MPLS LDP-IGP synchronization

As explained in my previous MPLS LSP post you use the IGP such as OSPF or IS-IS its CEF FIB (Cisco Express Forwarding – Forwarding Information Base) to exchange MPLS labels with the neighbor router and install them into the LFIB (Label Forwarding Information Base).

Any distruption in IGP will result in a IGP recalculation and causes temporary disrupton in the LSP’s (Label Switched Paths) untill LDP exchanges labels again and the two routers are in sync.

Imagine that P_3 had a malfunction in the IGP protocol; or just returned from a reload. LDP will start exchanging labels after the IGP convergence is complete. Packets will arrive from P_2 towards P_3 even if LDP is not syncronized yet. Thus dropping those LSP packets.

So basically packetloss will occur because the actions of the IGP and LDP are not synchronized. When an IGP adjacency is established, the router begins forwarding packets using the new adjacency before the LDP label exchange completes between the peers on that link.

If an LDP session closes, the router continues to forward traffic using the link associated with the LDP peer rather than an alternate pathway with a fully synchronized LDP session.

Fortunatly Cisco implemented an LDP-IGP syncronization feature that both works on the mpls ip and the mpls ldp autoconfig commands.

By enabling the MPLS LDP-IGP synchronization the router will ensure that LDP is fully established before the IGP path is used for forwarding MPLS traffic.

To enable this feature you need to configure your IGP protocol; OSPF in this case. It’s advised to run this feature on all your routers for optimal LDP-IGP convergence.

PE_2#config t
PE_2(config)#router ospf 1
PE_2(config-router)#mpls ldp sync
PE_2(config-router)#end

PE_2#sh run | section router ospf
router ospf 1
 router-id 192.168.0.2
 mpls ldp sync
 log-adjacency-changes
 passive-interface default
 no passive-interface GigabitEthernet1/1
 no passive-interface GigabitEthernet1/2
 no passive-interface GigabitEthernet1/3
 no passive-interface GigabitEthernet1/4
 network 192.168.0.0 0.0.255.255 area 0
PE_2#

You can now safely go to bed without worring about IGP synchronization on your MPLS network!