I currently have a test environment set up, so that I can test udld and loopguard to prevent spanning tree loops.
Topology is as follows:
- Two (6500 running IOS) distribution switches with several VLANs doing hsrp.
- A single access switch (2948g CatOS) with a trunked uplink to each distribution switch.
In a correctly operating network, one of the uplink ports is forwarding all vlans and the other uplink port has them blocking on the access switch.
I've forced a spanning tree loop using VLAN100 so that VLAN100 is forwarding on BOTH of the access' switch uplink ports.
I've enabled udld on ALL uplink interfaces (yes, I've also done the set enable udld on the 2948). I've also enabled loopguard on all the uplink interfaces. I've checked each interface to make sure these are enabled, and they are.
Now, from my understanding, both udld and loopguard SHOULD detect a loop and shut down the port that should be blocking. Is that correct? Am I missing anything with how those work?
UDLD is generally used to detect single sided fiber connections and shut them down so that data does not get blackholed . It sends out hello typer packets and if it doesn't see a reply within a predtermined timeframe then it will shutdown that link...
The term "spanning-tree loop" itself is not appropriate. The role of STP is to prevent forwarding loops. In the setup you describe, STP is responsible for detecting a loop and preventing it. UDLD and loopguard are additional mechanism that can protect against a scenario where STP fails: the unidirectional link failure, as Glen stated. Under this scenario, the link is still declared as operational while there is only transmission possible in one direction. In the case where the designated BPDUs are lost (communication from the distribution to the access has failed), you could end up in a permanent one-way forwarding loop that STP cannot detect. The scenario is not obvious to recreate without special hardware or engineering commands.
You did not say how you succeeded in having a loop in your vlan 100. If it's by using BPDU filter for instance, you prevented STP and loopguard from operating. At the same time, UDLD does not detect any connectivity issue (because there is none) and the loop will carry one for ever;-)
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