Flex Links Query

[RoDDy]

So by implementing loop-guard, you're basically saying that this port should ALWAYS receive BPDUs and if the BPDUs stop then something is very wrong, block the link.

Yes correct (its a loop prevention mechanism to stop the port from going into forwarding state when it really shouldn't be, causing loops until the STP reconverges). When the blocked port is in loop inconsistent state it will stay that way until it starts receiving BPDUs again, and go back to a "normal blocked state". I wanted it to work exactly like that so that i wouldn't have to intervene with the link too much. I'll go with this over UDLD because of that fact as i believe that after it err-disables then you either have to manually bring it back up or wait for the set timer to expire.

dingdingding

its similar to UDLD in that its basically for catching unidirectional fibre links - have never seen loop guard triggered by faulty copper or RJ45 ports

Neither have I but my situation was kind of perfect for it. This was actually the first time i had to use it. Its only by chance that the secondary link is basically a wireless bridge. Usually we don't get any issues with the microwave but there is probably some interference that is causing the wireless to be disrupted even thought nothing is wrong with the equipment at the 2 ends.

I can't believe flex links are still a thing. I only remember reading about it prepping for the CCIE v4 written.

I had to cover flex links for my CCNA service provider exam.

3.5 Describe Ethernet link bundling, LACP, and PAgP and Flex Links

[NetworkGroover]

So by implementing loop-guard, you're basically saying that this port should ALWAYS receive BPDUs and if the BPDUs stop then something is very wrong, block the link.

If my memory serves me well, loopguard is only implemented on STP-blocking links, where you have the risk of them erroneously going into forwarding mode.

Right - you have to know your STP topology and place it on non-designated ports.  Usual use case is root and alternate ports where you're doing per-VLAN STP. With MST.... at least a straight-forward deployment... I'd guess it'd just be alternate ports.

[NetworkGroover]

dingdingding

its similar to UDLD in that its basically for catching unidirectional fibre links - have never seen loop guard triggered by faulty copper or RJ45 ports

I labbed loopguard going into effect by adding a bpdu-filter on the other end, so that might be one obscure scenario where you'd have a loop otherwise

Loop Guard and UDLD are for different purposes, but kinda work similarly/in tandem if I remember correctly - too lazy to dig right now.

You can simply think of Loop Guard as a safety mechanism for any situation where you have an unexpected loss of BPDUs causing the undesirable condition of a previously-discarding port to transition to forwarding creating a loop.  A failed STP process on a connected switch, a grey failure on the link between the two, etc. - any situation that causes a loss of BPDUs on an active link where you expect them.

[NetworkGroover]

Ah okay - I was trying to think what would happen if the port is connected to something else like a router or end host.

So by implementing loop-guard, you're basically saying that this port should ALWAYS receive BPDUs and if the BPDUs stop then something is very wrong, block the link.

Yeah you wouldn't have this on a port connected to a router or end host - no possibility of a loop there.  You shouldn't be transmitting BPDUs in either direction (in other words, turn on BPDU Guard or Root Guard here depending on preference - if it's a host.  Neither are needed on a router connection).