Scalability Limitations of Link-State Routing Protocols

Scalability Limitations of Link-State Routing Protocols
Link-state routing protocols alleviate 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 convergence, 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—
usually three or four times the Hello time—the neighbor is considered down, and the router will
then flood the update to its neighbors. This occurs before the new routing table is calculated,
so it saves time. Neighbors receive the update, copy it, flood it to their neighbors, and
then
calculate
the new routing table. This 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
new
information—not
the entire routing table. As a result, the update is 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 30 minutes
to two hours. These are often called paranoid updates.
These key differences permit link-state routing protocols to function well in large networks—
they really have little to no limitations when it comes to scaling, other than the fact that they’re a
bit more complex to configure than distance-vector protocols.