Bgp As Number Substitution And Soo - HP 10500 SERIES Configuration Manual

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If the PE needs to advertise to a CE the routes from other OSPF domains, it must indicate that it is
the ASBR, and advertise the routes using Type 5 LSAs.
Sham link
Generally, BGP peers carry routing information on the MPLS VPN backbone through the BGP extended
community attributes. The OSPF that runs on the remote PE can use the information to create Type 3
summary LSAs to be transmitted to the CEs. As shown in
1 and OSPF area1. They are connected to different PEs, PE 1 and PE 2. There is an intra-area OSPF link
called backdoor link between them. In this case, the route connecting the two sites through PEs is an
inter-area route. It is not preferred by OSPF because its preference is lower than that of the intra-area
route across the backdoor link.
Figure 29 Network diagram for sham link
To resolve the problem, you can establish a sham link between the two PEs so that the routes between
them over the MPLS VPN backbone become an intra-area route.
The sham link acts as an intra-area point-to-point link and is advertised through the Type 1 LSA. You can
select a route between the sham link and backdoor link by adjusting the metric.
The sham link is considered the link between the two VPN instances with one endpoint address in each
VPN instance. The endpoint address is a loopback interface address with a 32-bit mask in the VPN
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 must configure it manually. In addition, the local VPN
instance must have a route to the destination of the sham link.
When configuring an OSPF sham link, redistribute OSPF VPN routes to BGP, but do not redistribute BGP
routes to OSPF to avoid routing loops.

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.
Figure
29, both site 1 and site 2 belong to VPN
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