continues to operate normally. All traffic flows from Router-C through Router-B to the hosts attached to
Router-A and Router-D.
Figure 14: Floating Summary Route Applied for Dual-Homed Remotes
However, if the link between Router-D and Router-E fails, the network may dump traffic into a black hole
because Router-E continues to advertise the default route (0.0.0.0/0) to Router-C, as long as at least one link
(other than the link to Router-C) to Router-E is still active. In this scenario, Router-C still forwards traffic to
Router-E, but Router-E drops the traffic creating the black hole. To avoid this problem, you should configure
the summary address with an administrative distance on only single-homed remote routers or areas in which
only one exit point exists between the segments of the network. If two or more exit points exist (from one
segment of the network to another), configuring the floating default route can cause a black hole to form.
Split Horizon for an EIGRP Process
Split horizon controls the sending of EIGRP update and query packets. When split horizon is enabled on an
interface, update and query packets are not sent for destinations for which this interface is the next hop.
Controlling update and query packets in this manner reduces the possibility of routing loops.
By default, split horizon is enabled on all interfaces.
Split horizon blocks route information from being advertised by a router on any interface from which that
information originated. This behavior usually optimizes communications among multiple routing devices,
particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and SMDS),
situations can arise for which this behavior is less than ideal. For these situations, including networks in which
you have EIGRP configured, you may want to disable split horizon.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
OL-30423-03 247
Implementing EIGRP
Split Horizon for an EIGRP Process
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