The address-family in the BGP configuration allows you to define a specific behavior of BGP with regards to many supported Layer3 protocols:
IPv4 networks for unicast
IPv4 networks for multicast
IPv6 networks for unicast
IPv6 networks for multicast
CLNS (NSAP) networks
The BGP treats all these address families individually, as if it has been configured in a separate instance for each of them. These individual instances do not leak one into each other (even if they are of the same basic type). This way, you can configure a single BGP process to maintain several databases of different networks.
The BGP, as a multiprotocol routing protocol, can carry different types of routing information in a single BGP session. That means, however, that for each of these network types, you need to somehow tell the BGP which neighbors also support the same protocol type, which routes to inject, how are the individual networks going to be filtered or their attributes modified, and so on... That is done in the address-family context.
For example, a simple IPv4/IPv6/NSAP BGP could be configured as follows:
A single BGP process in this case is configured with three address families: IPv4, IPv6, and NSAP (the OSI stack). There are two neighbors defined, one by its IPv4 address, the second by its IPv6 address. In my example, both these addresses actually correspond to a single neighboring router but of course they do not have to. Now, in the address-family ipv4 context, it is configured that the we will exchange IPv4 routing information with the neighbor reachable under the IP 188.8.131.52. In the address-family ipv6 context, it is configured that we will exchange IPv6 routing information with the neighbor 2001:12::1. And finally, in the address-family nsap context, it is stated that this router will also exchange the routing information for CLNS networks with the router 184.108.40.206. All these three different Layer3 protocols and their routing information will be maintained by this single BGP process but it was necessary to specify which neighbors speak which protocols. In addition, if we needed to use route-maps, prefix-lists and other attribute manipulations for a particular address type, we would also need to define them for particular neighbors in their respective address-family contexts.
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