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236 Chapter 7: OSPF, EIGRP, and IGRP
Split Horizon
When using physical interfaces on a broadcast network, it is not a good idea to send information
about a route to the same router from which you learned it. Split horizon does not allow a router
to propagate routes learned from an interface out the same interface. IGRP does not advertise
network reachability to a neighbor from which the route was learned. Split horizon prevents
routing loops.
When designing nonbroadcast multiaccess networks that use logical interfaces, such as Frame
Relay , you might want to disable split horizon. If an interface is configured with secondary IP
addresses and split horizon is enabled, updates might not be sourced by every secondary
address. One routing update is sourced per network number, unless split horizon is disabled.
Figure 7-10 shows an example of split horizon.
Figure 7-10
Bit Splitting
In this example, split horizon must be disabled for network 128.125.0.0 to be advertised into
network 131.108.0.0, and vice versa. These subnets overlap at Router C, interface S0. If split
horizon were enabled on serial interface S0, it would not advertise a route back into the Frame
Relay network for either network.
IGRP Variance
IGRP variance is used to balance two links at different speeds. It is recommended that the links
be similar in all respects, with the exception of speed. For example, the links should have the
same encapsulation and the same maximum transmission unit size. IGRP supports up to four
unequal-cost load-balancing paths at a time.
Determining EIGRP Routing, Characteristics, and
Convergence
EIGRP is an enhanced version of IGRP. The same distance-vector technology found in IGRP
is also used in EIGRP. All features in IGRP are available in EIGRP. However, EIGRP has
Area 131.108.0.0
131.108.1.0
131.108.2.0
up to
131.108.63.0
Area 131.108.64.0
131.108.64.0
131.108.65.0
up to
131.108.127.0
Area 131.108.128.0
131.108.128.0
131.108.129.0
up to
131.108.191.0
Area 131.108.192.0
131.108.192.0
131.108.193.0
up to
131.108.254.0
87200333.book Page 236 Wednesday, August 22, 2001 2:37 PM