Juniper JUNOSE, Configuration Manual

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advertise this capability only to peers for which the neighbor send-label command
has been configured.
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 Switching and Packet Forwarding on page 209
■
MPLS Label Distribution Methodology on page 218
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MPLS Interfaces and Interface Stacking on page 227
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Topology-Driven LSPs on page 241
ECMP Labels for MPLS
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.
ECMP Labels for MPLS
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Chapter 2: MPLS Overview