Sunday, December 19, 2004

Multi-homing to a single provider

In practice, the Border Gateway Protocol (BGP) is mainly used by ISP's and enterprises for connecting their autonomous systems (AS) to multiple other ASes, such as upstream providers, peers, and customers. An organization wishing to obtain better connectivity can do so by connecting to more than one upstream provider (multi-homing) and announcing its address space using BGP.

When using more than one connection to the same upstream provider, BGP is a logical choice for the routing protocol, since it supports load balancing and redundancy, and provides a clear separation between responsibilities and administrative domains of the provider and the customer, unlike an IGP would. However, the AS allocation guidelines (described in RFC 1930) preclude the use of a dedicated AS number for an organization connected in that way, since there is no need to exchange routing information with more than one party, i.e., there is no separate routing policy.
RFC 2270 proposes to use a single AS number for all customers multi-homed to the same (single) provider, preferably one of the private AS numbers, 64512 to 65535. In this way, there is no unnecessary use of AS numbers by organizations who do not strictly need them. The customer can use BGP to announce its address space, which will then be announced to the rest of the world by its provider. Despite the non-unique (and possibly private) AS number, one still has the advantages of BGP such as a fine grained control over routing announcements and preferences.The RFC also describes some of the implications of using this scheme, such as the need to announce a default route to the customer AS, effects of changes in connectivity, as well as points regarding aggregation and registering routes in a registry.

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