ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
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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.