[c-nsp] Operational impact of switching from ingress to egress replication mode

[Benjamin Lovell]

The primary thing to worry about here is the mcast packet rate not number of mroutes. Replication change will cause all mcast packets to be punted to CPU for a short period(few 100 msec or so). I have seen this go badly under the following conditions. 

1) High rate of mcast traffic. few Gigs per sec.
2) BFD will tight timers in the 750msec range. 

When mcast started getting punted the CPU spiked to the moon. Input queues got smashed. BFD and therefor OSPF dropped. RPF interfaces changed and it took almost 30min for the box to calm back down. 

If you have a moderate to high mcast packet rate I would not count on this being a hitless change. 

-Ben


On Sep 21, 2010, at 3:01 PM, John Neiberger wrote:

> If I recall correctly, we have over 500 mroutes. I was just speaking
> to a Cisco engineer that works with us about this. I think I'm going
> to save this change until last. We have a lot of etherchannels and we
> want to convert those to routed links with ECMP first, then we'll
> switch over to egress replication. It sounds like we shouldn't have
> more than a couple of seconds of impact.
> 
> Thanks!
> John
> 
> On Tue, Sep 21, 2010 at 12:46 PM, Tim Stevenson <tstevens at cisco.com> wrote:
>> Hi John,
>> Switching replication modes basically purges the hardware of all mroutes and
>> those will be reprogrammed based on the current software state. It will be
>> potentially disruptive for all mroutes, but the exact amount of traffic
>> loss/blackholing would depend on the rate of each stream at the time, and
>> the overall amount of time it takes to reprogram. For a few 100 mroutes, I
>> would not expect much impact.
>> 
>> Hope that helps,
>> Tim
>> 
>> At 11:30 AM 9/21/2010, John Neiberger averred:
>> 
>>> We're running 12.2(33)SRC2, I believe. It's actually experimental code
>>> and the exact version is overwritten with another code.
>>> 
>>> On Tue, Sep 21, 2010 at 12:04 PM, Jeffrey Pazahanick <jeffpaz at gmail.com>
>>> wrote:
>>>> John,
>>>> Having switched back and forth a few times, I never noticed more than
>>>> a 1-2 second outage.
>>>> What version of code are you on?
>>>> 
>>>> On Tue, Sep 21, 2010 at 11:59 AM, John Neiberger <jneiberger at gmail.com>
>>>> wrote:
>>>>> We're going to be doing a whole bunch of maintenance tonight during a
>>>>> maintenance window. One of the many things on our plate is to switch
>>>>> from ingress replication mode to egress on some 7600s that have a few
>>>>> hundred multicast routes on them. We know there is going to be at
>>>>> least a minor blip while things settle down after making the change,
>>>>> but I wanted to see if anyone on the list has done this and what the
>>>>> operational impact was. I've heard there will be slight interruption
>>>>> in traffic, but what sort of interruption are we talking about? Are we
>>>>> speaking about a second or two?
>>>>> 
>>>>> I'm asking because we're trying to decide if we want to split this out
>>>>> to another night. If the disruption is minor and the risk is low then
>>>>> we'll do it tonight. Otherwise, we might choose to do it on a separate
>>>>> night.
>>>>> 
>>>>> Any thoughts?
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>> 
>> 
>> 
>> Tim Stevenson, tstevens at cisco.com
>> Routing & Switching CCIE #5561
>> Distinguished Technical Marketing Engineer, Cisco Nexus 7000
>> Cisco - http://www.cisco.com
>> IP Phone: 408-526-6759
>> ********************************************************
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>> 
>> 
>> 
> 
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