From: Marius Venter (marius@aict.co.za)
Date: Thu May 11 2006 - 11:48:32 ART
Hi Sami
Sorry I am only replying now.
The internet in Europe apparently took a beating this morning and I could
not get to my mails for some time.
I am also getting ready for my CCIE attempt tomorrow morning so here goes an
abbreviated explanation on SH (Spilt Horizon)
There are many quirks around IGP's.
If I listen to what you are saying, I don't think SH is the reason for
losing marks, unless you only tested from your hub router.
If you have R1 connected to R2 and R2 connected to R3 with all the
interfaces in NBMA mode sharing 1 subnet and you either run EIGRP or RIP
over this network you will experience SH issues.
What will happen is that all updates from R1 will get to R2 and R2 will
assume R1 is connected to R3 since NBMA is supposed to be fully meshed, and
because of this R2 will not forward the routing updates from R1 to R3.
SH was put in place to prevent routing loops and only because of this do we
need to disable SH on R3 to accommodate a "BAD" design. (Partially meshed
NBMA design)
Cisco recommends using sub interfaces with unique /30 subnets.
If you do your pings from R3 you will find that all works just fine even
with SH enabled.
If you cannot ping from R1 to R3's loopback or advertised routes you know
that SH is the culprit.
Disable SH on R3 and test again.
If you are running multiple subnets over sub interfaces, SH will not be an
issue.
Do not forget to apply the same logic to IPV6 in RIPng over Frame
Hope this clears it up.
Marius Venter
From: Sami Salim [mailto:cisco_obsessed@yahoo.ca]
Sent: Thursday, May 11, 2006 9:07 AM
To: Marius Venter
Subject: split-horizon issue
Hi Marius,
It seems having every router able to ping all other routers at the pod does
not guarrantee scoring the IGP 100%. I am trying to figure out why I lost
some scores in the IGP, I found no reason other than Split-horizon at RIP
and EIGRP interfaces.
What is your opanion. Are you aware of a book or a URL explains the issue,
when to enable/disable it.
Appreciate your help.
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