background image
Scalability Features of Routing Protocols
sweet scenario, and the larger the network, the worse it gets, because a
greater percentage of bandwidth is needed for routing updates.
As the size of the routing table increases, so does CPU utilization, because
it takes more processing power to calculate the effects of topology changes
and then converge using the new information. Also, as more routes populate
a routing table, it becomes increasingly complex to determine the best path
and next hop for a given destination. The following list summarizes the scal-
ability limitations inherent in distance-vector algorithms:
Network convergence delay
Increased CPU utilization
Increased bandwidth utilization
Scalability Limitations of Link-State Routing Protocols
Link-state routing protocols assuage the scalability issues faced by distance-
vector protocols because the algorithm uses a different procedure for route
calculation and advertisement. This enables them to scale along with the
growth of the network.
Addressing distance-vector protocols' problem with network conver-
gence, link-state routing protocols maintain a formal neighbor relationship
with directly connected routers that allows for faster route convergence.
They establish peering by exchanging Hello packets during a session, which
cements the neighbor relationship between two directly connected routers.
This relationship expedites 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 predetermined hold time, the neighbor is
considered down, and the router will then flood the update out all physical
interfaces. This occurs before the new routing table is calculated, so it saves
time. Neighbors receive the update, copy it, flood it out their interfaces, and
calculate the new routing table. The procedure is followed until the
topology change has been propagated throughout the network.
It's noteworthy that the router sends an update concerning only the
information--not the entire routing table. So the update is a lot smaller,
which saves both bandwidth and CPU utilization. Plus, if there aren't any
network changes, updates are sent out only at specified, or default, intervals,
Copyright ©2001 SYBEX , Inc., Alameda, CA