[c-nsp] 6500 Sup720 high CPU load - RP LES Fragmentation unsupported

[Bernhard Schmidt]

Sukumar Subburayan wrote:

Hi,

>> The physical ports (6704-10GE and 6748-GE-TX) are configured to MTU 
>> 9216 as well, but the SVI is configured to 1500 Bytes. I guess the 
>> size is not checked on incoming packets on SVI.
> 
> OK, this makes sense. When MTU is configured to 9216 on the ingress, we 
> will check to see if the packet is less than that on the input interface 
> (the 6748-GE-TX linecard) and when we try to L3-switch the packet, we 
> see the SVI interface is 1500 bytes. We probably are punting the packet 
> to the RP for fragmentation.
> 
> Can you check to see if you configure 'mtu 9216' on your SVI interface, 
> that the CPU comes down?

The high CPU load is on the other box (ingress 10GE routed 9216 bytes, 
egress L3 SVI 1500 bytes, L2 10GE-Trunk 1500 bytes). The box where the 
packet enters the network (ingress L2 GE 9216 bytes, ingress L3 SVI 1500 
bytes, egress 10GE routed 9216 bytes) does not have any CPU issues. I 
would have expected it to just drop the packets, but I guess MTU != MRU 
on Cisco (as long as you don't hit hardware limits of course). We are 
talking about 1512 bytes IP (1530 Bytes Ethernet frame with FCS).

Filer ----- Router ----- Router ----- Host
MTU  L2 9216     L2/L3 9216   L2/L3 1500
      L3 1500                ^ CPU-Load

The inconsistent MTU configuration on the first router is definitely an 
error (I don't know why it was configured that way), but it is not 
causing us any issues at the moment. The fragmentation is happening at 
the second box. If I increased the L3 MTU on the SVI on the first router 
the large packet would still pass and need to be fragmented.

To remove the fragmentation I would have to set the L2 and L3 egress 
ports on the second box to something larger, but the infrastructure 
behind these ports is not entirely Jumbo-frames compatible.

We will see what NetApp has to say about this.

>> The traffic I've seen was incoming on WS-X6748-GE-TX. Interestingly 
>> there are no giants listed on the interface in "sh int", but the giant 
>> counter on the other router (where ingress is 10GE routed) is 
>> increasing. Different IOS though.
> 
> There are some differences in the way 'giant's are reported on the 
> linecards and particularly if the ports are configured as trunk-port vs 
> switchport.
> 
> For ports configured as trunk ports, we will not report giant packets 
> for packets upto the size of 1548 bytes.

The interface reporting the giants is 10GE "no switchport" on 
12.2(33)SRA, the one not reporting is GE "switchport mode access" on 
12.2(18)SXD6.

Regards,
Bernhard