Chapter 50
| IP Routing Commands
Border Gateway Protocol (BGPv4)
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on the capabilities advertised in these messages. Open messages include
information about the BGP version number in use, the peer’s AS number, the
hold time, the BGP identifier (i.e., loopback address or the highest value of all
the BGP speaker’s interfaces), and optional parameter length.
◆ UPDATE – These messages are used to announce or withdraw IP prefixes, and
include the following components: withdrawn route length, withdrawn routes,
total path attributes length, path attributes, and network layer reachability
information.
◆ NOTIFICATION – These messages are used to indicate error conditions. The
underlying TCP session is closed after a notification message is sent.
◆ KEEPALIVE – These messages are sent at a set interval and are used to verify that
the BGP session is active. The hold timer is reset upon receipt of a KEEPALIVE or
UPDATE message. If the hold time is set to zero by both peers, a BGP session can
be kept open without generating KEEPALIVE messages.
Route Aggregation and Dissemination
In the Internet, the number of destinations is larger than most routing protocols
can manage. It is not possible for routers to track every possible destination in their
routing tables. To overcome this problem BGP relies on route aggregation, whereby
multiple destinations are combined in a single advertisement. Routers receiving
this information, treat the combined destinations as a single destination, thus
reducing the number of individual routes that must be remembered. This also
reduces the network overhead required to transmit update packets and maintain
routing tables.
In BGP, route aggregation combines the address blocks for networks from two or
more ASes into a supernet, and transmits this information to a downstream AS. This
supernetted address block is less specific, and only lists the AS number of the AS
where the supernetting was done. The Atomic_Aggregate attribute indicates that
attributes for more specific paths are not included in the aggregated route, and the
Aggregator attribute indicates the AS and router where the aggregation was done.
The aggregator node will now serve as a proxy, using the more specific routes it still
maintains in its own routing table.
After inbound routes have been aggregated, the BGP speaker can propagates this
information based on export policies for individual neighbors or for defined router
groups, using route maps or other more precise routing criteria.
Internal BGP
Scalability
An iBGP speaker cannot advertise IP prefixes it has learned from one iBGP speaker
to another neighboring iBGP speaker. iBGP therefore requires full-mesh
connectivity among all iBGP speakers. For local networks containing a large
number of speakers, this requirement may be difficult to meet. There are several
commonly used approaches to resolving this problem, including route reflectors,
confederations, and route servers.
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