197
The BGP AS number substitution feature allows geographically different CEs to use the same AS
number. If the AS_PATH of a route contains the AS number of a CE, the PE replaces the AS number
with its own AS number before advertising the route to that CE.
After you enable the BGP AS number substitution feature, the PE performs BGP AS number
substitution for all routes and re-advertises them to connected CEs in the peer group.
Figure 62 Application of BGP AS number substitution and SoO attribute
As shown in
779H
Figure 62
, both Site 1 and Site 2 use the AS number 800. AS number substitution is
enabled on PE 2 for CE 2. Before advertising updates received from CE 1 to CE 2, PE 2 substitutes
its own AS number 100 for the AS number 800. In this way, CE 2 can correctly receive the routing
information from CE 1.
However, the AS number substitution feature also introduces a routing loop in Site 2 because route
updates originated from CE 3 can be advertised back to Site 2 through PE 2 and CE 2. To remove
the routing loop, you can configure the same SoO attribute on PE 2 for CE 2 and CE 3. PE 2 adds the
SoO attribute to route updates received from CE 2 or CE 3, and checks the SoO attribute of route
updates to be advertised to CE 2 or CE 3. The SoO attribute of the route updates from CE 3 is the
same as the SoO attribute for CE 2, and PE 2 does not advertise route updates to CE 2.
For more information about the SoO attribute, see
Layer 3—IP Routing Configuration Guide
.
232B
MPLS L3VPN FRR
MPLS L3VPN Fast Reroute (FRR) is applicable to a dual-homed scenario, as shown in
780H
Figure 63
. By
using BFD to detect the primary link, FRR enables a PE to use the backup link when the primary link
fails. The PE then selects a new optimal route, and uses the new optimal route to forward traffic.
MPLS L3VPN FRR supports the following types of backup:
•
VPNv4 route backup for a VPNv4 route.
•
VPNv4 route backup for an IPv4 route.
•
IPv4 route backup for a VPNv4 route.
MPLS backbone
VPNv4_Update: 10.1.0.0/16
RD: 100:1
AS_PATH: 800
EBGP_Update: 10.1.0.0/16
AS_PATH: 100, 100
PE 1
PE 2
CE 3
AS 800
Site 2
EBGP_Update: 10.1.0.0/16
AS_PATH: 800
AS 800
Site 1
AS 100
CE 1
CE 2
