EIGRP SPLIT HORIZON

[Ben Alex]

Hi all:

See diagram for more details:

Nex 1 is only learning 40.40.40.0/24 from router C as per show ip eigrp topology 40.40.40.0/24

Note bandwidth set on 40.40.40.0 is 128K

If I disable split horizon on router VPN fa0/1 then I am good.Nex 1 sees both.

So far so good.

Also, if i decide to leave split horison on and shut C, then I see the route towards the VPN.

What I don't understand is that if I leave Split horizon on and change the bandwidth of 40.40.40.0 on KING to 100000 from 128.

Split horizon is no longer in effect and Nex1 can see both 40.40.40.0 ( one from C and one from VPN)

This is driving me a little bit crazy. The JPG attached shows the Feasible distance and reported distance also.

Why is split horzon no longer in effect when the bandwidth is set to 100000( I was playing with this value it looks like below 92000 split horizon is on and greater than 92000 split horizon if off)

Thanks for taking the time to read.

[Giuseppe Larosa · Accepted solution]

Hello Ben,

split horizon rule says : do not advertise out interface X a prefix  A.B.C.D that you have learned on same interface X

your VPN router has two distinct layer 3 interfaces one in network segment with Nex1 and Nex2 and one towards KING that is the other router that can advertise the IP subnet 40.40.40.0/24

until VPN router sees a better route via NEX1 it does not advertise a route for 40.40.40.0/24

by increasing BW on KING router you are actually decreasing the EIGRP metric of the route originated by KING

EIGRP metric = 256  * ( sum of delays in tens of microseconds) +  256 * (10^7/mininumBW in kbps)

the minimum BW on path if all links are FE is the one you give to 40.40.40.0/24 on KING router

by increasing BW you reach a point where VPN router can prefer the route generated by KING router

As a result of this on Nex1 you can see both advertisements but this time VPN routers sends the route originated at KING router!

Edit:

at beginning when you have disabled split horizon on VPN router you have probably seen the route sent back by VPN router, but the route was the one originated at router C when BW=128 kbps. Later as explained above the scenario changes.

The route is sent out by VPN router as it prefers the one generated by KING router and not because split horizon fails.

Hope to help

Giuseppe

[Ben Alex]

Giuseppe:

Thank you so much for taking the time to explain.

"until VPN router sees a better route via NEX1 it does not advertise a route for 40.40.40.0/24"

I was thinking that with the bandwidth of 128K, the VPN router would still advertise 40.40.40.0 to Nex1 and let Nex1 decide not to use it since RD is less that FD. As a result it would appear in the eigrp topoloy table of Nex1.

So is that a fundamental rule not to advertise until I have a better metric?

Thanks again

[Giuseppe Larosa · Accepted solution]

Hello Ben,

>> So is that a fundamental rule not to advertise until I have a better metric?

yes, if you are using a route sent by neighbor RX you don't need to advertise it back to RX translates this in terms of "out of the interface I have heard the route I'm using"

VPN router makes a clear choice when BW=128 on KING router and so it does not advertise a route it wouldn't use itself

This is a basic concept in distance vector routing protocols.

In some cases to improve reliability it would be better to send back a route with infinite metric (poison reverse) to mean : "I'm using the route you have sent me if you miss that route don't think of using me to reach that destination I rely on you"

EIGRP has more advanced features to deal with topology changes (DUAL). but that is a different matter.

Hope to help

Giuseppe

[Ben Alex]

You certainly helped a lot. You are a great contributor to this forum, Give up the good job.

Thanks