H3C Technologies H3C S6800 Series Switches - Manual (Page 192)

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BGP AS number substitution
BGP detects routing loops by examining AS numbers. If EBGP runs between PE and CE, you must assign
different AS numbers to geographically different sites to ensure correct transmission of routing
information.
The BGP AS number substitution feature allows physically dispersed CEs to use the same AS number. The
feature is a BGP outbound policy and affects routes to be advertised.
With the BGP AS number substitution feature, when a PE advertises a route to a CE, if an AS number
identical to that of the CE exists in the AS_PATH of the route, the PE replaces it with its own AS number.
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 58 Application of BGP AS number substitution
As shown in Figure 58 , both Site 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 a routing policy on PE 2 to add the SoO attribute to route updates
received from CE 2 and CE 3 so that PE 2 does not advertise route updates from CE 3 to CE 2.
NOTE:
The device does not support adding the SoO attribute to routes.
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 a 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.
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