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.

As shown in Figure 49, both Site and Site 2 use the AS number 800. AS number substitution is enabled on PE 2 for CE 2. Before advertising updates received from CE 1 to CE 2, PE 2 substitutes its own AS number 100 for the AS number 800. In this way, CE 2 can correctly 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 CE 2. To remove the routing loop, you can configure a routing policy on PE 2 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.

	NOTE: The device does not support adding the SoO attribute to routes.