after a layer2 loop in our LAN environment I decided to configure udld and loop-guard, but I got a bit mixed up as to where loop guard should be configured.
We have 2x 65xx acting as VTP servers (layer2 connection between them). Each one is root for a number of VLANS. The udld part is easy. I have configured it on all optical connections.
For switches forming triangles with the 65xx I have enabled "spanning-tree guard loop" on the uplinks.
What about switches forming a square topology with the 65xx. Is it sufficient to enable loop-guard on only the uplinks, or shoud it also be enabled on link between the two access switches.
I have read a number of cisco documents and in some it is stated to enable loop-guard on all non-designated ports and in other it says enable it everywhere. In practice, do I enable it everywhere or is it enough to just go with the uplinks? Ports will be designated for one vlan and non-designated for the other, so there is no point in figuring out the exact role of the ports.
Thank you in advance,
The BPDU Loop Guard is a prevention mechanism that tries to avoid switching loops caused by a sudden stop of BPDU arrival on a port, and subsequent transition of this port to the Designated Forwarding role/state. Now, if you think about ports that rely on timely arrival of BPDUs to maintain their current role/state, these are Root port, Alternate port, and Backup port. Especially the Alternate and Backup ports are meant to be Discarding, and should they cease to receive BPDUs, they will move into Designate Forwarding, possibly creating a loop.
Therefore, the BPDU Loop Guard should be applied at least to ports that are Alternate (a Backup port is on a shared-type link, and Loop Guard runs only on point-to-point links). However, because in a per-VLAN environment each trunk port can be in diverse roles/states for individual VLANs, I suggest simply configuring the Loop Guard on all ports using the global configuration level command spanning-tree loopguard default.
Perhaps you know this document - it discusses the Loop Guard feature in a closer detail.