Understanding the network.pdf - Back to Home

internal/Internet access router where the internal network and the multiple Internet
links are all homed into the same router. This approach does have some drawbacks.
The first is the fact that there is no hardware redundancy; if the internal gateway
fails, you are completely down. To overcome this and still maintain a single gateway
for end-stations and servers, use HSRP (or VIRP) with two gateway routers. The
second drawback has to do with security. Internet gateways are managed by you
and in part by your ISP. It is usually not a good idea to have your ISP accessing your
internal network gateway router.
After the connectivity has been established between the internal gateway router
and the Internet gateway routers, you want to establish a local BGP process on your
gateway router. This is started using the global configuration EXEC command
. If you are planning to use BGP to provide reachability
information over the Internet, you will need a registered AS number from an
Internet Addressing Authority (IAA), such as the American Registry for Internet
Numbers (ARIN). For unannounced BGP (for internal use only), any number
between 32768 and 64511 will do. For this example, we are going to announce
networks 12.14.116.0 /22 and 192.168.180 /22.
Before we start configuring our BGP process, we first need to establish routes to our
remote-AS BGP neighbors. In most cases, you will peer with the router on the other
end of your Internet link. In this case, the routers are (ATT) 12.127.40.65 and (PSI)
192.144.36.1. We can accomplish peering with static routes:
ASBR-Master#config t
ASBR-Master(config)#ip route 12.127.40.64 255.255.255.252 12.14.116.3
ASBR-Master(config)#ip route 192.174.36.0 255.255.255.252 12.14.116.2
Now, let's enable our BGP process. The process ID is the AS number, which, in this
case, is 789. The process ID is used for communication with other BGP processes. If
you are planning to run BGP on more than one internal router, be sure that the same
AS is used:
ASBR-Master(config)#router bgp 789
ASBR-Master(config-router)#
After the process starts, we add the networks we want announced. Like OSPF, BGP
uses netmasks as part of the network number announcements. This means that you
can announce parts of networks as VLSM subnets, as CIDR address ranges, or as