R0106-HP MSR Router Series MPLS Configuration Guide(V7)

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The BGP AS number substitution function 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 function, the PE performs BGP AS number substitution
for all routes and re-advertises them to connected CEs in the peer group.
Figure 58 Application of BGP AS number substitution and SoO attribute
As shown in Figure 58, 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 function 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.
MPLS L3VPN FRR
MPLS L3VPN Fast Reroute (FRR) is applicable to a dual-homed scenario, as shown in Figure 59. By using
BFD to detect the primary link, FRR enables the PE router to use the backup link when the primary link fails.
The PE router 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, and 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