Hello, everyone.
I didn’t find a lesson for this so I am creating a new topic if you don’t mind
I’ll do my best to keep this brief.
Cisco and some resources state this regarding IGP synchronization with LDP:
IGPs do not form an adjacency across a link if the LDP session is not established first across that link. (OSPF does not send out Hellos on the link.)
This part is only true under two circumstances. First of all, if we already have an IGP session and the LDP session goes down, the adjacency does not go down, it remains up with max-metric
Cisco:
When an IGP adjacency is established on a link but LDP IGP synchronization is not yet achieved or is lost, the IGP advertises the max-metric on that link.
Second of all, if we don’t have an IGP adjacency yet and are waiting for LDP to come up, if the local router realizes that it has no route for the peer’s transport address, it will bring the IGP adjacency up regardless in the hope that it will provide it with a route.
If the LDP peer is not reachable, the IGP will then establish the adjacency regardless of LDP session state after the timeout has occurred.
(it will of course increase the metric on this path to prevent it from being used until LDP syncs).
The highlighted part above makes sense. If we have just a single connection to our LDP peer, we won’t learn about any routes regarding this peer until the IGP adjacency comes up. Without this, we would never establish the adjacency and never learn any routes at all.
My question is about the holddown timer. In the common cases I’ve mentioned above, the IGP adjacency establishes and we learn routes which is great. However, at what point could we end up stuck with no adjacency at all, only to rely on the holddown timer to fix this issue?
I can only think of one possible scenario.
- If the LDP peer is reachable, the IGP won’t build the adjacency unless LDP synchronizes. This is the opposite behavior of when the LDP peer is unreachable. If the LDP session here doesn’t establish due to some kind of problem, the IGP adjacency will never come up. So after the holddown timer expires, the IGP adjacency comes up (although the link is still advertised as max metric to prevent blackholes)
If the LDP peer is unreachable, the IGP brings the adjacency up which makes sense so I don’t see much use for it here.
Thank you.
David