BGP, LDP, and RSVP-TE can contribute routes to the table. LDP adds all destinations
that can be reached by means of labels learned from downstream LDP neighbors.
RSVP-TE adds only MPLS tunnel endpoints. BGP also adds routes to the tunnel table in
certain cases. Routes added by any of these protocols include the effective metric.
For example, in a BGP/MPLS VPN topology, LDP or RSVP-TE adds routes to the tunnel
routing table for all available tunnels. BGP performs a lookup in the tunnel routing table
so that it can resolve indirect next hops.
You can clear the routes from the tunnel routing table. You might do this, for example,
to reapply routing policies when the policies are changed.
Related Topics MPLS Forwarding and Next-Hop Tables Overview on page 233•
• Clearing and Refreshing IPv4 Dynamic Routes in the Tunnel Routing Table on page 325
• Clearing and Refreshing IPv6 Dynamic Routes in the Tunnel Routing Table on page 325
• Explicit Routing for MPLS Overview on page 235
Explicit Routing for MPLS Overview
MPLS offers two options for selecting routing paths:
•
Hop-by-hop routing
•
Explicit routing
In explicit routing, the route the LSP takes is defined by the ingress node. The path consists
of a series of hops defined by the ingress LSR. Each hop can be a traditional interface,
an autonomous system, or an LSP. A hop can be strict or loose.
A strict hop must be directly connected (that is, adjacent) to the previous node in the
path. A loose hop is not necessarily directly connected to the previous node; whether it
is directly connected is unknown.
The sequence of hops comprising an explicit routing LSP may be chosen in either of the
following ways:
•
Through a user-defined configuration, resulting in configured explicit paths. When you
create the explicit route, you must manually configure each hop in the path.
•
Through a routing protocol–defined configuration, resulting in dynamic explicit paths.
When the routing protocol (IS-IS or OSPF) creates the explicit path, it makes use of
the topological information learned from a link-state database in order to compute
the entire path, beginning at the ingress node and ending at the egress node.
Consider the MPLS domain shown in Figure 55 on page 236. Without explicit path routing,
the tunnel is created hop by hop along the following path:
LSR 1 –> LSR 3 –> LSR 4 –> LSR 7
235Copyright © 2010, Juniper Networks, Inc.
Chapter 3: MPLS Overview
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