OSPF Load balance not working on ES20 Cisco 7609

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Jan 28th, 2010
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Hi All,

I noticed this problem on my router. I use 3 links routed using OSPF (using same cost), but the utilization is not the same, moreover the difference is very significant.

I have one link utilized upto 650 Mbps, while the other two are just 330 Mbps and 130 Mbps. Do you know how to make them more balanced?

I am using Cisco 7609 running IOS SRD2a on ES20-GE-3CXL. I have read that per-destination load balance can cause traffic to be shared uneuquallu across the links, so then I tried to change the load balance setting to per-packet. but unfortunately, the command is not there. I also wanted to try to not using CEF, but I remembered that it is impossible as I am running MPLS on my network. Is there any solution to my problem?



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Sergei Vasilenko Mon, 02/08/2010 - 21:54
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Hello Prima,

On this platform with a PFC (and DFCs, if present), hardware Layer 3 switching uses per-flow

load balancing based on IP source and destination addresses.
Per-flow load balancing avoids the packet reordering that can be necessary with per-packet load balancing.
For any given flow, all PFC- and DFC-equipped switches make exactly the same load-balancing decision,
which can result in nonrandom load balancing.
The Cisco IOS CEF ip load-sharing per-packet, ip cef accounting per-prefix, and ip cef accounting non-recursive commands on the MSFC apply
only to traffic that is CEF-switched in software on the MSFC. The commands do not affect traffic that is hardware Layer 3 switched on the PFC or
on DFC-equipped switching modules.

See - Configuring Hardware Layer 3 Switching

Potentially, you could improve the load balancing ratio using not a L3 IGP balancing on 3 separate Phy links,
but if possible combine them into EtherCahnnel.
This will allow you to use L2, L3 and L4 info during balancing.

See descripcion on CCO at the "Configuring EtherChannels":
EtherChannel load balancing can use MAC addresses or IP addresses. EtherChannel load balancing can also use Layer 4 port numbers.
EtherChannel load balancing can use either source or destination or both source and destination addresses or ports.
The selected mode applies to all EtherChannels configured on the router. EtherChannel load balancing can use MPLS Layer 2 information.


Configuring EtherChannel Load Balancing
Router(config)# port-channel load-balance {src-mac | dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port | dst-port | src-dst-port}

Router# show etherchannel load-balance

But again even w EtherChannel cause of the way how the balancing hash is computed
with 3 links in bundle you will not be able to reach equal ratios -
one link will be utilised less then the other 2.
You might get ~40%, 40%, 20% distribribution of flows.
In case of even # 2,4,8 of Phy Links in Bundle the distribution of flows could be more equal.



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