OSI Layer 3: Routed and Routing Protocols
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2.
Replies to the Hello packets include all routes in the neighbor router's
topology database, including the metrics. Routes that are learned from
the originating router are not included in the reply.
3.
The originating router acknowledges the update to each neighbor via
an ACK packet.
4.
The topology database is then updated with the newly received infor-
mation.
5.
Once the topology database is updated, the originating router then
advertises its entire table to all the new neighbors.
6.
Neighbor routers acknowledge the receipt of the route information
from the originating router by sending back an ACK packet.
These steps are used in the initialization of EIGRP neighbors; they change
somewhat when only updates are sent to existing neighbors.
Route Calculation and Updates
Because EIGRP uses distance-vector and link-state information when calcu-
lating routes by using the DUAL algorithm, convergence is much faster
than with IGRP. The trick behind the convergence speed is that EIGRP cal-
culates new routes only when a change in the network directly affects the
routes contained in its route table. To make that a little clearer, look at
Figure 2.14, in which you see three routers meshed, and each has an Ethernet
segment connected as well.
It is important to understand the difference between accepting a routing
update and calculating a new route. If a change occurs to a network that is
directly connected to a router, all of the relevant information is used to
calculate a new metric and route entry for it. After the router calculates the
new route, it is advertised to the neighbors.
Using Figure 2.14 as the example, assume that Ethernet 0 on Router C is
very congested because of high traffic volumes. Router C then uses the dis-
tance and link information to calculate a new metric for network
172.16.30.0
. With the new metric in place, the change is propagated to
Routers A and B. To understand completely, you need to recognize that the
other routers don't do any calculation--they just receive the update. Routers
A and B don't need to calculate a new route for network 172.16.30.0
because they learn it from Router C.
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