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Distance-Vector Protocols
Route invalid timer Determines the length of time that must expire (90
seconds) before a router determines that a route has become invalid. It
will come to this conclusion if it hasn't heard any updates about a partic-
ular route for that period. When that happens, the router will send out
updates to all its neighbors, letting them know that the route is invalid.
Route flush timer Sets the time between a route becoming invalid and
its removal from the routing table (240 seconds). Before it is removed
from the table, the router notifies its neighbors of that route's impending
doom. The value of the route invalid timer must be less than that of the
route flush timer. This is to provide the router with enough time to tell its
neighbors about the invalid route before the routing table is updated.
RIP Updates
The distance-vector routing algorithm passes complete routing tables to
neighbor routers. The neighbor routers then combine the received routing
table with their own routing tables to complete the internetwork map. This
is called routing by rumor, as a router receiving an update from a neighbor
router believes the information about remote networks without actually
finding out for itself.
It is possible to have a network with multiple links to the same remote net-
work. If that is the case, the administrative distance is first checked. If the
administrative distance is the same, it will have to use other metrics to deter-
mine the best path to use to that remote network.
RIP uses only hop count to determine the best path to an internetwork. If
RIP finds more than one link to the same remote network with the same hop
count, it will automatically perform a round-robin load balance. RIP can
perform load balancing for up to six equal-cost links.
However, a problem with this type of routing metric arises when the two
links to a remote network are different bandwidths but the same hop count.
Figure 2.3, for example, shows two links to remote network
Since network is a T1 link with a bandwidth of 1.544Mbps,
and network is a 56K link, you would want the router to choose
the T1 over the 56K link. However, since hop count is the only metric used
with RIP routing, they would both be seen as equal-cost links. This is called
pinhole congestion.
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