Nexus 5548 & SPAN 10Gb

[s.fasel]

Hi,

we have 4 Nexus 5548 in our datacenter and we want use "span function" on a 10Gbits source interface to a 10Gbits destination interface, but in your documentation "Cisco Nexus 5000 Series NX-OS System Management Configuration Guide, Release 5.1(3)N1(1)", we can read:

On the Nexus 5500 series, SPAN traffic is rate-limited to 1Gbps by default so the switchport monitor rate-limit 1G interface command is not supported. Also, to avoid impacting monitored production traffic:

• SPAN is rate-limited to 5 Gbps for every 8 ports (one ASIC).
• RX-SPAN is rate-limited to 0.71 Gbps per port when the RX-traffic on the port exceeds 5 Gbps.

My question:  SPAN traffic is really limited at 1Gbps on these devices ?  If yes, it's therefore not possible to capture correctly (via a SPAN interface) a traffic from a server connected on a 10Gb interface (with a traffic up to 5 Gbps) because one part of this traffic will be dropped, it's correct ?

thanks a lot for your help

Best Regards

Sam

[vdsudame]

your understanding is correct.

Vin.

[s.fasel]

Hi Vin,

thank for your response.

but then, what is the usefulness of this function, while on the Nexus 5500 almost all servers are connected in 10Gb and that their traffic is often greater than 1Gbps (mainly storage data)?

it's not possible to do troubleshooting these servers with the traffic capture via SPAN..... and the tshark embedded on the Nexus 5500 cannot also do that...

Best Regards

Sam

[vdsudame · Accepted solution]

span in general is oversubscribed feature. span traffic gets rate limited only during period of congested span. Congestion of span could occur under following circumstances:

- when we have total of more than 10gig traffic being monitored over multiple 10gig ports to one 10gig port (span destination oversubscription)

- when we are monitoring more than 5gig per port with multiple such ports to single 10gig port (fabric link oversubscription)

if this is just single 10gig to single 10gig port span, then we should be able to span upto 5gig

without a problem, after which rate limiting would kick in for the reasons you mentioned before.

Vin

[s.fasel]

Hello Vin,

thanks a lot for your response... Now I understood ..

Best Regards

Sam