Leigh Anne Chisholm writes:
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.
--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."
Brian's reply:
You have just cut from a cisco document that proves I am accurate. This says
all interfaces are by default multipoint unless specifically configured
point-to-point. This is talking about physical interface configuration not
multipoint subinterface configuration, and you have just proved I am correct
that Physical interfaces configured for frame-relay by default disable
split-horizon. Also if you want a distance-vector protocol to work correctly
on a Frame-relay hub & spoke model split-horizon must be disabled on the
hub(unless the hub is configured for point-to-point to each spoke)
>>>Brian
>From: "Leigh Anne Chisholm" <[EMAIL PROTECTED]>
>Reply-To: <[EMAIL PROTECTED]>
>To: "Brian Lodwick" <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>,
><[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
>Subject: RE: FR + Poison Reverse...
>Date: Tue, 27 Feb 2001 11:03:21 -0700
>
>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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> > > >
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> > > >
> > > > Brian Feeny,CCDP,CCNP+VAS Scarlett Parria
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