From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Tue Feb 25 2003 - 17:50:16 GMT-3
At 1:52 PM -0500 2/25/03, OhioHondo wrote:
>Peter
>
>For troubleshooting purposes I find it handy to have an idea of where in the
>ISO stack a problem might exist. In this e-mail string someone suggested RIP
>runs over UDP. I don't believe that to be true
AFAIK, I may have been the only person to say that. Why do you think
it's untrue?
UDP Port 520. See the RFC. Do I need to get a note from Charley
Hedrick or Gary Malkin?
"RIP is a UDP-based protocol. Each host that uses RIP has a routing
process that sends and receives datagrams on UDP port number 520.
All communications directed at another host's RIP processor are sent
to port 520. All routing update messages are sent from port 520.
Unsolicited routing update messages have both the source and
destination port equal to 520. Those sent in response to a request
are sent to the port from which the request came. Specific queries
and debugging requests may be sent from ports other than 520, but
they are directed to port 520 on the target machine."
>and if it is not true then
>this can be mis-leading to someone that is trying to understand the
>technology. As far as passing the test, it may not be necessary to
>absolutely know how something works.
What do you mean "how something works?" If you mean what protocols
run over what, you'd better know that unless you want weird and
wonderful results from access lists.
But trying to coerce a concept NOT developed with OSI in mind into
the OSI reference model has nothing to do either with why something
is designed (theory) or how it works (principles).
>
>I send my perceptions out to this forum to be corrected if I am wrong. That
>helps me learn. If I am right then I am helping someone else in their
>pursuit to understand the technology -- for the test and for practical use.
>;)
>
>-----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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