If you don't want to read all of my quotes, look for the portion contained therein
enclosed by double asterisks (**).
>From Cisco's site:
"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 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.**" This excerpt can be found at:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios11/cbook/ciproute.htm#xtocid16743169
A few weeks ago, this same discussion was brought up. Someone posted the following
link outlining "Hub And Spoke Frame Relay Sample
Configuration - Dynamic (OSPF) IP Routing". Here's an excerpt from the link:
"In general, it is good practice to use subinterfaces for partially-meshed frame relay
networks. A frame relay network designed with
subinterfaces scales much easier to future expansion. Referring to the example,
subinterfaces allow routing updates to exchange
between Boston and Chicago through Atlanta. **Without subinterfaces, Boston is unable
to receive routing updates from Chicago and
vice versa creating a condition known as split-horizon.**" This excerpt can be found
at:
http://www.cisco.com/warp/public/779/smbiz/service/configs/framerelay/fr_ip_ospf.htm
Now this is in contrast to what I know of OSPF and in contrast to the first quote I've
included. Quite frankly, I would think that
the problems encountered in the Frame Relay example have more to do with the
non-broadcast nature of Frame Relay than split-horizon
so I think whomever wrote this may have misunderstood why subinterfaces enable routing
updates--and I think perhaps this is why
there's so much confusion over whether or not split-horizon plays any role in OSPF.
Brian L:
I think you've got it backwards as to when split-horizon operates on an interface. By
default, all Cisco serial interfaces are
multipoint unless specifically configured to be point-to-point. With multipoint,
you'd want split-horizon enabled if you were using
a broadcasting routing protocol. Here's two excerpts from Cisco's site:
--Excerpt 1--
"Note: For TCP/IP, Cisco routers can disable split-horizon limitations on all frame
relay interfaces and multipoint subinterfaces
and do this by default. However, split-horizon cannot be disabled for other protocols
like IPX and AppleTalk. These other protocols
must use subinterfaces if dynamic routing is desired."
--Excerpt 2--
"Cisco serial interfaces are multipoint interfaces by default unless specified as a
point-to-point subinterface. Though less common
than point-to-point subinterfaces, it is possible to divide the interface into
separate virtual multipoint subinterfaces."
"Multipoint interfaces/subinterfaces are still subject to the split-horizon
limitations as discussed above. All nodes attached to a
multipoint subinterface belong to the same network number. Typically, multipoint
subinterfaces are used in conjunction with
point-to-point interfaces in cases where an existing multipoint frame relay cloud is
migrating to a subinterfaced point-to-point
network design. A multipoint subinterface is used to keep remote sites on a single
network number while slowly migrating remote
sites to their own point-to-point subinterface network."
"Figure 4 shows serial 0.1 as a multipoint subinterface connecting to three different
locations. All devices on the multipoint
subinterface belong to the same network number (100). Site E has migrated off of the
multipoint network to its own point-to-point
subinterface network (200). Eventually, all remote sites can be moved to their own
point-to-point subinterface networks and the
multipoint subinterface will not be necessary."
Both of these excerpts can be found at:
http://www.cisco.com/warp/public/779/smbiz/service/knowledge/wan/subifs.htm
-- Leigh Anne
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> Brian Lodwick
> Sent: February 27, 2001 10:21 AM
> To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> Subject: RE: FR + Poison Reverse...
>
>
> OSPF uses split-horizon? I don't think this is correct. I hate to be quick
> to reply but there is no reason for OSPF to use split horizon. OSPF sends
> updates to all adjacent neighbors, and in NBMA , and Broadcast network types
> the DR forwards the updates on.
> Split-horizon is used to keep routing loops from happening for a
> Distance-vector protocol not a link state protocol.
> There is an issue split-horizon presents when using a distance-vector
> protocol in a frame-relay hub spoke topology. As you can imagine (if you
> know how a distance-vector protocol diseminates updates) the hub will need
> to send the update back out of the interface it received it on so that the
> other spokes will receive the update. This will not be allowed to happen if
> split-horizon is enabled. The spokes will not be an issue.
