Bgp Routing Information; Bgp Messages And Mpls Labels - Cisco CRS Configuration Manual

Implementing MPLS Layer 3 VPNs
• A local PE router (for example, PE1 in the figure below) needs to know the routes and label information
for the remote PE router (PE2).
This information can be exchanged between the PE routers and ASBRs in one of two ways:
Alternatively, the route reflector can reflect the IPv4 routes and MPLS labels learned from the ASBR to the
PE routers in the VPN. This reflecting of learned IPv4 routes and MPLS labels is accomplished by enabling
the ASBR to exchange IPv4 routes and MPLS labels with the route reflector. The route reflector also reflects
the VPN-IPv4 routes to the PE routers in the VPN. For example, in VPN1, RR1 reflects to PE1 the VPN-IPv4
routes it learned and IPv4 routes and MPLS labels learned from ASBR1. Using the route reflectors to store
the VPN-IPv4 routes and forward them through the PE routers and ASBRs allows for a scalable configuration.
Figure 35: VPNs Using eBGP and iBGP to Distribute Routes and MPLS Labels

BGP Routing Information

BGP routing information includes the following items:
• Network number (prefix), which is the IP address of the destination.
• Autonomous system (AS) path, which is a list of the other ASs through which a route passes on the way
to the local router. The first AS in the list is closest to the local router; the last AS in the list is farthest
from the local router and usually the AS where the route began.
• Path attributes, which provide other information about the AS path, for example, the next hop.

BGP Messages and MPLS Labels

MPLS labels are included in the update messages that a router sends. Routers exchange the following types
of BGP messages:
◦ Internal Gateway Protocol (IGP) and Label Distribution Protocol (LDP): The ASBR can redistribute
the IPv4 routes and MPLS labels it learned from eBGP into IGP and LDP and from IGP and LDP
into eBGP.
◦ Internal Border Gateway Protocol (iBGP) IPv4 label distribution: The ASBR and PE router can
use direct iBGP sessions to exchange VPN-IPv4 and IPv4 routes and MPLS labels.
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
Exchanging IPv4 Routes with MPLS labels
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