When dealing with BGP, route stability
is an important issue. There is a close relationship between Internet
route stability and the method used to inject routes into BGP. 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 that they identify. Statically injected routes are constantly
maintained by the BGP routing tables, regardless of the status of the
networks that they identify. Thus, while a dynamic advertisement will
cease if the network being advertised no longer exists, a static
advertisement will not. Each method has pros and cons, as you will see in this section.
Dynamically injected information can be further
divided into purely dynamic redistribution, in which all the IGP routes
are redistributed into BGP using the redistribute command, and
semi-dynamic
redistribution, in which only certain IGP routes are to be injected into
BGP using the 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 (RIP, OSPF, EIGRP, and so on) to be
redistributed into BGP. Dynamic redistribution offers ease of
configuration: All internal IGP routes dynamically flow into BGP,
regardless of the protocols being used.
The semi-dynamic 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 more selective than a completely dynamic method.
You control which of the IGP-learned routes are advertised by BGP.
Unfortunately, a network
command is necessary for each route prefix, so if you are dealing with a
large number of prefixes, maintaining a semi-dynamic configuration is
impractical. In fact, the Cisco IOS limits you to 200 network
statements. Ultimately, a semi-dynamic configuration provides greater
administrative control but dramatically increases administrative overhead.
As you have seen, BGP assumes that prefixes
specified by the network
command exist in an IGP domain and will verify this by checking for them
in the routing table. If an IGP has not learned about a local route, BGP
will not advertise it. Of course, you can use the no
synchronization command to disable this
verification, but in doing so, you risk allowing a router to advertise
networks that it can not reach.