Need help on split horizon

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Jul 17th, 2008
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Hi Experts,


Can anybody give me brief introductions on Router Passive interface & Split Horizon.


1. What is the reason for configuring a passive interface on a router?


2.What is the correct statement best explain the split horizon rule?



Thanks in advance,

Naidu.

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lee.reade Thu, 07/17/2008 - 02:24
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Hi,


1. passive interface, means that a router will not send routing advertisements out of that interface, ie


ETH1--R1---ETH2


If we make ETH1 passive under routing protocol, then R1 will only send routing advertisements out ETH2. It can still receive the routing advertisements on ETH1 if any are sent to it by another router however.


We use passive-interface when we still want to advertise a specific network interface, but not to form any routing adjancencies over it.


In the example above you can still advertise the ETH1 link to ETH2 even thoough ETH1 is passive, but you will not advertise the ETH2 link out ETH1 since it is passive.


2. Split horizon basically means that a router will not advertise a routing update out of the interface in which it learned it from.


For example,


ETH1--R1--ETH2---ETH2---R2


R2 learns the ETH1 network from R1 via its ETH2 interface, so R2 will not advertise the R1 ETH1 network back to R1.


This is used for loop prevention in distance vector routing, ie rip, igrp.


HTH


LR

ilnaiduccna Thu, 07/17/2008 - 03:06
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Hi Lee,


Thanks for your well explaination on my queries.



Regards,

Naidu.

Hi Lee


Just a small point that may not have been clear from your post.


When running OSPF or EIGRP and you configure passive-interface on an interface, then that interface will neither send or receive routing updates.


While with passive-interface configured under RIP, RIP will still receive updates on an interface marked as passive but will not send any updates over this interface.


To the OP:


You can see this if you lab it up using a simple 2 router lab.



Best Regards,


Michael

petersrule2003 Thu, 07/17/2008 - 06:35
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passive interace command means different things for different routing protocols

for rip: updates will not be sent out that interface,but can still receive updates and the network will still be advertised

for ospf and eigrp: hello's will not be sent out that interface and neighbors will not form


split horizn rule means that any update that is recieved on an interface will not be sent out that same interface it is an attempt to prevent routing loops

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