3Com Switch 8800 Configuration Guide
Chapter 21 BGP Configuration
21-3
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A BGP speaker does not advertise the routes obtained from IBGP to its IBGP
peers.
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A BGP speaker advertises the routes obtained from IBGP to its IBGP peers (In the
Switch 8800, BGP and IGP are asynchronous.)
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Once the connection is set up, a BGP speaker will advertise all its BGP routes to
its peers.
II. Route selection policy
In the Switch 8800, these policies are adopted for BGP to select routes:
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First discard the routes unreachable to the next hop.
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First select the routes with the highest local preference.
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First select the routes rooted from the router itself.
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First select the routes with the least AS-paths.
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First select the routes with the lowest origin.
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First select the routes with the lowest MED value.
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First select the routes learned from EBGP.
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First select the routes advertised by the router with the lowest ID.
21.1.4 MBGP
I. 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.
II. 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:
