From: Peter van Oene (pvo@usermail.com)
Date: Tue Feb 25 2003 - 16:24:57 GMT-3
At 01:59 PM 2/25/2003 -0500, Jerry Haverkos wrote:
>I stand corrected on the RIP using UDP. (I learned something.) Apparently it
>uses port 520 in some instances. If anyone can explain when/how this is used
>I would appreciate it.
Rip uses UDP for transport in all cases. Port 520 is the designated UDP
port for RIP communications. Source ports are allowed to vary, but 520
should be the destination in all cases. I would highly suggest reading
2453 for more info. Here is a related excerpt.
"RIP is a UDP-based protocol. Each router that uses RIP has a routing
process that sends and receives datagrams on UDP port number 520, the
RIP-1/RIP-2 port. All communications intended for another routers's RIP
process are sent to the RIP port. All routing update messages are sent from
the RIP port. Unsolicited routing update messages have both the source and
destination port equal to the RIP port. Update messages sent in response to
a request are sent to the port from which the request came. Specific
queries may be sent from ports other than the RIP port, but they must be
directed to the RIP port on the target machine. "
Pete
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
>Peter van Oene
>Sent: Tuesday, February 25, 2003 11:31 AM
>To: ccielab@groupstudy.com
>Subject: RE: split-horizon & BGP
>
>
>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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