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Chapter 2
Routing Principles
EIGRP Convergence
Let's take a look at the convergence time of Enhanced IGRP (EIGRP). We
will again use Figure 2.6 to help describe the convergence process:
RouterD detects the link failure between D and F and immediately
checks the topology table for a feasible successor. RouterD does not
find an alternate route in the topology table and puts the route into
active convergence state.
RouterD sends a query out all interfaces looking for a route to the
failed link. Routers C and E acknowledge the query.
RouterC responds with no known link.
RouterE responds with a route to the failed link, but it is at a higher
cost than the original link.
RouterD uses the new link and places it in the topology table, which
is then placed in the routing table.
RouterD broadcasts this new route out all interfaces, and Routers C
and E acknowledge the update back to RouterD, with an update of their
own. These updates make sure the routing tables are synchronized.
RouterA convergence time is the total time of detection, plus the query
and reply time, plus the update time--about two seconds total. However, the
time can be higher.
OSPF Convergence
Using Figure 2.6 as a reference, let's now take a look at the convergence cycle
used in OSPF:
RouterD detects the link failure between Routers D and F, and a Des-
ignated Router (DR) election is held on the E0 interface, but no neigh-
bors respond. The route entry is removed from RouterD. A Link State
Advertisement (LSA) is sent out all interfaces on RouterD.
Routers C and E receive the LSA and forward the LSA packet out all
interfaces except for the interface where the LSA was received.
Copyright ©2001 SYBEX , Inc., Alameda, CA