Posted on October 19, 2018 9:00 pm
 |  Asked by Mario Mancini
 |  597 views
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Hi, I am in the need to intoduce some delay and other stuff on one ethernet of Arista switch. On linux box i know is possible to use netnem, for istance:

tc qdisc add dev ens4f4d1 root netem delay 3s

But on Arista switch this doesn’t work. [mario@Ari ~]$ sudo tc qdisc add dev et45 root netem delay 3s RTNETLINK answers: No such file or directory

maybe the problem could be on the kenel module, it is not possible to find “sch_netem”

On Arista is possible to find only “sch_prio” [mario@Ari ~]$ sudo lsmod | grep sch sch_prio 3685 54 There is another way to introduce delay on Ethernet on Arista switch? Or enable sch_netem without issue? Because the switch is on production.

Thanks in advance, Mario

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Posted by Mark Berly
Answered on October 31, 2018 5:31 pm

Arista EOS uses hardware drivers to interact with the silicon doing the actual packet forwarding, to this end the module you are referencing will not work for anything switched at the ASIC level. I would also discourage you from using anything that would ‘delay’ packets via software kernel as this data has to be kept somewhere while it is delayed, you are using control plane resources to store / delay packets instead of to service the control plane protocols.

Example:
delay for 3 seconds assuming constant 100Mbs stream of traffic, that would equal to ~38M of memory used just to store the packets delayed

If you want to introduce latency and delay I would suggest using a device specifically designed for this purpose.

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