3Com Switch 8800 Family Configuration Guide

BGP/MBGP Overview 363
Route selection policy
In the implementation of Switch 8800 Family series, these policies are adopted for
BGP to select routes:
■ First discard the routes unreachable to the next hop.
■ First select the routes with the highest local preference.
■ First select the routes rooted from the router itself.
■ First select the routes with the least AS-paths.
■ First select the routes with the lowest origin.
■ First select the routes with the lowest MED value.
■ First select the routes learned from EBGP.
■ First select the routes advertised by the router with the lowest ID.
MBGP MBGP overview
As described at the beginning of this chapter, BGP, as the practical exterior
gateway protocol, is widely used in interconnection between autonomous
systems. The traditional BGP-4 can only manage the routing information of IPv4
and has limitation in inter-AS routing when used in the application of other
network layer protocols (such as IPv6 etc).
In order to support multiple network layer protocols, IETF extended BGP-4 and
formed MBGP (Multiprotocol Extensions for BGP-4, multiple protocols extension
of BGP-4). The present MBGP standard is RFC2858.
MBGP is backward compatible, that is, a router supporting BGP extension can be
interconnected with a router that does not support it.
MBGP extension attributes
In the packets BGP-4 uses, three pieces of information related to IPv4 are carried in
the update packet. They are network layer reachability information (NLRI),
Next_Hop (The next hop address) in path attribute and Aggregator in path
attribute (This attribute includes the BGP speaker address which forms the
summary route).
When multiple network layer protocols are supported, it is necessary for BGP-4 to
reflect the information of the specified network layer protocol to NLRI and the
Next_Hop. Two new routing attributes are introduced in MBGP:
■ MP_REACH_NLRI: Multiprotocol Reachable NLRI, used to advertise reachable
routes and the next hop information.
■ MP_UNREACH_NLRI: Multiprotocol Unreachable NLRI, used to delete
unreachable routes.
These two attributes are optional non-transitive. Therefore, the BGP speaker that
does not provide multiple protocols ability will ignore the information of them nor
transfer them to other peers.
Address family
The network layer protocols are differentiated by address families in BGP. See
RFC1700 (assigned numbers) for the possible values of these address families.