3Com S7906E - Switch, Configuration Manual

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Figure 1-17 Network diagram for sham link
To solve 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 need to configure it manually. In addition, the local VPN
instance must have a route to the destination of the sham link.
BGP AS Number Substitution
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.
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 above
principle.