Charles Hooper

Thoughts and projects from a site reliability engineer

Link Aggregation on Cisco Catalysts & Foundry Switches

LACP, or Link Aggregation Control Protocol, allows you to configure multiple ethernet ports to act as a single device. This is sometimes referred to as channel bonding or trunking. Link Aggregation provides several benefits: Increased bandwidth, load balancing, and allows you to create redundant ethernet links. If a link in your ethernet channel goes down, the switches, routers, or servers you have configured to use LACP will automatically fail over to the links that are still up and remain connected. With the right hardware and the right firmware, setting this up is very simple. On a Cisco Catalyst switch running IOS, once logged in, you will need to perform the following steps:

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cisco>enable  
Password: *enter password*  
cisco#config term  
cisco(config)#int Gi0/1  
cisco(config-if)#channel-group 1 mode active  
cisco(config-if)#channel-protocol lacp  
cisco(config-if)#int Gi0/2  
cisco(config-if)#channel-group 1 mode active  
cisco(config-if)#channel-protocol lacp  
cisco(config-if)#end  
cisco#write mem

It also helps to label your ports and to be sure that your bonded ports are configured in the right VLAN or set up for VLAN trunking. If your other device is another Cisco Catalyst, then just repeat the steps above, connect your newly bonded ports, and disconnect any non-bonded ports connecting the two devices. However, if you are configuring a Foundry switch, your configuration is just as easy, but slightly different.

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BR-telnet@foundry>enable  
Password: *enter password*  
BR-telnet@foundry#config term  
BR-telnet@foundry(config)#interface ethernet 1  
BR-telnet@foundry(config-if-e1000-1)#link-aggregate active  
BR-telnet@foundry(config-if-e1000-1)#interface ethernet 2  
BR-telnet@foundry(config-if-e1000-2)#link-aggregate active  
BR-telnet@foundry(config-if-e1000-2)#end  
BR-telnet@foundry#write mem

Provided you don’t have a complicated VLAN set up, you’re all set! One thing you should note about the Foundry devices, is that you can only start your trunked ports on the first port of each group of 4. Explanation: On a 12-port switch, you would only be able to start port groups on ports 1, 5, and 9. But this does not mean that you can’t bond ports 1 & 2 — but it does mean that you can’t bond ports 3 & 4.

Recommended reading: * Link aggregation article on Wikipedia

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