>
> By default:
> Physical interface frame-relay setup split-horizon will be disabled.
> Subinterface frame-relay setup split-horizon will be enabled.
>
> Also keep in mind you cannot disable split-horizon on IPX RIP.
>
> Summation when you have a hub spoke frame-relay topology and you are using a
> distance-vector routing protocol you need to disable split-horizon on the
> hub. (which does leave you secceptible to the issue split-horizon was
> designed to fix)
>
> Or use a link-state protocol.
>
> Or you could setup point-to-point subinterfaces on the hub for each spoke.
>
> >>>Brian
>
>
> >From: "Maness, Drew" <[EMAIL PROTECTED]>
> >Reply-To: "Maness, Drew" <[EMAIL PROTECTED]>
> >To: "'Z'" <[EMAIL PROTECTED]>, Brian <[EMAIL PROTECTED]>,
> >[EMAIL PROTECTED]
> >Subject: RE: FR + Poison Reverse...
> >Date: Tue, 27 Feb 2001 08:36:57 -0800
> >
> >OSPF does use Split horizon. I don't think OSPF uses PR because PR sets
> >the
> >route to infinity and I'm not sure what an ' infinity' cost would mean in
> >OSPF.
> >
> >-----Original Message-----
> >From: Z [mailto:[EMAIL PROTECTED]]
> >Sent: Tuesday, February 27, 2001 12:47 AM
> >To: Brian; [EMAIL PROTECTED]
> >Subject: Re: FR + Poison Reverse...
> >
> >
> >But I thought that I'd seen or heard of using split horizon with
> >OSPF...maybe I'm thinking of Frame Relay...long day I guess...
> >
> >************************************************************
> >This has been an Eyez Only streaming e-mail broadcast...We are watching.
> >
> >NetEyez ~ CCNP, CCDA
> >
> >----- Original Message -----
> >From: "Brian" <[EMAIL PROTECTED]>
> >To: "Z" <[EMAIL PROTECTED]>
> >Cc: <[EMAIL PROTECTED]>
> >Sent: Tuesday, February 27, 2001 1:16 AM
> >Subject: Re: FR + Poison Reverse...
> >
> >
> > >
> > >
> > > split horizon and PR are both associated with distance vector protocols,
> > > OSPF is a link state protocol.
> > >
> > > Brian
> > > On Mon, 26 Feb 2001, Z wrote:
> > >
> > > > Hey Group,
> > > > I recently pondered something while at work and nobody could give
> >me
> >a
> > > > strait answer. Actually I feel a little embarrassed asking this due to
> >me
> > > > being an NP and I feel like I should know this. Guess this type of
> >thing
> > > > doesn't come up much around me. Question is: Can poison reverse (PR)
> >be
> >used
> > > > with OSPF? I know split horizon is used with it but I just cant see
> >why/how
> > > > poison reverse would. My definition of PR is that it sets the link to
> >the
> > > > max hop count and deems it unreachable, hence the term poison. I can't
> >see
> > > > how this would work with OSPF because it doesn't use a hop count.
> >Maybe
> >I'm
> > > > confused about PR. Does it set the link to the highest metric, and not
> >hop
> > > > count? Maybe I'm just used to hearing about PR in discussions of RIP
> >that
> > > > I'm assuming it set the hop count to the highest and has nothing to do
> >with
> > > > metric. Any clarity would help, thanks all...
> > > >
> > > > ...sorry for the rambling...
> > > >
> > > > ************************************************************
> > > > This has been an Eyez Only streaming e-mail broadcast...We are
> >watching.
> > > >
> > > > NetEyez ~ CCNP, CCDA
> > > >
> > > >
> > > >
> > > > _________________________________
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> > >
> > >
> > > -----------------------------------------------
> > > I'm buying / selling used CISCO gear!!
> > > email me for a quote
> > >
> > > Brian Feeny,CCDP,CCNP+VAS Scarlett Parria
> > > [EMAIL PROTECTED] [EMAIL PROTECTED]
> > > 318-222-2638 x 109 318-222-2638 x 101
> > >
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> > > Fax 318-221-6612
> > >
> > >
> >
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