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Design considerations guide for SPAN?

With Cisco having so many product lines and they all use different hardware and software, is there one place that informs the viewer of span considerations by switching platform.  Here let me explain some more.  If I choose a 3750x I know the forwarding plane and SPAN is handled by the dopler ASIC (now UA Asic).  But if I go to a 3750g that SPAN is handled in software and uses the shared resources and does not have dedicated hardware for handling SPAN.  It gets even more complex when you go to a chassis based switch because then you need to know the supervisor SPAN capability and the line card SPAN capability.


The reason for this request is there is a myth that SPAN tears down networks.  Being a seasoned pro in networking I remember the catalyst 5k's running catos and yes they would bring down the network, but then again a simple spanning-tree issue brought down the network as well (and we still use spanning-tree).  So I am looking for a definitive resource, a single stop for all things SPAN for ALL of Cisco's products.


I doubt anything like this exists, but it is worth a shot...if you have any good resources on this subject can you please respond with a link.


Joseph W. Doherty
Hall of Fame Expert

You're asking about L2 spanning tree, not Cisco's SPAN (Switched Port Analyzer) port feature, correct?

Nope.  I am asking about SPAN, commonly referred to as mirroring or monitoring.  SPAN is used quite often for security solutions and I cannot find a single page that goes over how SPAN (switchport analyzer) is handled by the switching platforms.  Again as I mentioned in some they are handled in hardware, some in software, in chassis based solutions it might be a combination depending on line card and supervisor.


So really I mean SPAN, not spanning-tree.

Ok, gotcha.  Your mention of ". . . (and we still use spanning-tree).", on my first reading, threw me off a bit.

Unfortunately, often you don't see a lot of documentation on how Cisco supports different features on their platforms, as, no doubt, it's likely often considered proprietary information.

Occasionally the best source I've seen for such "public" Cisco information is what Cisco often provide during Cisco's Live presentations.

I suspect the differences in SPAN, RSPAN and ERSPAN might come into play too.



Do you have a reference for the documentation you mention where SPAN is done in software?

I have never heard of that on Catalyst switches(3750 models)

As far as I know all newer switches does SPAN in hardware, hence the limitation on the number of span sessions, and only devices supporting GRE in hardware supports ERSPAN.