[c-nsp] Uneven LACP load-balancing

[Benjamin Lovell]

Keegan,
If this were bad load sharing I could agree but the fact that one link  
shows zero utilization makes me doubt this. Plus he said he does have  
a good mix of src-dst IPs so low session count does not appear to be  
the issue. The link utilization really looks like a polarization like  
effect.

Brandon,
Just as a side note, "show ip cef exact-route" will show you the  
software CEF hash. It's not useful on hardware platforms to determine  
path selection as hardware will not necessarily make the same  
decision. "show mls cef exact-route" is what you want to use on these  
platforms. For distributed forwarding(DFC) platforms it may need to be  
checked by attaching to the ingress LC.

-Ben


On Nov 11, 2010, at 11:12 PM, Keegan Holley wrote:

> Port channels are prone to uneven distribution.  Assuming this isnt  
> a bug The sessions are probably being distributed evenly.  The  
> difference in throughput could be one or two hi bw sessions.  For  
> example an entire company nat'ed to one public IP.  Per session  
> hashing doesn't guarantee even traffic distribution.  How long have  
> been seeing this problem?  If it was a small number of sessions  
> things could look completely different tomorrow.  One thing you can  
> do is add more variables to the equation.  If it's supported you can  
> configure tcp port as part of the hashing algorithm.  This will  
> account for multiple sessions between the same two IP's.
>
> Sent from my iPhone
>
> On Nov 10, 2010, at 12:34 PM, Brandon Ewing <nicotine at warningg.com>  
> wrote:
>
>> I've got a weird problem that I hope someone can shed some light  
>> on.  We
>> have multiple 3560G's deployed currently, each utilizing 4 SFP's  
>> for uplink.
>> The switches are configured with 2 L2 port-channels, with a  
>> different SVI in
>> each port-channel pointing to our upstream router.  IE:
>>
>> g0/49 + g0/51 to core A, carries Vlan 100
>> g0/50 + g0/52 to core B, carries Vlan 200
>>
>> Group  Port-channel  Protocol    Ports
>> ------+-------------+----------- 
>> +-----------------------------------------------
>> 4      Po4(SU)         LACP      Gi0/49(P)   Gi0/51(P)
>> 5      Po5(SU)         LACP      Gi0/50(P)   Gi0/52(P)
>>
>>
>> We have two default routes, pointing out the above vlans:
>> switch#show ip cef 0.0.0.0 0.0.0.0
>> 0.0.0.0/0
>> nexthop 10.10.1.241 Vlan200
>> nexthop 10.10.1.245 Vlan100
>>
>> The etherchannel load-balancing method is set to src-dst-ip:
>> switch#show etherc load-balance
>> EtherChannel Load-Balancing Configuration:
>>       src-dst-ip
>>
>> EtherChannel Load-Balancing Addresses Used Per-Protocol:
>> Non-IP: Source XOR Destination MAC address
>> IPv4: Source XOR Destination IP address
>> IPv6: Source XOR Destination IP address
>>
>>
>> However, we are not seeing an even distribution of traffic among  
>> the 4
>> ports -- each L2 etherchannel is trasmitting on only one port:
>> switch#show control util
>> Port       Receive Utilization  Transmit Utilization
>> Gi0/49             1                    55
>> Gi0/50             6                    0
>> Gi0/51             1                    0
>> Gi0/52             12                   57
>>
>> Examination of flowstats from the core on the uplink interfaces  
>> shows a good
>> mix of src/dst IPs -- so why am I getting the polarization?
>>
>> Additionally, examining a test flow with the command line shows  
>> that it
>> SHOULD be working, but it's not:
>>
>> switch#show ip cef exact-route 172.16.79.186 192.168.42.183
>> 172.16.79.186 -> 192.168.42.183 => IP adj out of Vlan100, addr  
>> 10.10.1.245
>>
>> switch#test etherchannel load-balance interface po4 ip  
>> 172.16.79.186 192.168.42.183
>> Would select Gi0/51 of Po4
>>
>> However, g0/51 has no traffic on it, and hasn't for some time.  Can  
>> anyone
>> provide some clue?  This is occuring on multiple switches, and all  
>> switches
>> are running 12.2(50)SE1 ip services
>>
>> -- 
>> Brandon Ewing                                        (nicotine at warningg.com 
>> )
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