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New Member

Split horizon and locally generated routes

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800d97f7.html#wp1001168

If we have got this scenario :

hostname R1

!

!

int E1

ip address 10.1.1.1 255.255.255.0

ip summary-address rip 10.2.0.0 255.255.0.0

!---split horizon is enabled by default,therefor I am not going to write this command:"ip split-horizon"

router rip

version 2

network 10.0.0.0

The definition of the split horizon is :

(Split Horizon blocks routing update to be sent to over the same interface that the route was learned from).

To apply this definition to the scenario above, R1 should receive updates from another router, then split horizon will not let R1 to advertise updates from the same interface where it received them.--> that means split horizon should not affect routes generated by R1 itself, it only should affect routes received from another router.

But my confusion how can we satisfy the note below (quote from the link above)!!!!! :

"If split horizon is enabled, neither an autosummary address nor the interface summary-address is advertised".

6 REPLIES

Re: Split horizon and locally generated routes

Hi,

My understanding of split horizon is: "it prevents the router from advertising routes out any interface from which the route was learnt". In your case split horizon would affect routes originated from R1 as well.

If you are testing this is in a lab setup then you could configure an interface summary address, enable 'debug ip rip' and look at the rip updates and you would see the summary address getting suppressed by R1. Unfortunately, I don't have access to my lab at this time to post the output.

Hope that helps!

Regards,

Sundar

New Member

Re: Split horizon and locally generated routes

Thanks Sundar

((My understanding of split horizon is: "it prevents the router from advertising routes out any interface from which the route was learnt"))

Is your understanding different from mine ? I mean is there any disparity between my understanding and you understanding, ? because I feel both definition are same (coreect me if I am wrong)

((In your case split horizon would affect routes originated from R1 as well))

Does this conculsion rely the definition that you have stated ? or it rely on the test that you had done in a lab ?

((If you are testing this is in a lab setup then you could configure an interface summary address))

I will try to do that

Regards

New Member

Re: Split horizon and locally generated routes

Basically split horizon affects how received routes are being advertised, not the routes generated locally. So in your case, 10.2.0.0 255.255.0.0 will be the route advertised out of the interface.

The Cisco literature is a little bit confusing. I think it is meant to say the same as "everything received will not be advertised out of the same interface". Becasue RIP by default does classful auto-summary, so the "everything" here can only be anto-summarized route or the "ip summary-address" route configured on the neighboring RIP router.

You can verify it in lab using another router as R2 and configure several /24 networks on it and do a "ip summary-address". You should see 10.2.0.0 255.255.0.0 on R2 as received from R1 but not advertied from R2 to R1 again.

Let us know what you find out.

Gary

New Member

Re: Split horizon and locally generated routes

((So in your case, 10.2.0.0 255.255.0.0 will be the route advertised out of the interface)).

Did you mean to say that the above network was received (not generated locally ) by R1, and R1 tries to summarize this routes and sends them out of interface E1 ????

New Member

Re: Split horizon and locally generated routes

No. 10.2.0.0 255.255.0.0 is locally generated summary route, advertised out by R1. You should see the route on a neighboring RIP router as received from R1. With split horizon, the neighboring router will not advertise the route out of the same interface back to R1.

New Member

Re: Split horizon and locally generated routes

((No. 10.2.0.0 255.255.0.0 is locally generated summary route, advertised out by R1. You should see the route on a neighboring RIP router as received from R1. With split horizon, the neighboring router will not advertise the route out of the same interface back to R1)).

This is exactly how I understood split Horizon (your explaination is what I understood from the definition of split horizon).

Now my confusion as i have mentioned in my early post can we satisfy the note below :

((((("If split horizon is enabled, neither an autosummary address nor the interface summary-address is advertised".))))),,,,

In our scenario (example) autosummary and summary address (10.2.0.0 255.255.0.0 locally generated route) has been advertised by R1 to neighbor/s!!!!!!now the neighor/s (This is not my concern at this stage, my concern is R1) should not advertise the route back to R1 from the same interface it received.

Now according to the note, R1 should not have advertise 10.2.0.0

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