[c-nsp] Uneven LACP load-balancing

[Keegan Holley]

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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