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slow-path/fast-path traffic


Userlevel 6
Hello!

Am I right? Or if not - please correct me.

Slow-path forwarded traffic - which processed with ASICs
Fast-path forwarded traffic - which rocesses with CPU

Thank you!

7 replies

Userlevel 6
Hello Alexandr. Close reverse your thinking. Fast path is ASIC slow path is CPU. The only data that should be going slow path are packets to the switch, telnet, snmp, route protocols etc. If in band traffic is going through the CPU it usually is linked to a mistake configuration redirects as an example or the tables on the switch are full and thus the switch treats it as an unknown packet. Hope that helps P
Userlevel 6
Thanks a lot!
Userlevel 6
How can i determine if traffic is forwarded via slow-path?
And if slow path traffic occurs, how can i determine which systems (source ip - destination ip - maybe port) cause this traffic ?
Userlevel 7
"sh ipstats" is the command to look at potential slow-path traffic. You need to look at the "Forward" counter. Issue the command several times to see if the counter increases.

show ipstats | inc Forward
Userlevel 6
Hi Stephane,

is there no command which exactly tell me if slow-path traffic occur ?

What can i do if i see "show ipstats" forward counter increases - how can i see which traffic occurs this ?

Regards
Userlevel 6
M.Nees wrote:

Hi Stephane,

is there no command which exactly tell me if slow-path traffic occur ?

What can i do if i see "show ipstats" forward counter increases - how can i see which traffic occurs this ?

Regards

Hello, MAtthias!

You can take a capture of CPU's slow-path traffic and analize it with Wireshark.
Some like - #debug packet capture on interface Broadcom....

Thank you!
Userlevel 7
I'm not sure of how to capture that info. You certainly could tcpdump that traffic, being careful with the tool as the CPU is already forwarding traffic and so must be stressed. Maybe some debug commands exist, but I don't know them off the top of my head. Maybe people from our GTAC can help here.

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