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When to Use and When Not to Use BGP
Link-state routers establish a peering by exchanging Hello packets (also
known as link-state messages) during a session, which cements the neighbor
relationship between two directly connected routers. This relationship expe-
dites network convergence because neighbors are immediately notified of
topology changes.
Hello packets are sent at short intervals, typically every 10 seconds, and
if an interface fails to receive Hello packets from a neighbor within a pre-
determined hold time, the neighbor is considered down, and the router will
then flood the update out all physical interfaces. This occurs before the new
route table is calculated, so it saves time. Neighbors receive the update, copy
it, flood it out their interfaces, and then calculate the new routing table--this
procedure is followed until the topology change has been propagated
throughout the network.
Unlike distance-vector protocols, which send the entire routing table,
link-state routing protocols advertise only updates or changes, making the
messages much smaller, which saves both bandwidth and CPU utilization.
Plus, if there are no network changes, updates are sent out only at specified,
or default, intervals, which differ among specific routing protocols and can
range from as short as 30 minutes to as long as two hours. EIGRP, which is
a link-state routing protocol, sends updates only when there is a topology
change to a directly connected neighboring router. These updates are called
triggered updates.
IGPs can be used with BGP; however, there are certain instances where
BGP should be used and certain instances where it should not be used. In the
following sections, we will outline these instances.
When to Use and When Not to Use BGP
tatic or default routes, which were discussed earlier, may be used in
situations where the complexity of BGP is not required. First let's take a look
at when you should use BGP. The following scenarios are examples of when
BGP should be used:
When you need to send traffic through one AS to get to another AS
When the flow of data traffic out of your network must be
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