From: Peter van Oene (pvo@usermail.com)
Date: Tue Feb 25 2003 - 13:30:45 GMT-3
At 09:59 AM 2/25/2003 -0500, OhioHondo wrote:
>Howard
>
>I disagree with your "BGP is not an application" statement. The fact that it
>uses TCP means it uses Layer 4. The fact that it uses TCP ports means that
>it uses Layer 5 and it creates TCP sessions (Layer 6) with a communicating
>partner.
These discussions about which OSI layer an IP protocol fits into are really
quite fruitless. Conformance with OSI terminology was not a design goal
for BGP as far as I know.
>It is an entity communicating over a session and through the network. If
>that's not a definition of a Layer 7 (application) entity I don't know what
>is ;)
>
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Howard C. Berkowitz
>Sent: Monday, February 24, 2003 9:54 PM
>To: ccielab@groupstudy.com
>Subject: RE: split-horizon & BGP
>
>
>At 5:20 PM -0500 2/24/03, Jerry Haverkos wrote:
> >Howard
> >
> >I do not believe that the is a command to turn split-horizon on or off
> >available for BGP, especially not one that works at layer 3.
>
>First, let me be clear about some terminology and OSI references,
>which you may know.
>
>BGP is not an application. It is a connection-oriented network layer
>control program. That it happens to use reliable layer 4 transport
>is irrelevant to its payload function, which is totally network layer
>oriented. Connection-oriented routing protocol, connection-oriented
>transport mechanism.
>
>Split horizon is not a general problem of DV protocols, but of
>connectionless transports for the routing information. Split horizon
>also applies to routes, not link state information.
>
>It can be perfectly normal behavior to receive a self-originated LSA
>or LSP at a LS interface. Split horizon isn't needed because there
>are tiebreakers such as age.
>
>RIP and IGRP are multicast/broadcast and can cause loops if split
>horizon is not enforced. Since EIGRP first forms neighbor
>relationships and uses reliable transport, the split horizon issue is
>not nearly as significant. In any case, EIGRP has superior loop
>prevention mechanisms.
>
>Think of what the AS_PATH would look like if BGP returned an update
>to the AS from which it received it. There would be a loop in it,
>and it would be discarded. It definitely would be discarded at the
>receiver, and it's an implementer choice to check for loops before
>sending.
>
>
> >My point is
> >that BGP does not run at the layer 2 or layer 3 or even layer 4 part of the
> >stack.
>
>BGP _payloads_ do run in the management plane at layer 3, as do all
>other routing protocols. RIP, for example, runs over UDP, but again
>contains only layer 3 management information.
>
> >It is an application that exchanges data via an established BGP TCP
> >session. It is an application to application (BGP peer to peer) decision
>not
> >to send routes back to a peer that it received the routes from.)
> >
> >I do not believe that it has anything to do with the traditional idea that
> >split horizon does not allow updates, received over an interface, to be
>sent
> >back over that interface. ;)
> >
> >-----Original Message-----
> >From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> >Howard C. Berkowitz
> >Sent: Monday, February 24, 2003 3:13 PM
> >To: ccielab@groupstudy.com
> >Subject: RE: split-horizon & BGP
> >
> >
> >At 12:30 PM -0500 2/24/03, OhioHondo wrote:
> >>Since BGP runs as a higher layer protocol (on top of TCP) split horizon
> >does
> >>not apply.
> >
> >Why do you think TCP would make a difference in loop detection?
> >
> >BGP is not strictly a DV protocol. Its primary loop detection method
> >is examining incoming AS paths (i.e., path vectors) and rejecting
> >those that contain the local AS number.
> >
> >There are additional methods, for iBGP using confederations and RR's,
> >to reduce/eliminate transient internal loops/oscillation, but these
> >are probably outside the CCIE scope.
> >
> >It isn't completely clean, as BGP/PV is provably loop-free only when
> >additional policies are NOT used.
> >
> >>
> >>-----Original Message-----
> >>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
> >>Pedro Eira
> >>Sent: Monday, February 24, 2003 10:36 AM
> >>To: ccielab@groupstudy.com
> >>Subject: split-horizon & BGP
> >>
> >>
> >>Hello, Would split-horizon have any effect on BGP?Should I follow the
> >>same rules for BGP as I do for other DV routing protocols when
> >>split-horizon is involved?
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