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Routing Protocols
RIPv2 provides support for VLSM and CIDR. RIPv1 does not.
A network's convergence time is determined by the ability of RIP to prop-
agate changes within the network topology. Distance-vector protocols don't
use formal neighbor relationships between routers. A router using distance-
vector algorithms becomes aware of a topology change in two ways and ages
out entries in its routing information base (RIB):
When a router fails to receive a routing update from a directly con-
nected router
When a router receives an update from a neighbor notifying it of a
topology change somewhere in the network
Each routing protocol sends out routing updates at default intervals or at
a manually configured time interval. This means that when a topology
change occurs, using the defaults, it could take 90 seconds or longer before
a neighboring router realizes that there has been a link-state change and
switches to an alternate path. Ninety seconds can be an eternity to the net-
work, causing application timeouts and other problems for network users.
When the router does finally update its RIB with the change, it recalculates
its route table. Then instead of just advertising the change, it advertises its
entire table to all its neighboring routers.
Imagine having 50 routers advertising their entire routing tables and the
impact that this can have on the bandwidth in your network. It compounds
one problem with another. Not only do you lose a link that provides band-
width, but the more problems you have the worse it gets because a greater
percentage of bandwidth is needed for routing updates.
When the size of the routing table increases, so does the router's CPU uti-
lization. The reason is that it takes a lot more processing power to calculate
the routing table changes, converge using the new information, and advertise
its new table. This utilization can also be compounded by more routes pop-
ulating a routing table. The table becomes increasingly complex in order to
determine the best path and next hop for a given destination.
Copyright ©2001 SYBEX , Inc., Alameda, CA