From: Brian Dennis (bdennis@internetworkexpert.com)
Date: Mon Feb 21 2005 - 07:53:57 GMT-3
Since RIPv1 is incapable of carrying the subnet mask information, how
could R1 check the "mask" of the 10.50.31.0 route? Post a "show ip
route 10.50.31.0" from R1 and see if its not showing up as
10.50.31.0/28.
Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)
bdennis@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 775-745-6404 (Outside the US and Canada)
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
anantha S
Sent: Sunday, February 20, 2005 2:15 PM
To: Cisco certification
Subject: RIP v1 and v2 question
Hi all,
Question about Rip v1 router accepting a route of different mask from
the received interface which has same major network.
Topology:
----------------
10.50.13.128/27-------R1 (rip v1)-----10.50.13.0/28------R3 (rip
v2)---10.50.31.0/24
Should R1 learn 10.50.31.0/24 ?
Some reason R1 is learning the route. Can someone give me the reason ?
I thought it should check for the major network and mask in R1
against 10.50.31.0/24 versus 10.50.13.0/28 and it should reject the
update.
Below are the snippet i read ((i think its specific to rip version 1))
------------------------------------------------------------------------
----------------------
Send Updates
------------------------
Before Router 1 sends updates to Router 2, it checks the following:
b" Is the subnet information part of the same major net as the
interface that is sourcing the update?
o No. Router 1 summarizes at the major net boundary and advertises
the network.
o Yes. Does the network have the same subnet mask as the interface
that is sourcing the update?
o' No. Router 1 drops the network, and does not advertise it.
o' Yes. Router 1 advertises the subnet.
Receive Updates
---------------------------
When RIP or IGRP receive an update, they perform certain checks before
accepting the update and applying the subnet mask. Below is the
sequence of events that occurs before Router 2 accepts an update from
Router 1:
b" Is the subnet received in the update on the same major net as
the
interface that received the update?
o Yes. Router 2 applies the mask of the interface that received
the
update. If the advertised network has a host bit set in the host
portion of the update, Router 2 applies the host mask (/32). In the
case of RIP, it continues to advertise the /32 route to the subsequent
router, but IGRP does not.
o No. Do any subnets of this major net already exist in the
routing
table, known from interfaces other than the one that received the
update? Note that the network in this update should be a major net
unless the link between the two routers is an unnumbered link, in
which case it is possible for the update to contain subnet
information.
o' Yes. Router 2 ignores the update.
o' No. Router 2 applies a classful mask. If the update came across
an
unnumbered link and contains subnet information (bits in subnet
portion of network are set), then Router 2 applies a host mask. Please
refer to Understanding and Configuring the ip unnumbered Command for
unnumbered case examples.
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