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.

• 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 above principle.

Application of BGP AS number substitution

In above Figure both CE 1 and CE 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.

AS number substitution also applies to a PE connecting multiple CEs through different interfaces, such as PE 2 in above Figure, which connects CE 2 and CE 3.

Note:

For a multi-homed CE, that is, a CE connected with multiple PEs, the BGP AS number substitution function must be used in combination with the site-of-origin (SOO) function. Otherwise, routing loops may appear.