234 Chapter 7: OSPF, EIGRP, and IGRP
table to its neighbors periodically. Like RIP, IGRP is a classful protocol and summarizes at
network boundaries.
IGRP has several features that differentiate it from RIP and allow it to have greater functionality:
·
Scalability--IGRP can be configured for large internetworks. IGRP has a default network
diameter of 100 hops and can scale up to 255 hops.
·
Fast response to network changes--Unlike RIP and other distance-vector routing
protocols, IGRP sends updates when route topology changes occur.
·
Sophisticated metric--IGRP uses a composite metric that provides significant route
selection flexibility. Internetwork delay, bandwidth, reliability, and load are all factored
into the routing decision.
NOTE
By default, only bandwidth and delay are used.
·
Multiple paths--IGRP can maintain up to four unequal paths between a network source
and destination. Multiple paths can be used for redundancy or to increase available
bandwidth.
IGRP Features
As mentioned in Chapter 6, IGRP by default uses a combination of bandwidth and delay as a
routing metric:
IGRP metric = (C1
Ч bandwidth) + [(C2 Ч bandwidth) / (256-load) + (C3 Ч delay)]
C1, C2, and C3 are constants. By default, C2 = 0 and C1 and C3 = 1.
So, by default, IGRP metric = bandwidth + delay.
In addition to bandwidth and delay, IGRP can be configured to track the following metrics:
·
Reliability
·
Loading
·
MTU (static)
·
Hops (static)
CH07.fm Page 234 Wednesday, August 22, 2001 3:52 PM