Reference Guide

BGP session supports multiple address family interface (AFI) and sub address family interface (SAFI) combinations, BGP uses OPEN
message to convey this information to the peers. As a result, the IPv6 routing information is exchanged over the IPv4 peers and vice versa.
BGP routers that support IPv6 can set up BGP sessions using IPv6 peers. If the existing BGP-v4 session is capable of exchanging ipv6
prexes, the same is used to carry ipv4 as well as ipv6 prexes. If the BGP-v4 neighbor goes down, it also impacts the IPv6 route
exchange. If BGP-v6 session exists, it continues to operate independently from BGP-v4.
Multiprotocol BGPv6 supports many of the same features and functionality as BGPv4. IPv6 enhancements to MBGP include support for an
IPv6 address family and Network Layer Reachability Information (NLRI) and next hop attributes that use the IPv6 addresses.
Attributes
Routes learned using BGP have associated properties that are used to determine the best route to a destination when multiple paths exist
to a particular destination. These properties are called BGP attributes which inuence route selection for designing robust networks. There
are no hard-coded limits on the number of supported BGP attributes.
BGP attributes for route selection:
• Weight
• Local preference
• Multiexit discriminators
• Origin
• AS path
• Next-hop
Communities
BGP communities are sets of routes with one or more common attributes. Communities assign common attributes to multiple routes at the
same time. Duplicate communities are not rejected.
Selection criteria
Best path selection criteria for BGP attributes:
1 Prefer the path with the largest WEIGHT attribute, and prefer the path with the largest LOCAL_PREF attribute.
2 Prefer the path that is locally originated using the network command, redistribute command, or aggregate-address
command. Routes originated using a network or redistribute command are preferred over routes that originate with the
aggregate-address command.
3 (Optional) If you congure the bgp bestpath as-path ignore command, skip this step because the AS_PATH is not
considered. Prefer the path with the shortest AS_PATH:
• An AS_SET has a path length of 1 no matter how many are in the set
• A path with no AS_PATH congured has a path length of 0
• AS_CONFED_SET is not included in the AS_PATH length
• AS_CONFED_SEQUENCE has a path length of 1 no matter how many ASs are in the AS_CONFED_SEQUENCE
4 Prefer the path with the lowest ORIGIN type—IGP is lower than EGP and EGP is lower than INCOMPLETE.
5 Prefer the path with the lowest multiexit discriminator (MED) attribute:
• This comparison is only done if the rst neighboring AS is the same in the two paths. The MEDs compare only if the rst AS in the
AS_SEQUENCE is the same for both paths.
• Congure the bgp always-compare-med command to compare MEDs for all paths.
• Paths with no MED are treated as “worst” and assigned a MED of 4294967295.
Layer 3
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