8.2 Exchanging information with BGP-3

This document suggests the following guidelines for exchanging routing information between BGP-4 and BGP-3.

To provide for graceful migration, a BGP speaker may participate in BGP-3, as well as in BGP-4. Thus, a BGP speaker may receive IP reachability information by means of BGP-3, as well as by means of BGP-4.

A BGP speaker may inject the information received by BGP-4 into BGP-3 as follows.

If an AS_PATH attribute of a BGP-4 route carries AS_SET path segments, then the AS_PATH attribute of the BGP-3 route shall be constructed by treating the AS_SET segments as AS_SEQUENCE segments, with the resulting AS_PATH being a single AS_SEQUENCE. While this procedure loses set/sequence information, it doesn't affect protection for routing loops suppression, but may affect policies, if the policies are based on the content or ordering of the AS_PATH attribute.

While injecting BGP-4 derived NLRI into BGP-3, one needs to be aware of the potential information explosion when a given IP prefix denotes a set of consecutive A/B/C class networks. Injection of BGP-4 derived NLRI that denotes IP subnets requires the BGP speaker to inject the corresponding network into BGP-3. The local system shall provide mechanisms to control the exchange of routing information between BGP-3 and BGP-4. Specifically, a conformant implementation is required to support all of the following options when injecting BGP-4 received routing information into BGP-3:

• inject default only (0.0.0.0), no export of any other NLRI

• allow controlled deaggregation, but only of specific routes; allow export of non-aggregated NLRI

• allow export of only non-aggregated NLRI

The exchange of routing information via BGP-3 between a BGP speaker participating in BGP-4 and a pure BGP-3 speaker may occur only at the autonomous system boundaries. Within a single autonomous system BGP conversations between all the BGP speakers of that autonomous system have to be either BGP-3 or BGP-4, but not a mixture.