IGP and BGP Destinations

You can configure MPLS to control the paths that traffic takes to destinations within an AS.

When traffic engineering is for IGP destinations only, the MPLS host routes are installed in the inet.3 routing table (see Figure 12), separate from the routes learned from other routing protocols. Not all inet.3 routes are downloaded into the forwarding table. Packets directly addressed to the egress router do not follow the LSP, which prevents routes learned from LSPs from overriding routes learned from IGPs or other sources.

Traffic within a domain, including BGP control traffic between BGP peers, is not affected by LSPs. MPLS affects interdomain transit traffic only; that is, it affects only those BGP prefixes that are learned from an external domain. MPLS does not disrupt intradomain traffic, so IS-IS or OSPF routes remain undisturbed. If you issue a ping or traceroute command to any destination within the domain, the ping or traceroute packets follow the IGP path. However, if you issue a ping or traceroute command from Router 1 in Figure 10 (the LSP ingress router) to a destination outside of the domain, the packets use the LSP tunnel.

When traffic engineering for IGP and BGP destinations is enabled, the MPLS host routes are installed in the inet.0 table (see Figure 13) and downloaded into the forwarding table. Any traffic destined to the egress router could enter the LSP. In effect, it moves all the routes in inet.3 into inet.0, causing the inet.3 table to be emptied.

RSVP packets automatically avoid all MPLS LSPs, including those established by RSVP or LDP. This prevents placing one RSVP session into another LSP, or in other words, nesting one LSP into another.