[c-nsp] Network Address Response

Matlock, Kenneth L MatlockK at exempla.org
Fri Jun 26 10:42:19 EDT 2009


I don't have a test box handy I can try it on, but does it still exhibit
that behavior if you put a 'no ip directed-broadcast' in the interface
config of the 10.0.0.5 interface?

By default it's on, so it takes anything for the network or broadcast
Layer 3 addresses and spits them out as Layer 2 broadcasts, which the
router may be seeing/processing.

Ken Matlock
Network Analyst
Exempla Healthcare
(303) 467-4671
matlockk at exempla.org



-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Ray Burkholder
Sent: Friday, June 26, 2009 8:13 AM
To: 'Geoffrey Pendery'; 'Ms Geekgirl'
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] Network Address Response

> 
> No, I think he's asking why the router with address 10.0.0.5 responds
> to pings that have a destination IP of 10.0.0.4.  The echo request is
> targeted at a network address, not at the router.

Yes, that is the basis for my question.  I suppose to clarify further, a
/30
has four addresses:

* network portion
* link ip 1
* link ip 2
* broadcast

Pings to the device originating on the ingress side but destined to the
network portion of the egress side 
are responded to back out the ingress side with the link ip of the
ingress
side.

It appears that the router listens to the 'network address' portion of a
subnet.  Is this
good, bad, or ugly?  Should-it/can-it be turned off?

> 
> I've also observed this behavior (more than ICMP though - I have a
> router responding to SNMP and being discovered by our configuration
> management team, on the network address of one of its interfaces) and
> would like to know more about why...
> 

Ah, good, someone else has the same experience.


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