[j-nsp] Cut through and buffer questions

[james list]

Can you please share the output of:

show class-of-service shared-buffer

on your QFX5100 ?

Cheers
James

Il giorno ven 19 nov 2021 alle ore 11:58 Thomas Bellman <bellman at nsc.liu.se>
ha scritto:

> On 2021-11-19 09:49, james list via juniper-nsp wrote:
>
> > I try to rephrase the question you do not understand: if I enable cut
> > through or change buffer is it traffic affecting ?
>
> On the QFX 5xxx series and (at least) EX 46xx series, the forwarding
> ASIC needs to reset in order to change between store-and-forward and
> cut-through, and traffic will be lost until the reprogramming has been
> completed.  Likewise, changing buffer config will need to reset the
> ASIC.  When I have tested it, this has taken at most one second, though,
> so for many people it will be a non-event.
>
> One thing to remember when using cut-through forwarding, is that packets
> that have suffered bit errors or truncation, so the CRC checksum is
> incorrect, will still be forwarded, and not be discarded by the switch.
> This is usually not a problem in itself, but if you are not aware of it,
> it is easy to get confused when troubleshooting bit errors (you see
> ingress errors on one switch, and think it is the link to the switch
> that has problems, but in reality it might just be that the switch on
> the other end that is forwarding broken packets *it* received).
>
>
> > Regarding the drops here the outputs (15h after clear statistics):
> [...abbreviated...]
> > Queue: 0, Forwarding classes: best-effort
> >   Transmitted:
> >     Packets              :        6929684309        190446 pps
> >     Bytes                :     4259968408584     761960360 bps
> >     Total-dropped packets:              1592             0 pps
> >     Total-dropped bytes  :           2244862             0 bps
> [...]> Queue: 7, Forwarding classes: network-control
> >   Transmitted:
> >     Packets              :             59234             0 pps
> >     Bytes                :           4532824           504 bps
> >     Total-dropped packets:                 0             0 pps
> >     Total-dropped bytes  :                 0             0 bps
> > Queue: 8, Forwarding classes: mcast
> >   Transmitted:
> >     Packets              :           6553704            88 pps
> >     Bytes                :        5102847425        663112 bps
> >     Total-dropped packets:               279             0 pps
> >     Total-dropped bytes  :            423522             0 bps
>
> These drop figures don't immediately strike me as excessive.  We
> certainly have much higher drop percentages, and don't see much
> practical performance problems.  But it will very much depend on
> your application.  The one thing I note is that you have much
> more multicast than we do, and you see drops in that forwarding
> class.
>
> I didn't quite understand if you see actual application or
> performance problems.
>
>
> > show class-of-service shared-buffer
> > Ingress:
> >   Total Buffer     :  12480.00 KB
> >   Dedicated Buffer :  2912.81 KB
> >   Shared Buffer    :  9567.19 KB
> >     Lossless          :  861.05 KB
> >     Lossless Headroom :  4305.23 KB
> >     Lossy             :  4400.91 KB
>
> This looks like a QFX5100 or EX4600, with the 12 Mbyte buffer in the
> Broadcom Trident 2 chip.  You probably want to read this page, to
> understand how to configure buffer allocation for your needs:
>
>
> https://www.juniper.net/documentation/us/en/software/junos/traffic-mgmt-qfx/topics/concept/cos-qfx-series-buffer-configuration-understanding.html
>
> In my network, we only have best-effort traffic, and very little
> multi- or broadcast traffic (basically just ARP/Neighbour discovery,
> DHCP, and OSPF), so we use these settings on our QFX5100 and EX4600
> switches:
>
>     forwarding-options {
>         cut-through;
>     }
>     class-of-service {
>         /* Max buffers to best-effort traffic, minimum for lossless
> ethernet */
>         shared-buffer {
>             ingress {
>                 percent 100;
>                 buffer-partition lossless { percent 5; }
>                 buffer-partition lossless-headroom { percent 0; }
>                 buffer-partition lossy { percent 95; }
>             }
>             egress {
>                 percent 100;
>                 buffer-partition lossless { percent 5; }
>                 buffer-partition lossy { percent 75; }
>                 buffer-partition multicast { percent 20; }
>             }
>         }
>     }
>
> (On our QFX5120 switches, I have moved even more buffer space to
> the "lossy" classes.)  But you need to tune to *your* needs; the
> above is for our needs.
>
>
>         /Bellman
>
>