The router can place routes in its routing table by using an IGP to learn
the network topology. It uses its own table and calculates its own routes. A
default (static) route can be configured, or a directly connected network can
advertise the route. BGP has a synchronization option that requires the
BGP's learned routes and the IGP's learned routes to synchronize before BGP
will advertise the IGP's learned network topologies.
BGP can also learn routes through the network from other BGP adver-
tisements, network statements, and redistribution of an IGP into a BGP.
Since redistribution can cause routing loops and route flapping, this method
is not recommended except in a lab scenario.
n a stub network, which we discussed earlier, there is only one way in
or out of the AS or network--there is no need to use another protocol to find
routes in the network. A static address mapping can be used for any
unknown routes. This means that if the router has determined that the des-
tination in a packet is not on the local network, it merely forwards the packet
to the static default mapping.
As networks grow, however, and there begin to be many routes through-
out the network, a static route becomes too difficult to maintain. This occurs
when the network has expanded to the extent at which a routing protocol
will scale well and is the point at which you want to discontinue using static
The increased network growth imposes a greater number of topology
changes in the network environment. The number of hops between end sys-
tems, the number of routes in the routing table, the various ways a route has
to be learned, and route convergence are all seriously affected by network
To maintain a stable routing environment, it's absolutely crucial to use a
scalable protocol. When the results of network growth manifest themselves,
whether your network's routers will be able to meet those challenges is up to
the routing protocol the routers are using. For instance, if you use a protocol
that's limited by the number of hops it can traverse, how many routes it can
store in its table, or even the inability to communicate with other protocols,
you have a protocol that will likely hinder the growth of your network.
Copyright ©2001 SYBEX , Inc., Alameda, CA