When BGP advertises labeled routes, it adds a label-to-next-hop mapping
(cross-connect) to the MPLS forwarding table. This mapping consists of the in label that
BGP allocates from the platform label space plus the MPLS next hop information related
to the labeled route's next hop.
BGP can also distribute labels for VPN routes in BGP/MPLS VPNs. In a BGP/MPLS VPN
network, BGP is used to exchange the routes of a particular VPN among the provider
edge routers attached to the VPN. To ensure that routes from different VPNS remain
distinct even if the VPNs use overlapping address spaces, an MPLS label is assigned to
each route within the VPN. BGP distributes both the VPN routes and the associated MPLS
label for each route.
The label mapping information for a particular VPN route is included in the same BGP
update message that distributes the route. The label is encoded into the NLRI field of
the attribute, and the SAFI field has a value of 128 to indicate that the NLRI contains both
an RD (route distinguisher) and a label.
For more information on BGP capabilities, see Configuring BGP Routing. For more
information on MP-BGP extensions, NLRIs, and BGP/MPLS VPNs, see Configuring
BGP-MPLS Applications..
Related Topics MPLS Label Distribution Methodology on page 227•
• MPLS Label Switching and Packet Forwarding Overview on page 218
• MPLS Interfaces and Interface Stacking Overview on page 236
• Topology-Driven LSPs Overview on page 255
ECMP Labels for MPLS Overview
MPLS supports equal-cost multipath (ECMP) labels. A maximum of 16 MPLS paths is
supported; 4 paths are available by default. On LERs, MPLS ECMP next hops can be used
in the IP routing table for non-VPN and VPN routes. On LSRs, an incoming label can point
to either an MPLS ECMP next hop or an IP ECMP.
The signaling protocol determines whether ECMP next hops are used. For example, LDP
can learn multiple labels for a route from different downstream peers (or one label from
a downstream peer that has parallel connections to the router). LDP then creates an
MPLS ECMP next hop that can be used in the IP routing table. If LDP also advertises a
label, then a forwarding entry is added to the MPLS forwarding table with the ECMP next
hop.
MPLS Connectivity and ECMP
When an MPLS ECMP is part of the tunnel being explored by an MPLS echo request, the
request packet takes one of the available ECMP paths. Probing FECs with different label
stacks can yield different ECMP paths. However, you cannot guarantee complete coverage
of all the ECMP paths.
You can use MPLS trace to determine which paths are present on an MPLS LSR. When
the TTL expires on an MPLS LSR, the echo reply that is returned includes a downstream
Copyright © 2010, Juniper Networks, Inc.242
JunosE 11.2.x BGP and MPLS Configuration Guide
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