H3C Technologies H3C S12500-X Series Switches - Manual (Page 128)

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Figure 45 Network diagram for sham link
To use the inter-area route, you can establish a sham link between the two PEs to change the inter-area
route to an intra-area route. The sham link is advertised in a Type 1 LSA as an intra-area point-to-point link.
You can also select the sham link or the backdoor link by adjusting their costs.
The sham link is considered a link between the two VPN instances. Each VPN instance has an endpoint
address of the sham link, which is a loopback interface address with a 32-bit mask in the VPN address
space. Different sham links of the same OSPF process can share an endpoint address, but sham links of
different OSPF processes cannot share an endpoint address.
BGP advertises the endpoint addresses of sham links as VPN-IPv4 addresses. Sham link routes cannot be
redistributed into BGP as VPN-IPv4 routes.
A sham link can be configured in any area and can only be manually configured. The local VPN
instance must have a route to the destination of the sham link.
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 function allows physically dispersed CEs to use the same AS number.
The function is a BGP outbound policy and affects routes to be advertised.
With the BGP AS number substitution function, 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 function, the PE performs BGP AS number substitution
for all routes and re-advertises them to connected CEs in the peer group.