EIGRP does split horizon and also poison reverse! Poison reverse isn't just
a theoretical thing that they make you learn for CCNA. ;-)
I have a Sniffer trace of an EIGRP router booting. It's on Ethernet.
There's just one other router on the Ethernet. (The behavior may have been
different if there had been multiple routers. I'll try that at some point.)
Both routers have multiple networks behind them.
\ /
- Old Router------Ethernet------New Router -
/ \
The new router boots and sends a multicast Hello.
The old router sends an update directly to the new router (unicast) listing
the networks behind it. Multiple networks are listed in the packet.
The new router ACKs.
Here's the answer to your question (sort of): The new router then sends an
update listing all the networks it just learned from the old router as
unreachable (delay = FFFFFFFF). It sends this twice, as both a multicast
and a unicast! (I see this as a strange implementation of poison reverse.)
The old router ACKs.
The new router sends an update directly to the old router (unicast)
specifying the networks that are behind it.
The old router ACKs.
The old router does the poison reverse thing.
Then they settle down and just send Hellos for a while.
Then I pulled one of the networks behind the old router. This caused the
old router to multicast a query asking if anyone can get to the dead
network. It's a query type of packet. It lists the dead network as
unreachable.
The new router ACKs.
The new router sends a response that the dead network is unreachable.
The old router ACKs.
I put the cable back. The old router sends a multicast update saying it can
get to that network again. It also sends the exact same information as a
unicast to the new router.
The new router ACKs.
You can guess what it does next!? The new router sends a multicast and
unicast saying that the network is unreachable (poison reverse thing.)
I hope you could follow all that and that it answered your questions
somewhat. Bottom line: EIGRP uses a mix of unicast and multicast and it
does split horizon and poison reverse. I'll try a network with more than
just two routers on Ethernet when I get a chance.
Priscilla
(At 03:43 PM 8/9/01, Leigh Anne Chisholm wrote:
>Don't forget that split-horizon plays a role here. I'm not sure if updates
>are limited to one route per packet... (I believe that they are judging from
>some debugs I've run but it's hard to tell exactly what you're seeing--are
>multiple lines from one packet?) but if a router receives information about
>a network it advertised, it will ignore the update. So it wouldn't really
>care if it received the information back. It got its ACK from the original
>information...
>
>
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
>Ole Drews Jensen
>Sent: Thursday, August 09, 2001 1:18 PM
>To: [EMAIL PROTECTED]
>Subject: RE: EIGRP: Adding a new router [7:15504]
>
>
>Thanks again Leigh,
>
>So I guess the neighbors WILL receive their own advertised routes back from
>Elvis, since it's multicasting its entire Route Table...
>
>Have a great day,
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> http://www.RouterChief.com
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> NEED A JOB ???
> http://www.oledrews.com/job
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>-----Original Message-----
>From: Leigh Anne Chisholm [mailto:[EMAIL PROTECTED]]
>Sent: Thursday, August 09, 2001 1:58 PM
>To: Ole Drews Jensen; [EMAIL PROTECTED]
>Subject: RE: EIGRP: Adding a new router [7:15504]
>
>
>EIGRP Update packets are used to communicate information about routes.
>Updates are transmitted only when there is a change in the topology.
>Updates contain only the changed information and are sent only to routers
>that require the information.
>
>Updates are multicast unless only one router requires the update
>information--then the update is unicast. Updates are also unicast when a
>neighbor is on a point-to-point link.
>
>In the case of Elvis coming up, all routers require its routing information
>so Elvis will multicast (and unicast to any point-to-point neighboring
>routers).
>
>
> -- Leigh Anne
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
>Ole Drews Jensen
>Sent: Thursday, August 09, 2001 12:19 PM
>To: [EMAIL PROTECTED]
>Subject: EIGRP: Adding a new router [7:15504]
>
>
>When adding a new router - Elvis - it is my understanding that the following
>happens:
>
>1)
>
>Elvis sends multicast HELLO packets out all interfaces.
>
>2)
>
>Neighbors that receives the HELLO packets replies with their entire Route
>Table, except routes learned from Elvis (however, they have not learned
>anything from Elvis yet).
>
>3)
>
>Elvis ACKnowledges the replies.
>
>4)
>
>Elvis adds all the replying neighbors to its Neighbor Database, and all the
>routing information to its Topology Database.
>
>5)
>
>After having calculated the best routes and put them in its own Route Table,
>Elvis will send the full Route Table out to all the neighbors.
>
>However, will it (A) send unicasts to each neighbor with all routes except
>the ones it have learned from that neighbor, or will it (B) send a multicast
>to 224.0.0.10 with all routes?
>
>Thanks,
>
>Ole
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> http://www.RouterChief.com
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> NEED A JOB ???
> http://www.oledrews.com/job
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=15557&t=15504
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