Re: Cisco ASR9902 SNMP polling ... is interesting

[Tom Beecher via NANOG <nanog () lists nanog org>]

> The control plane receives 100% of the packets, providing the control
> plane policies allow it to. The control plane is likely connected to the
> ASIC via a mix of a PCI-E interface (providing the programming interface
> and an emulated NIC) and/or a specialized NIC port.


Different vendors and platforms do this differently, but generally the
forwarding complex element has two egress paths ; one goes to the device
fabric used for through traffic, the other goes to the control plane.
Packets that egress to the fabric are usually chopped up into cells that
are reassembled at the forwarding complex connected to the
outbound interface.

The control plane connection is usually just a standard ethernet to an
internal control plane switch. The forwarding complex wraps up the packet
and transmits it that direction, and it's passed over to the RP/RE that
way. There is internal policing here to prevent elements from running
the CP over. Some of those are user configurable, others are not. ( Vendor
dependent.)

When you say 'the control plane receives 100% of the packets', it sort of
depends on what you define as the 'control plane'. That's usually defined
as 'did the packet get to the RE/RP to process it' There are many scenarios
by which this can break :

   - Interface buffers may be full
   - Interface buffers may be drained fast enough
   - Oversubscription of forwarding complex
   - Poorly designed QoS
   - Incorrect config/bugs of control plane policer on internal interface
   to CP switch
   - Central CPU (RE, RP, etc) overwhelmed
   - Internal CP switch manfunctioning




On Sun, Aug 3, 2025 at 12:35 AM Ryan Hamel <ryan () rkhtech org> wrote:

> Mel,
>
> The control plane receives 100% of the packets, providing the control
> plane policies allow it to. The control plane is likely connected to the
> ASIC via a mix of a PCI-E interface (providing the programming interface
> and an emulated NIC) and/or a specialized NIC port. If the CPU port is
> experiencing packet loss (my stance is very unlikely), that can be a
> separate discussion. I agree that an escalation is the appropriate response
> here, where TAC should try to reproduce the issue with Drew's config.
>
> Kind regards,
>
> Ryan Hamel
>
> ------------------------------
> *From:* Mel Beckman via NANOG <nanog () lists nanog org>
> *Sent:* Saturday, August 2, 2025 4:23 PM
> *To:* Tom Beecher <beecher () beecher cc>
> *Cc:* nanog () lists nanog org <nanog () lists nanog org>; Mel Beckman <
> mel () beckman org>
> *Subject:* Re: Cisco ASR9902 SNMP polling ... is interesting
>
> Caution: This is an external email and may be malicious. Please take care
> when clicking links or opening attachments.
>
>
> I’ll just let the incivility of you both stand.
>
> -mel
>
> On Aug 2, 2025, at 3:52 PM, Tom Beecher <beecher () beecher cc> wrote:
>
> 
> Mel-
>
> Saku did not call *you* any names. He called your *incorrect statements*
> in this thread 'bizzard drivel'. Which he is absolutely correct about.
> While your intentions may certainly have been to help, your statements here
> have been frankly dead wrong and did not accomplish that.
>
> Probably just want to take the L here.
>
>
> On Sat, Aug 2, 2025 at 5:34 PM Mel Beckman via NANOG <
> nanog () lists nanog org<mailto:nanog () lists nanog org>> wrote:
> Saku,
>
> What is actually appalling is that a member of NANOG calls “bizarre
> drivel” the honest and sincere attempts by other members to help identify
> the possible problem. There’s no cause to be uncivil, people can disagree
> without stooping to name-calling.
>
>  -mel
>
> > On Aug 2, 2025, at 11:46 AM, Saku Ytti via NANOG <nanog () lists nanog org
> <mailto:nanog () lists nanog org>> wrote:
> > On Sat, 2 Aug 2025 at 21:02, Tom Beecher via NANOG
> > <nanog () lists nanog org<mailto:nanog () lists nanog org>> wrote:
> >
> > > I don't have in depth knowledge of Cisco's SNMP implementations, or even
> > > the ASR platform specifically, but if Cisco TAC is telling you this is
> > > 'normal', they are completely full of shit, and you should click any and
> > > every 'escalate' button you can find.
> > >
> > > This almost sounds like a default control plane DDOS policer / LPTS ,
> > > something like that.
> >
> > There are various complicated reasons for this, LPTS policer is
> > unlikely culprit, but possible. Bug search will show various DDTS with
> > poor SNMP performance outcome, most of them are unrelated to LPTS.
> >
> > But absolutely correct, the right solution is to escalate. In common
> > case this would be SE from your account team, who would fight for you
> > internally.
> >
> >
> > It is appalling that OP came to nanog after correctly suspecting TAC
> > is gaslighting them, some community member piled on with what can only
> > be described as a bizarre drivel.
> > --
> >  ++ytti
> > _______________________________________________
> > NANOG mailing list
> https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.nanog.org%2Farchives%2Flist%2Fnanog%40lists.nanog.org%2Fmessage%2F7KXUNRGFI5OEVSDEDU2OL5VMY5NBGQCV%2F&data=05%7C02%7Cryan%40rkhtech.org%7C935f5b9276c34753763108ddd21bb022%7C81c24bb4f9ec4739ba4d25c42594d996%7C0%7C0%7C638897738572035533%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6ZnOfMhjhXcOW6xJQgBOLUTCz9tS4Uzyb8esw9zjkww%3D&reserved=0
> <https://lists.nanog.org/archives/list/nanog () lists nanog org/message/7KXUNRGFI5OEVSDEDU2OL5VMY5NBGQCV/>
> _______________________________________________
> NANOG mailing list
>
> https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.nanog.org%2Farchives%2Flist%2Fnanog%40lists.nanog.org%2Fmessage%2FCF3QHVTISL6LDFTOWG4E3KK54QEDHUIY%2F&data=05%7C02%7Cryan%40rkhtech.org%7C935f5b9276c34753763108ddd21bb022%7C81c24bb4f9ec4739ba4d25c42594d996%7C0%7C0%7C638897738572091179%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=rMHLcrZ21hVS2zLMHWW2nmH%2FZoF%2FPm3gZdU1ViywGQc%3D&reserved=0
> <https://lists.nanog.org/archives/list/nanog () lists nanog org/message/CF3QHVTISL6LDFTOWG4E3KK54QEDHUIY/>
> _______________________________________________
> NANOG mailing list
>
> https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.nanog.org%2Farchives%2Flist%2Fnanog%40lists.nanog.org%2Fmessage%2FOJ7ICXLSPFND32X2XS2U7XIWA6DALSIF%2F&data=05%7C02%7Cryan%40rkhtech.org%7C935f5b9276c34753763108ddd21bb022%7C81c24bb4f9ec4739ba4d25c42594d996%7C0%7C0%7C638897738572130874%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=J8MV4YiEOgQROlfuT5ij7baERA6aF8bH0Tm%2Bg2%2FMKC0%3D&reserved=0
> <https://lists.nanog.org/archives/list/nanog () lists nanog org/message/OJ7ICXLSPFND32X2XS2U7XIWA6DALSIF/>
_______________________________________________
NANOG mailing list 
https://lists.nanog.org/archives/list/nanog () lists nanog org/message/X7TXH57M7WJ5MXQY3MGB4ZGGFXFL5ENS/