MPLS - "when/how" does the router build the LFIB

[vv0bbLeS]

Hello all,

I'm trying to determine the exact order of operations a router takes when you run mpls ip on the router, specifically when/how the router builds the LFIB table.

The first step I have is that the router takes all the routes in the RIB and assigns a label to each non-/32 route (label numbers start at 16 and go up), and the router puts these "labeled routes" in a new table called the LIB (Label Information Base).

Once the router builds the LIB table, the router builds the LFIB table, but I'm not sure how - I've read two methods:

1. The router builds the LFIB by combining the RIB and LIB entries
2. The router builds the LFIB by taking entries from the FIB table (in this method, the FIB already had labels pulled in from the LIB table before the LFIB was built, e.g. show ip cef x.x.x.x detail and you can see the labels)

So, is the LFIB built from the RIB/LIB combination, or is the LFIB built from the FIB label entries?

Thanks again!

[MHM Cisco World]

Fib is same as rib except fib is for data plane and rib is for control plane 

Same as lib and lfib.

MHM

[Joseph W. Doherty]

I couldn't (at least quickly) find the answer to your question.

For general (and dated?) Cisco IOS operations, you'll normally find variants of: https://www.slideshare.net/slideshow/cisco-ios-order-of-operation/16372702 or https://blog.router-switch.com/2013/01/whats-the-order-of-operations-for-cisco-ios/.

Neither of which, unfortunately, mention MPLS.