To elaborate on Kim's answer:
OSPF Area Border Routers act much like routers running a distance vector
protocol when it comes to type-3 LSAs from other OSPF areas.
1. An ABR will never re-advertise a learned type-3 LSA back into area 0
(split-horizon)
2. ABRs ignore LSAs created by other ABRs when learned through a
non-backbone area.
- Prevents an ABR from choosing a non-backbone path to area 0
These rules can be overcome by a virtual link between ABRs in the event of a
backbone area partition.
On Mon, Aug 17, 2009 at 2:06 PM, Kim Pedersen <[email protected]> wrote:
> Hi,
>
> The only "split-horizon"- like thing i can come up with consists of
> ABR's not propagating any information learned from a LSA type 3 from
> another ABR through a non-backbone area, back into the backbone (area 0)
> again. In this manner i guess you could say "dont advertise back into
> area 0 something the area learned from area 0 in the first place".
>
> Dont know if thats what you are referring to.
>
> Sincerely,
> Kim Pedersen
>
> desmond Black wrote:
> >
> > Explanation given is to mean that there is no Split-Horizon Rule in OSPF.
> >
> > I dont want to confuse, but there is a Split-Horizon equivalent logic
> > of Distance-Vector R'tng Protocol in OSPF but cannot be termed
> > explicitly as Split-horizon.
> > In this logic if the Next-Hop of the Prefix is in the Area to which
> > the ABR is going to advertise the route [in form of LSA] OR the Prefix
> > itself is in that LSA...... do not advertise it into that Area.
> >
> > Desmond Black,
> > In Pursuit of CCIE!!
> > India
> >
> > ----- Original Message -----
> > From: [email protected]
> > To: "desmond Black"
> > Cc: ��ս�� , [email protected]
> > Subject: Re: [OSL | CCIE_RS] How to disable OSPF splite horizon ?
> > Date: Mon, 17 Aug 2009 16:25:14 +0000 (GMT)
> >
> > can you give a bit more detail on the ospf split horizon pls...
> >
> > ----- Original Message -----
> > From: desmond Black
> > Date: Monday, August 17, 2009 10:28 am
> > Subject: Re: [OSL | CCIE_RS] How to disable OSPF splite horizon ?
> > To: 邵战新 , [email protected]
> >
> > > Split-Horizon is needed in Distance vector Protocols since they
> > > do not
> > > have complete view of the topology and therefore to avoid loops
> > > tehy do
> > > not send back the routes originated by advertising peer back to
> > > it. RIP
> > > actually uses Split-horizon with Poison-Reverse just for making
> > > it sure.
> > > Since Protocols like OSPF & ISIS do have complete view of the
> > network
> > > they can make the decision themselves when the routes advertised
> > > by them
> > > will have had covered most "COST" and therefore would never get
> into
> > > Routing table.
> > >
> > > ----- Original Message -----
> > > From: 邵战新
> > > To: [email protected]
> > > Subject: [OSL | CCIE_RS] How to disable OSPF splite horizon ?
> > > Date: Mon, 17 Aug 2009 19:12:07 +0800
> > >
> > > I got this open end question in ccie lab exam. Although I
> > > search it ,
> > > I can't get answer. Who can know it ? Thanks
> > >
> > > ---------------------------------------------------------------
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> > >
> > > Desmond Black,
> > > In Pursuit of CCIE!!
> > > India
> > >
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>
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