Cisco Multiservice Switching Networks

The previous section discussed routing protocol concepts including IGP convergence. However, ATM MPLS networks need a two-stage convergence:

• IGP Convergence Core IP routing convergence.

• MPLS Convergence Re-establishment of label mappings.

After the IGP converges, MPLS ATM eLSRs must resignal for label mappings. This is because the label distribution is downstream on demand, and the label retention is conservative. So in the event of a change in the next-hop, a new label must be requested by signaling downstream.