Subject: Re: Route Aggregation

From: "Balaji R (Networking) - CTD, Chennai." <[email-masked]>
Date: Tue, 23 Apr 2002 20:14:50 +0530

Eric,

Yes, if the router lies in the routing path of the next hop router, loop
will ensue. I was talking about a special case of this, where the sending
router happens to be the next hop of the next hop router (but it does not
really matter if it is or not :-)). Split horizon only takes care of looping
during control information exchange. I was referring to data packet bouncing
back and forth between the two. (In the case that wu pointed out, the
routing table updates were already received and aggregated. We are only
interested in what happens after the aggregation. Split horizon will only
prevent this aggregated information from being sent back to the same router
that originated it and not the data looping that occurs after aggregation.
).

Eric - I did not understand how split horizon will create a black hole? Can
you please explain?

Thanks,
Balaji.R.

-----Original Message-----
From: Eric Gray [mailto:[email-masked]]
Sent: Tuesday, April 23, 2002 8:03 PM
To: [email-masked]
Subject: Re: Route Aggregation

Balaji R (Networking) - CTD, Chennai. wrote:

>-----Original Message-----
>From: wu min [mailto:[email-masked]]
>Sent: Tuesday, April 23, 2002 6:05 AM
>To: [email-masked]
>Subject: Route Aggregation
>
>Hi,
>
>I have some questions and I hope someone out there can help.
>
>1) I understand that when a router tries to route a packet it must use the
>packet's destination address to search for the longest match  prefix entry
>in it routing table to determine how and where the packet is supposed to be
>forwarded. Supposely the router break this rule and use a prefix which is
>not the longest match regardless of the existent of a longer one, what
would
>be the consequences assuming that the next hop router for the chosen prefix
>is  different from the longest match prefix? Is this safe?
>        Not all the time. If the other router thinks that you are the next
>hop for the longer prefix, this will result in a routing loop.
>
I doubt that this is ever 'safe' - though you may get away with it.

In order to get away with it, other routers must not have a preferred route
such that the packet will subsequently arrive at the same router.  The case
where the peer router thinks the local router is the next hop will not
often
result in a routing loop - it would be a black hole instead - because
'split-
horizon' behavior will prevent it from returning the packet to the router
from which it has received it.  But there are a ton of other ways in which
the packet may end up arriving at the local router again, resulting in a
loop.

>...

--
--
Eric Gray (mailto:[email-masked])
http://www.mindspring.com/~ewgray

---

From: Eric Gray <[email-masked]>
Date: Tue, 23 Apr 2002 13:29:00 -0400

Balaji,

    Apologies for perhaps using the wrong term.

    The same concept that applies to control messages should
apply to data.  What conceivable advantage is there to sending
a packet back to the same box it was received from, using the
same (potentially virtual) interface as it was received on?  If
nothing has changed except for swapping the MAC addresses
around, what reason does a box have for believing the other
box will do otherwise than it did most recently?

    In some implementations, it is trivially easy to prevent
this from happening.  In such an implementation, packets
would be dropped rather than forwarded in a loop.  Thus
an upstream router (WRT this route) would be sending
packets to a peer that will simply drop them.  This is one
of the definitions for a black hole.

    As I understand it, routers treat packets received from
 their next hop with respect to the packet's destination as
if the destination is unreachable.