Leigh Anne Chisholm,
Actually by default on IP interfaces:
Split-horizon is disabled on Frame-relay physical interfaces
Split-horizon is enabled on Frame-relay point-to-point and multipoint
subinterfaces
(but who cares it's configurable)
There is a difference between a multipoint physical interface and a
multipoint subinterface. Althought there aren't too many differences (static
mapping and dynamic mapping allowed on both) there are subtle differences.
My brother and I went over this yesterday and wondered what the differences
are. What we came down to was that the big difference is simple
-subinterfaces function is to provide a means to logically separate a
physical interface. The subtle differences like the default split-horizon
setting and default OSPF network type aren't the big differences. You can
build a fully scaleable hub spoke frame model using a physical multipoint
interface on the hub and point-to-point subinterfaces on the spokes. The
advantage of using the Multipoint subinterface on the hub? You would only
need 1 physical interface for 2 hub spoke frame models. I am interested in
learning more about other things you can do with the hub as a multipoint
subinterface to control traffic, such as for filtering or traffic shaping.
>>>Brian
>From: "Leigh Anne Chisholm" <[EMAIL PROTECTED]>
>Reply-To: <[EMAIL PROTECTED]>
>To: "Brian Lodwick" <[EMAIL PROTECTED]>, "Cisco@Groupstudy. Com"
><[EMAIL PROTECTED]>
>Subject: RE: FR + Poison Reverse...
>Date: Tue, 27 Feb 2001 12:41:07 -0700
>
>Whoops! When I posted Excerpt 1, I missed the part in the first sentence
>that
>seems to imply that frame-relay interfaces and multipoint interfaces
>disable
>split-horizon BY DEFAULT (when using TCP/IP).
>
>When I took ICRC so many years ago, I remember discussing split-horizon but
>don't recall any explicit mention that split-horizon is disabled by default
>for the IP routing protocols (RIP and IGRP). I remember discussing this
>issue
>as well in CIT but I don't think the exception to the rule was noted
>either--but because I wasn't aware of it, it may have been something that
>went
>in one ear and out the other.
>
>It's interesting to note that in the current ICND curriculum, I know the
>section that talks about configuring subinterfaces indicates that
>"multipoint
>subinterfaces act as NBMA network so they do not resolve the split horizon
>issue". I don't have the entire official ICND curriculum available, but I
>don't notice anything specifically mentioning the exception.
>
>So, in summary - we're both right with respect to multipoint interfaces:
>
>With multipoint (or physical) interfaces: split horizon is ENABLED by
>default
>unless you are routing IP across the link. Then it is DISABLED. If you're
>routing using RIP, IGRP, or EIGRP to route IP, it's DISABLED. If you're
>routing IPX using RIP, EIGRP, NLSP, it's ENABLED. It's also enabled if
>routing AppleTalk.
>
>With point-to-point subinterfaces, split horizon is ENABLED but isn't much
>of
>an issue since it's a point-to-point link.
>
>
> -- Leigh Anne
>
> > -----Original Message-----
> > From: Brian Lodwick [mailto:[EMAIL PROTECTED]]
> > Sent: February 27, 2001 11:26 AM
> > To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> > Subject: RE: FR + Poison Reverse...
> >
> >
> > 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_o
>spf.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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