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When there is a change in the network, the
router that learned about the change advertises it to its neighbors by
multicasting an update packet with the change. If the update packets are
to notify the neighbors that a router was added to the network, then the
process described in the previous "Discovering Neighbors" and "Discovering
Routes" sections occurs. However, if the update packet says that a
link has a worse metric, or is no longer available, the router
must find an alternative path.
To obtain an alternative path, the router
that lost the link looks for a new feasible successor in its topology
table. If a feasible successor exists, it is promoted to a successor and
added to the routing table, and then used. The topology table is then
recalculated to determine whether there are any new feasible successors,
based on the new successor's feasible distance.
If a feasible successor is not available,
the following process is performed.
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- The router (router A) flags the failed
route as in an "active" state in the topology table. When
routes are operating well, they are in "passive" state.
- Router A looks for an alternative path
by sending out a query packet to all its neighbors to learn whether they
have a path to the given destination. The query packet is multicast out
every interface except the one from which the dead link was learned, thus following the split horizon rule.
Because the router expects a reply to the
query from each neighbor, it tracks the sending and receiving of these
packets from topology table.
In Figure , for example, no
feasible successor exists because no router's advertised distance is
less than router B's feasible distance. As a result, the router must
query its neighbors to find new successors and feasible successors. The
route to network 7 changes from passive to active state.
- If a neighbor has a feasible successor
that does not use the querying router, or no route at all to the
destination, it unicasts a Reply packet to the requestor indicating the
appropriate information.

If a neighbor that receives the query is
using the querying router as its feasible successor, then it sends its
own Query packet to its neighbors, which creates a query ripple effect
through the network until a major network boundary is met with, or until
the router is on the autonomous system boundary (the end of EIGRP
routers). In Figure , for example, you see router B send a
query to its next network.
- When the query router receives
replies, it reacts, based on the answer in the reply:
- If the reply included a successor or
feasible successor, the information is put into its topology table
and the querying router waits until all replies are received. It
then recalculates the topology table and adds the successor(s) to
the routing table. The route returns to a passive state in the
topology table and routing can continue.
- If none of the replies includes a
successor or feasible successor, the querying router removes the
active route from its topology and routing tables.
If one or more routers to which a query is
sent do not respond with a reply within the active time of 180 seconds,
EIGRP tears down the neighbor relationship with this rogue router and puts
routes that used the rogue router into an active state. The querying
router then generates queries for the route(s) it lost through the rogue
router.
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Lab
Activity |
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In
this lab, you will learn how to use VLSM and IP
unnumbered with the EIGRP routing protocol. |
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