From: Kenneth Wygand (KWygand@customonline.com)
Date: Wed Jan 21 2004 - 00:09:20 GMT-3
Split horizon issues usually come into play with Frame Relay circuits (as you mentioned). The following are the split horizon defaults for Frame Relay interfaces:
Physical FR Int: Split Horizon is DISABLED
Multipoint FR Int: Split Horizon is DISABLED
Point-to-point FR Int: Splic Horizon is ENABLED
If you have three routers (A, B and C) connecting into a FR cloud, where a PVC exists between A and B and between B and C, the FR interface type on B will be multipoint. Split horizon will be DISABLED. It needs to be disabled to allow A routes to propegate to C and visa versa (because B is sending out the updates to the other router through the same interface is received the update on from the first router). In this case, you must leave split horizon DISABLED, *BUT* you must filter inbound on A and C so that they discard all networks they advertised to B (B will advertise them back because split horizon is DISABLED, and it cannot be enabled due to the previous mentioning about how routes need to pass from A to C and visa versa).
Other cases are pretty straightforward. On routers A and C, for example, they each only connect to a single router. The configuration will work with these interfaces configured for Physical FR, Multipoint FR or Point-to-point FR, but again the Split Horizon defaults will change. If you are using Physical FR or Multipoint FR on A or C, you must explicitly ENABLE split horizon.
The general rule I use is this: Ask yourself - is there any reason I would want to send information back out this interface (in other words, are there any other devices attached to this interface other than the router originating the update)? If so, just DISABLE split horizon. If not, just ENABLE split horizon.
Hope this helps,
Ken
-----Original Message-----
From: nobody@groupstudy.com on behalf of David Porta
Sent: Tue 1/20/2004 9:46 PM
To: ccielab@groupstudy.com
Cc:
Subject: Split horizon
I have searched the archives, and there is a lot of information about
this subject. Unfortunaly some of the information seems to be
contradicting, so I am wondering if some one could please explain this
one to me:
I understand that Split Horizon is used by Distance Vector protocols in
order to prevent routing loops usind the famous "Don't tell me what I
have told you" explanation.
Now, since Frame Relay disables split horizon by default, here is where
expertise come in to play.
Where do we enable or disable split horizon?
For Point to Point connections...
For Point to Multipoint connections:
In the hub ...
In the spokes ...
Also, when doing mutual redistribution, do we need to disble split
horizon?
Thank you in advance for your help clarifying these items.
David
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