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*Keypoints:IPX uses SAP advertisements to perform network updates.Know what the above IPX monitoring commands do.Be sure to know the above table of compared encapsulation types.Use "show ipx interface" to display the IPX address assignments on a router.Routing Protocol TypesDistance Vector Concept
Distance vector based routing algorithms pass periodic copies of a routing table from router to router.
Regular updates between routers communicate topology changes. Each router receives a routing table
from its direct neighbor and increments all learned routes by one.
This is the way that the algorithm learns the internetwork topology, via second hand information. Dis-
tance Vector algorithms do not allow a router to know the exact topology of an internetwork.
RIP and IGRP are Distance Vector Routing Protocols.
Distance Vector Topology Changes
When the topology in a distance vector network changes, routing table updates must occur. As with the
network discovery process, topology change notification must occur router to router.
Distance Vector protocols call for each router to send its entire routing table to each of its adjacent
neighbors. When a router receives an update from a neighboring router, it compares the update to its
own routing table. If it learns about a better route (smaller hop count) to a network from its neighbor,
the router updates its own routing table.
Problems with Distance Vector
Distance Vector routing protocols are prone to Routing Loops and counting to infinity. Routing loops
can occur if the internetwork's slow convergence on a new configuration causes inconsistent routing
entries.
Counting to infinity continuously loops packets around the network, despite the fundamental fact that
the destination network is down.
To over come these you can implement several different options:
·
Defining a maximum number of hops - Specify a maximum distance vector metric as infin-
ity. 16 with RIP and 256 with IGRP.
·
Split Horizon - If you learn a protocol's route on an interface, do not send information about
that route back out that interface.
·
Route Poisoning - Information past out on an interface is marked as unreachable by setting
the hop count to 16 for RIP
·
Hold Down Timers - Routers ignore network update information for some period of time.
The timers can been reset when:
1.
The timer expires.
2.
Infinity is finally defined as some maximum number.
3.
Another update is received indicating that the original route to the network has been re-
stored.