Note
The order of comparison determined in Step 2 is important. Consider the case where you have three paths—A, B, and C. When Cisco NX-OS compares A and B, it chooses A. When Cisco NX-OS compares B and C, it chooses B. But when Cisco NX-OS compares A and C, it might not choose A because some BGP metrics apply only among paths from the same neighboring autonomous system and not among all paths.
The path selection uses the BGP AS-path attribute. The AS-path attribute includes the list of autonomous system numbers (AS numbers) traversed in the advertised path. If you subdivide your BGP autonomous system into a collection or confederation of autonomous systems, the AS-path contains confederation segments that list these locally defined autonomous systems.
Cisco NX-OS supports multiple address families. Multiprotocol BGP (MBGP) can carry different sets of routes depending on the address family. For example, BGP can carry one set of routes for IPv4 unicast routing, one set of routes for IPv4 multicast routing, and one set of routes for IPv6 multicast routing. You can use MBGP for Reverse Path Forwarding (RPF) checks in IP multicast networks.
Note
Because Multicast BGP does not propagate multicast state information, you need a multicast protocol, such as Protocol Independent Multicast (PIM).
You need to use the router address-family and neighbor address-family configuration modes to support Multiprotocol BGP configurations. MBGP maintains separate Routing Information Bases (RIBs) for each configured address family, such as a unicast RIB and a multicast RIB for BGP.
A Multiprotocol BGP network is backward compatible, but BGP peers that do not support multiprotocol extensions cannot forward routing information, such as address family identifier information, that the multiprotocol extensions carry.
Table 1-9 lists the BGP default parameters; you can alter BGP default parameters as necessary.
Table 1-9 Default BGP Parameters
Table 1-10 shows NX-OS feature license required for BGP. For more information, visit the Cisco NX-OS Licensing Guide.
Table 1-10 Feature-Based Licenses for Cisco NX-OS
BGP has the following configuration limitations:
The dynamic AS number prefix peer configuration overrides the individual AS number configuration inherited from a BGP template.
If you configure a dynamic AS number for prefix peers in an AS confederation, BGP establishes sessions with only the AS numbers in the local confederation.
BGP sessions created through a dynamic AS number prefix peer ignore any configured eBGP multihop time-to-live (TTL) value or a disabled check for directly connected peers.
Configure a router ID for BGP to avoid automatic router ID changes and session flaps.
Use the maximum-prefix configuration option per peer to restrict the number of routes received and system resources used.
Configure the update source to establish a session with BGP/eBGP multihop sessions.
Specify a BGP policy if you configure redistribution.
Define the BGP router ID within a VRF.
If you decrease the keepalive and hold timer values, you might experience BGP session flaps.
The BGP minimum route advertisement interval (MRAI) value for all iBGP and eBGP sessions is zero and is not configurable.
Tables 1-11 through 1-13 describe the most-used BGP configuration commands. For a full list of the commands, refer to the Nexus Unicast Routing Configuration Guide listed in the reference section at the end of the chapter.
Table 1-11 BGP Global-Level Configurations
Table 1-12 BGP Routing-Level Configurations
Table 1-13 BGP Verification and BGP Clear Commands
Figure 1-5 shows the network topology for the configuration that follows, which demonstrates how to configure Nexus BGP for IPv4 and IPv6.
Figure 1-5 BGP Network Topology
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