CCNA Certification

CCNA, CCNP, CCIE Certification News

EIGRP And Split Horizon

EIGRP is a major topic for your CCNA and CCNP studies, and one basic skill you’ll need to pass your Cisco certification exams is to identify situations where you need to enable or disable split horizon.

EIGRP commands tend to be a little different than those used with other protocols, so let’s take a look at how EIGRP and split horizon interoperate.

R1 is our hub router, with R2 and R3 as the spokes. There are no subinterfaces, and each router is advertising a single loopback network using its router number for each octet. R1 will see both R2 and R3’s loopback network, but the spokes will not have a route to the other spoke’s loopback. “show ip route eigrp” verifies this.

eigrp1.gif

EIGRP runs split horizon by default, making it impossible for R1 to forward an advertisement to R2 regarding R3’s loopback. Likewise, R1 cannot advertise R2’s loopback address to R3. We could configure two subinterfaces on R1 to resolve this issue, but here we’re going to disable split horizon instead.

eigrp2.gif

Note that disabling split horizon resulted in the EIGRP adjacencies being torn down. They came back up 20 – 35 seconds after being torn down according to the timestamps, but that’s a good detail to keep in mind!

The routing tables of each spoke should now show the loopback network configured on the remote spoke.

eigrp3.gif

Be careful when disabling split horizon. In this scenario, R1 can and will advertise routes out Serial0 that were learned about on that interface in the first place, and that’s not always desirable. Split horizon is enabled by default for a reason, so be careful when disabling it!

Advertisements

April 2, 2007 - Posted by | CCNA, CCNP, Education, EIGRP, Technology

No comments yet.

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: