Imagine a scenario where a router A has two possible ways to get to a destination, N. One of these is through router B and the other through router C. Now, router B advertises a better metric to N than router C. Therefore, as far as A's IGP is concerned, the next-hop for network N is through router B. Now, if you are also running LDP in this network, both routers B and C will advertise labels for network N to router A. The way A deals with these labels depends on label retention mode:
- if configured for conservative retention mode, A will only store the label received from B, since that is the next hop network N. It will discard the label from router C.
- if configured for liberal retention mode, A will store the labels received from both B and C
The advantage of using liberal retention mode is that the label associated with a non-preferred path is available should the best path to the destination change.
Thanks for the reply.I assume both conservative and liberal retention modes are dealing with labels in LFIB only.In both modes,in LIBs all the labels corresponding to both IGP paths will be stored...pls correct me if I am wrong..
My understanding of this is that in consevative retention mode, the label is not even stored in the LIB. And that makes sense - the LFIB only stores the used labels in both cases. The LFIB will be identical whether you use liberal or conservative retention mode. It's the LIB which is impacted by the retention mode.
Hope that helps - pls do rate the post if it does.
Short answer would be that LIBERAL mode consumes more space because it stores all available LSPs in LIB while only the best path is used in CONSERVATIVE mode.
Benefits are to be considered on both sides. Faster failover would be on side on LIBERAL because you already have prepared backup paths. Basically the same is benefit for CONSERVATIVE but from other point of view - because in the initial phase of label exchange you don't have to wait so long for the exchange completion and then to store them all in memory.
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