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In networks as complex as today's Internet,
route stability is a big issue. There is a close correspondence between
route fluctuations and the stability of the Internet access links on one
hand and how the routing information was injected into the Internet via
BGP on the other hand. Information can be injected into BGP dynamically or
statically. Dynamically injected routes come and go from the BGP routing
table, depending on the status of the networks they identify. Statically
injected routes are constantly maintained by the BGP routing tables,
regardless of the status of the networks they identify. Thus, while a
dynamic advertisement will cease if the network being advertised no longer
exists, a static advertisement would not. Each method has its pros and
cons, as you will see next.
Dynamically injected information can be
further divided into purely dynamic, where all the IGP routes are
redistributed into BGP (redistribute
command), and semidynamic, where only certain IGP routes are to be
injected into BGP (network
command). The distinction reflects both the level of user intervention and
the level of control in defining the routes to be advertised.
Information is injected dynamically into
BGP by enabling all the IGP routes to be redistributed into BGP. A variety
of IGPs are currently used in autonomous systems, including Routing
Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP),
Enhanced IGRP (EIGRP), Open Shortest Path First (OSPF), and Intermediate
System-to-Intermediate System (IS-IS). What dynamic redistribution offers
is ease of configuration: all internal IGP routes will dynamically flow
into BGP, regardless of the protocols being used.
A semidynamic method of injecting
information into BGP is to specify a subset of IGP networks to be
advertised by individually listing them for injection into BGP by using
the network
command. This method is less than completely dynamic because a list of all
the prefixes that need to be advertised must be maintained in the
router-the router is not automatically injecting all IGP routes into BGP.
If the list of prefixes is large, maintaining it would be impractical.
Besides, router implementations might put an upper limit on the number of
prefixes that can be listed. If the number of prefixes to be advertised is
larger than that upper-bound limit, dynamically redistributing the IGP
into BGP is a must.
BGP assumes that networks defined with the network
command are existing networks and will try to verify that by checking in
the IP routing table. If BGP does not find an exact match for these
networks, they will not be advertised. The verification step is smart
because advertising a network not connected to or unknown by the router is
misleading to other networks because the router cannot deliver traffic to
this network anyway.
Injecting routes into BGP via the network
command offers a more controlled way of route advertisement. Injecting
IGPs into BGP via redistribution could result in the side effect of
leaking unwanted or faulty information into BGP, as you will see next.
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