Forgot to comment on that Sherif. Here is some detail that may explain.
LDP-IGP Sync: Is there any equivalent config in IOS-XR for IOS's
IGP Sync hold-down time ? The short answer is, No.
IOS IGP defers adj formation on an upcoming interface until LDP sync
status is achieved on the interface. To limit this adj holddown, an
"holddown" config is provided in IOS to limit/cap this [adj] holddown
period. IOS-XR implementation for LDP-IGP Sync is different from IOS as
IOS-XR IGPs do not tie-up their adj formation on a given interface to
intf LDP sync status. Instead, IGP still form their adj [as they will
form in the absence of this feature], but advertise max-metric for given
interface if LDP sync is not achieved yet. Due to this reason, there is
no "holddown" cfg required/implemented in IOS-XR IGP for LDP-IGP Sync
feature. Please note this will NOT cause any inter-op issues as LDP-IGP
sync is a local feature.
How MPLS LDP IGP Sync Works
Synchronization means that the LDP session is up across the link and the LDP label bindings have been learned. In that case, the interface can be used as outgoing interface in the RIB by the IGP.
When synchronization is not achieved ā this means typically that the LDP session is down, there are 2 cases:
- either the IGP makes the link the least favorable link or unusable
- either the IGP is held down on that interface
2 triggers for LDP IGP Sync:
- Link comes up:
- If the LDP peer is reachable (an alternate route exists), then the IGP adjacency is held down on the link
- If the LDP peer is not reachable (no alternate route exists), then the IGP adjacency is brought up on the link and advertises max-metric to reach the neighbor through that link
- LDP session goes down:
- The IGP advertises max-metric to reach the neighbor through that link
The link is made the least favorable link by OSPF by advertising it with a maximum link metric (65536 of hex 0xFFFF). This leads to having no routes/paths through this interface where LDP is broken, unless it is the only path available.
ISIS uses the same method, the link/IP prefix is advertised with link metric 16777214 (MAX LINK METRIC ā 1) when metric-style wide is in use or 63 (MAX LINK METRIC) when metric-style narrow is in use, in the ISIS LSP.
Alternatively, the IGP session across the link can be held down purposely, as long as synchronization is not achieved. The reason for keeping the IGP adjacency down, instead of bringing it up and using the maximum link metric, is to minimize the number of additional SPFs and link advertisement flooding which must be performed by the IGP. This maximizes the probability that only one SPF is required to respond to the interface UP event and therefore isolates this mechanism from delays which may occur due to SPF holddown timers.
This is different in IOS-XR. In the case where the link comes up and the LDP peer is reachable, IOS-XR does bring up the IGP adjacency and uses advertise max-metric.
Another difference in IOS-XR is the following. IOS-XR has also uses the LDP IGP sync feature to make TE tunnel participate in routing or forwarding only when the LDP targeted session corresponding to the TE tunnel is converged. This is configurable under IGP (OSPF or ISIS) as "mpls ldp sync-igp-shortcuts".
Does this jive with you?
cheers!
xander
