only
supported over PPP encapsulation, though, so you'd need to convert to
PPP encapsulation on all four serial interfaces involved here.

     -- Brett