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RouterG<br />

192.168.19.1<br />

Ingress router<br />

RouterJ<br />

192.168.17.1<br />

Transit router<br />

RouterK<br />

192.168.21.1<br />

Transit router<br />

RouterF<br />

192.168.16.1<br />

Egress router<br />

100032 IP<br />

Figure 14-3. Transit router swaps labels in the MPLS header<br />

RouterG<br />

192.168.19.1<br />

Ingress router<br />

RouterJ<br />

192.168.17.1<br />

Transit router<br />

RouterK<br />

192.168.21.1<br />

Transit router<br />

RouterF<br />

192.168.16.1<br />

Egress router<br />

IP<br />

Figure 14-4. Penultimate router pops label<br />

MPLS Routing and Forwarding Tables<br />

For standard routing operations, the JUNOS routing software uses the inet.0 unicast<br />

routing table, which contains prefixes learned from IGPs. For MPLS forwarding<br />

operations, by default the JUNOS software stores prefixes learned from a signaling<br />

protocol (either LDP or RSVP) in a separate routing table, inet.3. MPLS consults<br />

this table when making forwarding decisions.<br />

MPLS also creates another table, mpls.0, that contains the labels received and used<br />

by the local router to forward packets to the next-hop router.<br />

Signaling Protocols<br />

Labels are allocated and distributed throughout an MPLS network either manually or<br />

using a signaling protocol. It is time-intensive to set up and maintain static (manual)<br />

LSPs, so they are rarely used and are not discussed in this book. The JUNOS software<br />

supports two signaling protocols, LDP and RSVP.<br />

LDP was designed by the MPLS working group in the IETF to set up LSPs and distribute<br />

labels throughout an MPLS network. LDP relies on an IGP (either OSPF or<br />

IS-IS) for all routing decisions, and the LSPs it sets up always follow the IGP’s shortest<br />

path and change when the IGP’s path changes. These LSPs use the IGP to avoid<br />

loops. Because LDP is dependent on an IGP, LDP-signaled LSPs are limited in scope<br />

to the IGP’s domain and cannot cross AS boundaries.<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.<br />

Introduction | 483

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