6. Deployment Considerations

The extension proposed in this document provides a mechanism for a BGP speaker to advertise multiple paths over a BGP session. Care needs to be taken in its deployment to ensure consistent routing and forwarding in a network [ADDPATH].

The only explicit indication that the encoding described in Section 3 is in use in a particular BGP session is the exchange of Capabilities described in Section 4. If the exchange is successful [RFC5492], then the BGP speakers will be able to process all BGP UPDATES properly, as described in Section 5. However, if, for example, a packet analyzer is used on the wire to examine an active BGP session, it may not be able to properly decode the BGP UPDATES because it lacks prior knowledge of the exchanged Capabilities.

When deployed as a provider edge router or a peering router that interacts with external neighbors, a BGP speaker usually advertises at most one path to the internal neighbors in a network. In the case where the speaker is configured to advertise multiple paths to the internal neighbors, and additional information is needed for the application, the speaker could use attributes such as the Edge_Discriminator attribute [FAST]. The use of that type of additional information is outside the scope of this document.