[c-nsp] spanning-tree for local switching on ASR920

Mark Tinka mark.tinka at seacom.mu
Thu Oct 19 04:19:22 EDT 2017

On 19/Oct/17 09:46, Gert Doering wrote:

> I wasn't particularily asking for suggestions, but for "I have this 
> working, and this is how it looks like".  
> This box is unlike any other Cisco "switch-like thing" I've had in my 
> hands before, so it might very well be just not supported at all.

The ASR920, technically, is not an Ethernet switch. It just looks like one.

Treat more like an ASR1000 router, and you'll be just fine.

> There isn't anything else you *can* configure (except changing the
> global STP mode from pvstp to mst and back, and setup mst instances,
> which does not have an effecit either).
> So indeed, that's all there is, and the ASR920 MST/PVST+ documentation
> claims "there is nothing else you can do".
> *If* you have a vlan, you can do "spanning-tree vlan 10", but that is
> default anyway (= not showing up in the config), but since there is no
> "vlan 10" in the system, there's no spanning-tree instance for it either
> ("show vlans" comes up empty).

So Cisco's EVC/EFP infrastructure is very different from classic 802.1Q
Ethernet switching.

As you've figured out, VLAN ID's are not globally-significant on the
ASR920. This allows you to use the same VLAN ID several times on the
same device, which does not impose classic network-wide VLAN
limitations. Of course, most of this assumes you are using MPLS as the
upstream transport protocol.

The use of VLAN ID's on an ASR920 is really to map incoming traffic from
a downstream customer to a specific EFP, where upon an action is
performed on the traffic, e.g., run through a BDI for IP routing, attach
it to an x-connect for onward EoMPLS transport, e.t.c. The VLAN ID is
not used to separate traffic at a global level of the device, as it
happens in classic Layer 2 switching.

Hope this helps.

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