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Etherchannel question

Tricky question:

In the scenario where we have a etherchannel with only one port and traffic is passing by it, when  another physical port is put into the etherchannel, will all the traffic be"hashed" again and load-balanced by the algorithm over the 2 ports or will only new traffic be load-balanced  and the "old" traffic will still only use the first port (will there be a timeout or something for this traffic) ?

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Etherchannel question

Hello,  As soon as you add a port to a etherchannel, the etherchannel "hashing bits" will be immediately load-balanced - which includes old and new traffic.

Etherchannel hashing has 8 bits.For any given flow, hashing result (referred as RBH) will be one of the values in the range of 0-7.

when you have only one port then all hashing bits mapped to that port.

Say, Gig 1/1 is the only port, then hashing bits 1111 1111 ==> FF

This means, traffic mapped to any RBH will be Tx by Gig1/1

As soon as you add another port, Say Gig1/2, then hashing bits will be:

Gig 1/1 ==> AA ==> 1010 1010 (traffic with RBH 1, 3, 5, and 7 will be sent via this port)

Gig 1/2 ==> 55 ==> 0101 0101 (traffic with RBH 0, 2, 4 and 6 will be sent via this port)

Hope this helps.

- Yogesh

1 REPLY
Cisco Employee

Re: Etherchannel question

Hello,  As soon as you add a port to a etherchannel, the etherchannel "hashing bits" will be immediately load-balanced - which includes old and new traffic.

Etherchannel hashing has 8 bits.For any given flow, hashing result (referred as RBH) will be one of the values in the range of 0-7.

when you have only one port then all hashing bits mapped to that port.

Say, Gig 1/1 is the only port, then hashing bits 1111 1111 ==> FF

This means, traffic mapped to any RBH will be Tx by Gig1/1

As soon as you add another port, Say Gig1/2, then hashing bits will be:

Gig 1/1 ==> AA ==> 1010 1010 (traffic with RBH 1, 3, 5, and 7 will be sent via this port)

Gig 1/2 ==> 55 ==> 0101 0101 (traffic with RBH 0, 2, 4 and 6 will be sent via this port)

Hope this helps.

- Yogesh

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