So here’s the scene. You have an uplink that is constantly getting saturated due to backups (or whatever) and you need more than a Gigabit of throughput so that your network won’t be congested. No worries, you need to create an Etherchannel to bundle multiple Gig interfaces together and increase your throughput. So you put in your change request and sit down to create your EtherChannel.
As you add you physical ports to the channel group you notice the switch is throwing the following error:
CAPI_EC-4-GROUP_RATE_LIMITED: Adding interfaces of the same port-group (1-8) on WS-X6148A-GE-TX to an Etherchannel will not increase the channel throughput!
Crap! What does that mean?!?!
Well what is going on is this. Cisco like many other hardware manufacturers built their FastEthernet modules on the assumption that not every port on the FastEthernet 10/100/1000 module will be sending and receiving a full Gigabit of traffic at the same time. So like an Airliner, they over-subscribe the card. They engineer it so that every 8 ports are controlled by a single ASIC connecting them to the switch fabric. That ASIC connection to the switch fabric is only a Gigabit of throughput.
So here’s a big NOTE when planning your EtherChannel Config: When setting up EtherChannel on copper Gigabit modules ensure that the module is not oversubscribed. For example, a Cisco WS-x6148 48 port 10/100/1000 Ethernet module for the 6500 series switch is oversubscribed. It will throw an error when trying to configure Etherchannel across any contiguous 8 ports.
From Cisco’s Website:
Error Message CAPI_EC-4-GROUP_RATE_LIMITED: Adding interfaces of the same port-group
([dec]-[dec]) on [chars] to an Etherchannel will not increase the channel
Explanation This message indicates that ports on the indicated slot use over subscription causing the total throughput of the port channel to be limited by port-group.
Recommended Action If more throughput is required, either use ports from a card that does not use over subscription, or use ports from different cards or port-groupings on oversubscribed cards. For example, if the card has 48 ports in groups of eight, you can select ports 1, 9, 17, 25, 33, and 41 for the same port-channel.
So to alleviate this make sure you are using a WS-X6548 or better yet a WS-X6748 module. You will then be on your way to EtherChannel bliss and be forwarding packets like a banshee in no time!