(Rapid) Spanning Tree Protocol Questions

Hi Guys,

As some of you are aware I work with DSLAMs not routers/ switches. I am currently working on a new deployment and some bright spark at the customer sight has been asking about DSLAM interaction with Spanning Tree Protocol, which I know nothing about.

I've been asked:

A) TO ensure spanning tree protocol (STP) and IEEE 802.1w rapid spanning tree protocol (RSTP) should be supported on the backhaul links.

to configure the RSTP parameters such that the DSLAM is never becomes the route bridge.

As we don't support these I am going to have explain why they are not needed for a DSLAM.

Can anyone explain why a device on the edge of an ethernet network would need to be involved with STP - especially as a DSLAM only ever feeds into 1 ethernet Switch.

And what is all about????

Cheers for any help you can offer.

br

R

As long as your DSLAM has no special configurations for STP, then it's up to those responsible for the ethernet network to determine which switch(es) are the root bridge. This is done on their part by increasing the priority on the switch(es). This stuff is really cool, and with PVST (Per VLAN Spanning Tree) or Multiple Spanning Trees, 802.1s, can allow them to make highly available and load balanced topologies.

So basically tell them you are compliant with their request. Also, I'm assuming the DSLAM's ethernet interface is a layer 3 router interface and not a switchport. If it is a routing interface, then your DSLAM isn't even running STP! And that's how its supposed to be.

Thanks for the reply jwj,


Unfortunately it isn't as simple as that. The DSLAM can be configured either as a Layer 3 device (Basic routing) or as a layer 2 switch - switching DSL ports to ethernet ports, with traffic contained within VLANS.

If it is configured as a layer 2 switch it will just ignore STP.

br

R

If it's configured as a layer 2 switchport, it will participate in STP unless configured otherwise. STP is a layer 2 protocol . If it's configured as a layer 3 routing interface, there will be no instance of spanning tree.

Unfortunately when configured for layer 2 it does ignore STP as it doesn't have any STP protocol stacks so STP messages are dropped.

R

How is your DSLAM configured? I've never touched one, but I am very familiar with L2/L3 switching, especially with the Catalyst line of switches. Is your ethernet interface set as a switchport? I'm not even sure this is possible with DSLAMs. Or are you using integrated routing and bridging (IRB) or another bridging method?

But back to the point of this thread. If your DSLAM does not recognize spanning tree, and is dropping all BPDUs, then your device will not become the root bridge. It could have a system MAC address of 0000.0000.0001, and it still wouldn't be elected because its not participating in the spanning tree process.