EIGRP Split Horizon
Lab Objective:
The objective of this lab exercise is for you to learn and understand the
effects of Split Horizon in a typical hub and spoke topology.
Lab Purpose:
Configuring and troubleshooting Split Horizon is a fundamental skill. EIGRP is
an advanced Distance Vector protocol, and as such uses Split Horizon to prevent
routing loops. Split Horizon mandates that EIGRP will not send updates back out
of the interface on which they were received. While this default feature is
generally a good thing, it can have a disastrous effect on traditional hub and
spoke topologies. As a Cisco engineer, as well as in the Cisco CCNA exam, you
will be expected to know how to address Split Horizon issues in EIGRP.
Certification Level:
This lab is suitable for CCNA certification exam preparation
Lab Difficulty:
This lab has a difficulty rating of 8/10
Readiness Assessment:
When you are ready for your certification exam, you should complete this lab in
no more than 20 minutes
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IMPORTANT NOTE: In order to configure Frame Relay between two routers in your lab, you will need THREE routers! The first two routers will be regular routers, and the third will need to be configured as a Frame Relay switch. This can be any Cisco router that has at least two Serial interfaces. Please refer to Appendix B: Cabling and configuring a Frame Relay Switch For Three Routers for the appropriate configuration to issue on the Frame Relay switch. |
Lab Topology:
Please use the following topology to complete this lab exercise:
Task 1:
This lab will only be performed
on R1, R2, and R3. Configure the hostnames on routers R1, R2, and R3 as above.
Task 2:
Configure the switch in the
topology with the hostname Sw1. Enable F0/1, F0/2, and F0/4 on Sw1 so that when
you bring up the router interfaces connected to those switch ports they can come
up.
Task 3:
Configure IP addresses on the
Fa0/0 interfaces on R1, R2, and R3. Make sure you enable these interfaces and
they are up.
Task 4:
Configure Frame Relay on R1,
R2, and R3. Use the IP addresses in the topology for their respective Serial
interfaces. Use the default Frame Relay encapsulation of Cisco. Configure static
Frame Relay maps between R1, R2, and R3, so that each router has a static Frame
Relay map to the other two routers on the Frame Relay network.
Task 5:
Enable EIGRP in AS 1760 on R1,
R2, and R3 for all the subnets configured on those respective routers. Be sure
to prevent EIGRP from automatically summarizing at Classful network boundaries.
Task 6:
If you have configured
everything as requested, you will not be able to see the 10.1.1.0/24 route via
EIGRP on R3 nor will you be able to see the 172.16.3.0/25 route via EIGRP on R2.
However, R1 will have both routes. Verify that this is the case using the
appropriate commands.
Task 7:
Based on your studies, you know
that the reason you are not seeing the 10.1.1.0/24 route via EIGRP on R3 and the
172.16.3.0/25 route via EIGRP on R2 is because these routes are both sent to R1
via EIGRP, but since Distance Vector protocols do not send routing information
back out of the same interface they received it, R1 will not send the routing
information for 10.1.1.0/24 to R3 or the routing information for 172.16.3.0/25
since the routing information was received on the same interface. To prevent
this from happening, disable this default feature.
Task 8:
Now verify that you can see the
10.1.1.0/24 route via EIGRP on R3 and the 172.16.3.0/25 route via EIGRP on R2.
Ping 10.1.1.1 from R3 and 172.16.3.3 from R2 to verify network connectivity.
