Bgp As Number Substitution And Soo; Mpls L3Vpn Frr - HP HSR6800 Configuration Manual

BGP AS number substitution and SoO

Because BGP detects routing loops by AS number, if EBGP runs between PEs and CEs, you must
assign different AS numbers to geographically different sites to ensure correct transmission of the
routing information.
The BGP AS number substitution function allows physically dispersed CEs to use the same AS
number. The function is a BGP outbound policy and functions on routes to be advertised.
With the BGP AS number substitution function, when a PE advertises a route to a CE of the specified
peer, if an AS number identical to that of the CE exist in the AS_PATH of the route, it is replaced with
that of the PE.
After you enable the BGP AS number substitution function, the PE re-advertises all routing
information to the connected CEs in the peer group, performing BGP AS number substitution based
on the previous principle.
Figure 82 Application of BGP AS number substitution and SoO
EBGP_Update: 10.1.1.1/32
AS_PATH: 800
AS 800
Site 1
In Figure 82, both Site and Site 2 use the AS number of 800. AS number substitution is enabled on
PE 2 for CE 2. Before advertising updates received from CE 1 to CE 2, PE 2 finds that an AS number
in the AS_PATH is the same as that of CE 2 and hence substitutes its own AS number 100 for the AS
number. In this way, CE 2 can normally 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 CE2. To remove the
routing loop, you can configure a routing policy on PE2 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.

MPLS L3VPN FRR

MPLS L3VPN Fast Reroute (FRR) is applicable to a dual-homed scenario, as shown in
MPLS L3VPN FRR supports the two backup modes, backup between two VPNv4 routes and backup
between one IPv4 route and one VPNv4 route.
AS 100
PE 1
MPLS backbone
VPNv4_Update: 10.1.0.0/16
RD: 10.1.1.1/32
AS_PATH: 800
CE 1
257
CE 3
PE 2
AS 800
Site 2
EBGP_Update: 10.1.1.1/32
AS_PATH: 100, 100
CE 2
Figure 83.