From: Ayman Hamza (ayhamza@cisco.com)
Date: Mon Sep 30 2002 - 14:04:37 GMT-3
Paul;
Split horizon is need even if you have two routers connected alone in
a point to point connectivity. Let's examine this;
(128.1.2.0/24)Ethernet0--R1-----serial-----R2-----Ethernet1(128.1.1.0/24)
If Ethernet0 of R1 went down, and split horizon is turned off on serials of
R1 and R2, then R2 is still sending the R1/s ethernet network(128.1.2.0/24)
to R1. Be aware that R2 won't detect that R1's ethernet interface
is down till three times the update period interval, as it won't see this subnet
in the regular periodic routing updates coming from R1, which is called
Invalid interval .
So now, before R2 reaches the invalid interval (three times the update
interval value) for the subnet (128.1.2.0/24), it is still sending that
subnet to R1 , because:
1- split horizon is still disabled, and
2- this subnet in R2 routing table is marked as valid route.
So R1 receives this update from R2(THAT'S R2 CAN REACH SUBNET 128.1.2.0/24,
WHICH IS ACTUALLY DOWN!!!!!). so R1 will believe this and try to install this
route and destine any packets to this subnet to R2, but ofcourse the packet
will be lost for sure, as the update coming from R2 is a fake one because
of split horzon is disabled.
This situation will continue till R2 gives up sending update for (128.1.2.0/24)
subnet as it reaches invalid timer, and the max hop for this subnet
on R1 reaches infinity- unreachable - . Then R1 will send this subnet
as unreachable subnet and will be removed from R2 routing table after the
fllush timer expires.
Split Horizon is needed to be disabled when you have """Multipoint"""
interface that you want to send out an update or a route you have learnt
it on the same ""Multipoint interface""
R2
|
|
R1------Multipoint interface-----Frame relay Cloud
|
|
|
R3
R1 is the HUB.Any updates from R2 to R1, needed to be advertised to R3 via R1.
So you have to disable split horizon on multipoint interface of R1.
Be aware that : By default the Physical interface is Multipoint interface, and
the dialer iterface as well " interface Dialer " .
Regards;
Ayman
> From nobody@groupstudy.com Mon Sep 30 18:40:24 2002
> From: "Paul Grey" <paul@greyboy.org>
> To: <ccielab@groupstudy.com>
> Subject: Split-horizon Issue
> Date: Mon, 30 Sep 2002 17:24:17 +0100
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>
> Group
>
>
>
> By default split-horizon is turned off on physical frame-relay i/fs.
>
>
>
> So if I'm running a DV routing protocol, e.g. RIP between two routers, A
> and B, on physical FR then there should be no problem.
>
>
>
> If router B is mutually redistributing between RIP and say, OSPF, still
> no problem as OSPFs AD is 110 compared to RIPs AD of 120. So any
> reflected routes will not get fed back into OSPF.
>
>
>
> However, should I out of good practice enable split-horizon on physical
> FR in this case.
>
>
>
> I know if RIP is replaced with IGRP then things change and you would
> have to enable SH. I'm also aware of SH issues in hub-spoke
> arrangements.
>
>
>
> Any thoughts?
>
>
>
>
>
> TIA
>
>
>
> Paul
>
>
>
> ________________________________________________
>
>
>
> Paul Grey
>
>
>
> paul@greyboy.org
>
>
>
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