Understanding the network.pdf - Back to Home

For many users, the usefulness of BGP is questionable. BGP, althoughconsidered a
routing protocol, could be more aptly described as a network reachability protocol.
Its focus is not on announcing how to reach a destination, but rather on how to get
somewhere that can reach the destination. With this goal in mind, let's review the
three types of routing BGP is used for:
• Inter-autonomous system routing—A collection (two or more) routers from
different autonomous systems exchange BGP information with the goal of
maintaining a complete reachability table for the entire intranetwork.
• Intra-autonomous system routing—Used by peer-routers connected to the
same autonomous system to exchange external AS information.
• Pass-through autonomous system routing—Used to exchange BGP
reachability information between a collection of routers belonging to the
same or different ASs across an uninvolved third-party AS. In this case, the
ASs exchanging reachability information are dependent on the third-party
AS's IGP protocol to transport the BGP traffic.
As stated earlier, almost all applications of BGP occur within the context of the public
Internet. Most private enterprise networks do not have these types of requirements,
and those that do can safely accomplish their goals with OSPF or EIGRP. However,
with businesses'growing dependency on the Internet and the hyper-growth of many
corporate intranets, the usefulness of BGP is finally reaching the private network. It
is particularly applicable to environments where access to multiple private/public
networks links is required. Because BGP was designed to manage the Internet, it is
powerful, and as you can imagine, can be quite complex to configure. With this in
mind, we will look at some basic inter-autonomous and intra-autonomous BGP
configurations.
Inter-Autonomous BGP
For most BGP applications, the goal is to provide network-reachability information
for your local network between two or more ISP gateways. In this situation, your
goal is not to configure a large BGP routing policy. All you want is to have Internet
access redundancy. An easy way to achieve this goal is to use ships in the night (SIN)
routing. SIN routing exists where a router is running multiple dynamic protocols,
without redistributing them into one another. The effect is that only the SIN router
processes a complete internal and external network routing table. The other internal
network routers use the SIN router as the default gateway, so they blindly forward
all of their external traffic to the SIN router, which then uses its full routing table to
decide the best next hop. When configuring BGP for the SIN routing context, you
want to set up the BGP process on your internal network's gateway router instead of
configuring BGP on each of your Internet access gateways.
Figure 10.6 illustrates a SIN routing router topology configuration example.