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Chapter 7
BGP's Basic Components
BGP keeps its acquired routing table information separate from the IGP's
routing tables. BGP literally steals information the IGPs have learned of their
local network environments and stored on their routing tables. BGP handles
the translation of information from one routing protocol to another routing
protocol when multiple routing protocols are used in an AS.
Cisco supports two different types of algorithms for IGPs to find and cal-
culate paths through the network. These two types are distance-vector and
link-state routing protocols. In the next two sections, we'll take a look at
these in greater detail.
Distance-Vector Protocols
BGP is considered an advanced distance-vector protocol. Distance-vector
protocols such as RIP were designed when network topologies were small.
Distance-vector refers to a routing protocol that uses hop counts or vectors
to determine the distance from one device in the network to another. Each
device that a packet must encompass to get to another destination is consid-
ered a hop. For example, if you have to go through two routers to get to a
destination node, then there are three devices including the destination node,
making it three hops away from your local workstation or two hops from
your local router.
In small networks (less than 100 routers) where the environment contains
less than 15 hops (the count-to-infinity restriction) between any destination,
the network topology is much more forgiving of routing updates and calcu-
lations, and distance-vector protocols perform pretty well. Scaling a distance-
vector protocol to a larger network creates higher convergence times, high
router overhead CPU utilization, and increased bandwidth utilization, all of
which become factors that hinder scalability.
Other drawbacks to distance-vector protocols include no support for
Variable Length Subnet Masks (VLSM) or for Classless Interdomain Rout-
ing (CIDR) and that they don't take into account the speed of each link. For
example, if the network topology contains two ways to a destination, one
contains an ISDN link (128K) and the other is a Frame Relay link (768K) cir-
cuit. The Frame Relay link contains an extra hop. What this means is that a
distance-vector protocol would choose the slow boat, which is the ISDN
link, because it calculates only the hops to a destination and doesn't care that
there is a faster way to the destination. Your 12MB ZIP file will now take an
additional hour to get to its destination.
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