Maximizing Hop Count - Juniper J2300 User Manual

J-series™ Services Router User Guide
Figure 59: Distance-Vector Protocol
C=1
D=2
E=3
In Figure 59, routers A and B have RIP enabled on adjacent interfaces. Router A
has known RIP neighbors routers C, D, and E, which are 1, 2, and 3 hops away,
respectively. Router B has known RIP neighbors routers X, Y, and Z, which are
1, 2, and 3 hops away, respectively. Every 30 seconds, each router floods its
entire routing table information out all RIP-enabled interfaces. In this case,
flooding exchanges routing table information across the RIP link.
When router A receives routing information from router B, it adds 1 to the hop count
to determine the new hop count. For example, router X has a hop count of 1, but
when router A imports the route to X, the new hop count is 2. The imported route
also includes information about where the route was learned, so that the original
route is imported as a route to router X through router B with a hop count of 2.
When multiple routes to the same host are received, RIP uses the distance-vector
algorithm to determine which path to import into the forwarding table. The
route with the smallest hop count is imported. If there are multiple routes
with the same hop count, all are imported into the forwarding table, and
traffic is sent along the paths in round-robin fashion.

Maximizing Hop Count

The successful routing of traffic across a RIP network requires that every node in
the network maintain the same view of the topology. Topology information is
broadcast between RIP neighbors every 30 seconds. If router A is many hops
away from a new host, router B, the route to B might take significant time to
propagate through the network and be imported into router A's routing table. If
the two routers are 5 hops away from each other, router A cannot import the
route to router B until 2.5 minutes after router B is online. For large numbers of
hops, the delay becomes prohibitive. To help prevent this delay from growing
arbitrarily large, RIP enforces a maximum hop count of 15 hops. Any prefix that
is more than 15 hops away is treated as unreachable and assigned a hop count
equal to infinity. This maximum hop count is called the network diameter.
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RIP Overview
Routing information
A
Route table A
C=1
D=2
E=3
X=2
Y=3
Z=4
X=1
B
Y=2
Z=3
Route table B
X=1
Y=2
Z=3
C=2
D=3
E=4