From: Nawaz, Ajaz (Ajaz.Nawaz@bskyb.com)
Date: Wed Jan 24 2007 - 07:34:27 ART
<http://www.freesoft.org/CIE/RFC/1812/48.htm>
Source Route Options
A router MUST be able to act as the final destination of a source route. If
a router receives a packet containing a completed source route, the packet
has reached its final destination. In such an option, the pointer points
beyond the last field and the destination address in the IP header addresses
the router. The option as received (the recorded route) MUST be passed up to
the transport layer (or to ICMP message processing).
In the general case, a correct response to a source-routed datagram
traverses the same route. A router MUST provide a means whereby transport
protocols and applications can reverse the source route in a received
datagram. This reversed source route MUST be inserted into datagrams they
originate (see [INTRO:2] for details) when the router is unaware of policy
constraints. However, if the router is policy aware, it MAY select another
path.
Some applications in the router MAY require that the user be able to enter a
source route.
A router MUST NOT originate a datagram containing multiple source route
options. What a router should do if asked to forward a packet containing
multiple source route options is described in Section [5.2.4.1].
When a source route option is created (which would happen when the router is
originating a source routed datagram or is inserting a source route option
as a result of a special filter), it MUST be correctly formed even if it is
being created by reversing a recorded route that erroneously includes the
source host (see case (B) in the discussion below).
DISCUSSION
Suppose a source routed datagram is to be routed from source S to
destination D via routers G1, G2, Gn. Source S constructs a datagram with
G1's IP address as its destination address, and a source route option to get
the datagram the rest of the way to its destination. However, there is an
ambiguity in the specification over whether the source route option in a
datagram sent out by S should be (A) or (B):
(A): {>>G2, G3, ... Gn, D} <--- CORRECT
(B): {S, >>G2, G3, ... Gn, D} <---- WRONG
(where >> represents the pointer). If (A) is sent, the datagram received at
D will contain the option: {G1, G2, ... Gn >>}, with S and D as the IP
source and destination addresses. If (B) were sent, the datagram received at
D would again contain S and D as the same IP source and destination
addresses, but the option would be: {S, G1, ...Gn >>}; i.e., the originating
host would be the first hop in the route.
=============
Hth,
Ajaz Nawaz
CCIE#15721
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Tom
Hanks
Sent: 24 January 2007 07:00
To: ccielab@groupstudy.com
Subject: "IP Source-route" command
Hi,
Is there a document on this particular command that explains its
applications and how its used ? I looked on the Cisco website but could not
locate relevant info.
How is this command used in the real world other than extended ping
options ?
Regards
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