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List:       ethereal-users
Subject:    RE: [Ethereal-users] Ping packet sizes
From:       "Stewart, Damien" <damien.r.stewart () adi-limited ! com>
Date:       2005-10-31 23:48:14
Message-ID: 9BC810ABB541D511B1C200508B6095F50D4FAD98 () charon ! gi ! adi-limited ! com
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Hi Joe,

Yes, both directions are implied in the command by default with the latest
version of IOS. I am monitoring the SPANed port using Ethereal and ping from
another workstation to yet another workstation. My Ethereal box is
effectively in the middle.

Damien Stewart

> -----Original Message-----
> From: Joe Elliott [mailto:joe@inetd.com] 
> Sent: Monday, 24 October 2005 10:50 AM
> To: Ethereal user support
> Subject: RE: [Ethereal-users] Ping packet sizes
> 
> Hello Damien,
> 	When I see errors like this I first ensure that the 
> SPAN configuration is correct, ie you have the keywork 'both' 
> in the IOS command. Its important that when you mirror a port 
> to get inbound and outbound streams forwarded.
> 
> Of course when you mirror/SPAN VLANs you run into  
> double/triple counting or worse.
> 
> Ethereal only tells you what it sees.
> 
> Run:
> 
> # tcpdump host <serverIP> ip proto \icmp
> 
> on the monitoring host and then ping the server your 
> monitoring from another PC with a count of 1. You should  
> only see the echo request/reply once (2 packets). If you see 
> anything else, you have a SPAN configuration issue.
> 
> I use this test at every customer site to ensure my setup and 
> it always finds the problem.
> 
> Hope this helps .. Joe.
> 
> -- 
> __o       _~o       __o
> "Know your Network"           `\<,      `\<,      `\<,
> 
> ______________________________________(*)/_(*)__(*)/_(*)__(*)/
> _(*)________
> Im a 21st Century Digital Boy ... I aint got a life, but I 
> got lotsa toys.
> *************** Joe Elliott  joe@inetd.com  AOL:xqos  
> ********************
> -   NetContExt  - sniffer trace forensics - tcp follow 
> stream analysis   - 
> -  Extract data files and Images from tcpdump & ethereal 
> packet payloads -
> Inetd.Com Network analysis solutions http://www.inetd.com
> 
> --------------------------------------------------------------
> ------------
> 
> 
> On Mon, 24 Oct 2005, Stewart, Damien wrote:
> 
> > Date: Mon, 24 Oct 2005 10:30:43 +1000
> > From: "Stewart, Damien" <damien.r.stewart@adi-limited.com>
> > Reply-To: Ethereal user support <ethereal-users@ethereal.com>
> > To: 'Ethereal user support' <ethereal-users@ethereal.com>
> > Subject: RE: [Ethereal-users] Ping packet sizes
> > 
> > Hi there,
> > 
> > Yes I am aware that Ethereal can't see all of the packet when it's 
> > running on a machine the packet is generated from. However, in this 
> > particular case, when I noticed the discrepancy between 
> ping request 
> > and ping replies, Ethereal was monitoring a SPAN session on a Cisco 
> > router. To my understanding, the router copies data from 
> one specified 
> > port to another. In short, this is like plugging an 
> Ethereal box into 
> > an unswitched hub, correct?
> > 
> > So I still can't account for the missing bytes on the request. Are 
> > there any known issues with SPAN sessions altering packets, that is 
> > knocking off the odd byte here and there?
> > 
> > It's a minor issue, but it would be nice to know exactly in what 
> > situations Ethereal will correctly report packet sizes
> > 
> > Regards,
> > 
> > Damien.
> > 


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