Bgp As Number Substitution And Soo - HP 12500 Series Configuration Manual

address space on the PE. Different sham links of the same OSPF process can share an endpoint address,
but that of different OSPF processes cannot.
BGP advertises the endpoint addresses of sham links as VPN-IPv4 addresses. A route across the sham
link cannot be redistributed into BGP as a VPN-IPv4 route.
A sham link can be configured in any area. You need to configure it manually. In addition, the local VPN
instance must have a route to the destination of the sham link.
NOTE:
When configuring an OSPF sham link, redistribute OSPF VPN routes to BGP, but do not redistribute BGP
routes to OSPF to avoid route loops.

BGP AS number substitution and SoO

Since 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 will be replaced with that
of the PE.
NOTE:
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 64 Application of BGP AS number substitution and SoO
EBGP_Update: 10.1.1.1/32
AS_PATH: 800
AS 800
Site 1
In Figure 64, both Site 1 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 CE3 can be advertised back to Site 2 through PE 2 and CE2. To remove the
AS 100
PE 1
MPLS backbone
VPNv4_Update: 10.1.0.0/16
RD: 10.1.1.1/32
AS_PATH: 800
CE 1
211
CE 3
PE 2
AS 800
Site 2
EBGP_Update: 10.1.1.1/32
AS_PATH: 100, 100
CE 2