| Author |
When shall I disable split-horizon?
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| mosam 2002-10-18, 10:08 am |
| Is there a rule of thumb for enable/disable split-horizon. I only disable it when using one or more secondary IPs on an interface that sends RIP, IGRP or EIGRP updates (I use eigrp parameter in case of EIGRP updates).
Will this be enough as a general rule? Or is there any other concerns I shall bear in mind?
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| netport 2002-10-21, 1:29 pm |
| You would also need to disable it when using frame-relay. | |
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| Why? I can use subinterfaces in FR with no need to disable split-horizon, since it is virtually a different interface with a different source address.
Is there a specific reason for disabling split-horizon in FR interfaces? | |
| netport 2002-10-22, 12:00 am |
| straight off of cisco's website
Enable or Disable Split Horizon
Normally, routers that are connected to broadcast-type IP networks and that use distance-vector routing protocols employ the split horizon mechanism to reduce the possibility of routing loops. Split horizon blocks information about routes from being advertised by a router out of any interface from which that information originated. This behavior usually optimizes communications among multiple routers, particularly when links are broken. However, with nonbroadcast networks (such as Frame Relay and SMDS), situations can arise for which this behavior is less than ideal. For these situations, you might want to disable split horizon. This applies to IGRP and RIP.
If an interface is configured with secondary IP addresses and split horizon is enabled, updates might not be sourced by every secondary address. One routing update is sourced per network number unless split horizon is disabled.
To enable or disable split horizon, use one of the following commands in interface configuration mode: Command Purpose
ip split-horizon
Enable split horizon.
no ip split-horizon
Disable split horizon.
Split horizon for Frame Relay and SMDS encapsulation is disabled by default. Split horizon is not disabled by default for interfaces using any of the X.25 encapsulations. For all other encapsulations, split horizon is enabled by default.
See the "Split Horizon Examples" section at the end of this chapter for examples of using split horizon. | |
| MadChef 2002-10-22, 5:47 am |
| quote:
Originally posted by mosam
Why? I can use subinterfaces in FR with no need to disable split-horizon, since it is virtually a different interface with a different source address.
Is there a specific reason for disabling split-horizon in FR interfaces?
You're right, there is no split horizon issue when using point to point subs since they're all treated as discreet interfaces, thus split horizon doesn't apply. It DOES apply when using multipoint subs and when using FR on a physical interface. Split horizon is disable by default on physical FR interfaces and enabled by default on multipoint subs.
Caslow treats FR's impact on various routing protocols better than any other book I know of. I highly recommend that book if only for the FR & OSPF on NBMA sections.
MadChef | |
| SureshHomepage 2002-11-05, 1:06 am |
| Hi,
Here is the quick method to remember the split related issues. This actually happens automatically.
Physical Interface(split enabled) -->Enable Frame relay (split disabled)-->Create Subinterface(split enabled again).
You would need to enable exclusively the split horz when you got 'point to multi' at the hub and if running distance vector protocols like EIGRP and RIP. Its not enough if you type 'ip split enable' this will not solve the issue unless you type-in "ip split-horizon eigrp 100 enable". Beware!
Species like OSPF are never affected by tis the split issue coz they are link state types. |
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