Bgp Cost Community Support For Eigrp Mpls Vpn Pe-Ce With Back-Door Links - Cisco ASR 9000 Series Routing Configuration Manual

BGP Cost Community
The preceding route policy applies a cost community number of 1 to the 10.8.0.0 route. By default, the path
learned from ASBR1 is assigned a cost community number of 2147483647. Because the path learned from
ASBR2 has a lower-cost community number, the path is preferred.

BGP Cost Community Support for EIGRP MPLS VPN PE-CE with Back-door Links

Back-door links in an EIGRP MPLS VPN topology is preferred by BGP if the back-door link is learned first.
(A back-door link, or route, is a connection that is configured outside of the VPN between a remote and main
site; for example, a WAN leased line that connects a remote site to the corporate network.)
The "prebest path" point of insertion (POI) in the BGP cost community feature supports mixed EIGRP VPN
network topologies that contain VPN and back-door links. This POI is applied automatically to EIGRP routes
that are redistributed into BGP. The "prebest path" POI carries the EIGRP route type and metric. This POI
influences the best-path calculation process by influencing BGP to consider the POI before any other comparison
step. No configuration is required. This feature is enabled automatically for EIGRP VPN sites when Cisco
IOS XR software is installed on a PE, CE, or back-door router.
For information about configuring EIGRP MPLS VPNs, see the Cisco ASR 9000 Series Aggregation Services
Router MPLS Configuration Guide.
This figure shows how cost community can be used to support backdoor links in a network.
Figure 2: Network Showing How Cost Community Can be Used to Support Backdoor Links
The following sequence of events happens in PE1:
1 PE1 learns IPv4 prefix 10.1.1.0/24 from CE1 through EIGRP running a virtual routing and forwarding
(VRF) instance. EIGRP selects and installs the best path in the RIB. It also encodes the cost-extended
community and adds the information to the RIB.
2 The route is redistributed into BGP (assuming that IGP-to-BGP redistribution is configured). BGP also
receives the cost-extended community from the route through the redistribution process.
3 After BGP has determined the best path for the newly redistributed prefix, the path is advertised to PE
peers (PE2).
4 PE2 receives the BGP VPNv4 prefix route_distinguisher:10.1.1.0/24 along with the cost community. It
is likely that CE2 advertises the same prefix (because of the back-door link between CE1 and CE2) to
PE2 through EIGRP. PE2 BGP would have already learned the CE route through the redistribution process
along with the cost community value
5 PE2 has two paths within BGP: one with cost community cost1 through multipath BGP (PE1) and another
with cost community cost2 through the EIGRP neighbor (CE2).
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
36
Implementing BGP