[Snort-users] SPAN groups and network taps

Jefferson, Shawn Shawn.Jefferson at ...14448...
Thu May 21 13:12:36 EDT 2009

Typically how are people setting up their snort machines with network taps?  One tap per link you want to monitor into one NIC on the snort machine?

Any recommendations on hardware and techniques to accomplish tapping 4 1GB links into one Snort sensor, minimizing the amount of NICs required on the snort sensor?

From: David Thomason [mailto:david at ...14585...]
Sent: May 21, 2009 9:39 AM
To: Jefferson, Shawn
Cc: David Thomason; snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] SPAN groups and network taps

That is two per switch.  You can have a total of two full duplex, (RX/TX) SPAN ports per switch or four half duplex, (2 x RX only + 2 x TX only) SPAN ports.

David Thomason
Thomason Technologies, LLC

On May 21, 2009, at 10:51 AM, Jefferson, Shawn wrote:


So when you say two full duplex span ports, does that mean two full duplex SOURCE ports, and is that two per switch, or per span group?

I have 4 ports that I want to mirror to one port that snort watches-currently that only one of those ports is setup as full duplex, the others are received traffic only.  The total aggregated bandwidth is less than 1GB for the four ports.

Depending on whether the 6500 can actually support more than 2 full duplex span ports per switch will change what I'll need in the way of network taps/port aggregator devices, I think.

From: David Thomason [mailto:david at ...14585...]
Sent: May 20, 2009 5:51 PM
To: Jefferson, Shawn
Cc: David Thomason
Subject: Re: [Snort-users] SPAN groups and network taps


I'm not an expert when it comes to Cisco, but I do know that the 6500 can support two full duplex span ports.  As far as limitations, that really depends on how much traffic you are sending to SPAN port.  It is possible to overflow a 1G Span port with more than 1G of data.  In this case the switch starts dropping packets to the Span port.  The span port gets the lowest priority of service, but overflowing the SPAN port can impact performance on the entire switch.

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