EtherChannel /Port Aggregation

Answered Question
Jun 11th, 2008

On Cisco documentation on EtherChannel, it states "A Gigabit EtherChannel bundles individual Gigabit Ethernet links into a single logical link that provides the aggregate bandwidth of up to eight physical links."

My question is if I bundle 5(Gbps) ports to one EtherChannel logica interface, would this provide a total of 5x5=25Gbps for the link? If so, does that mean the switch would be capable of sending and receiving packets at 25Gbps on one port?

Any clarification on this is greatly appreciated!

Thanks in advance,

sK

I have this problem too.
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Correct Answer by Jon Marshall about 8 years 6 months ago

sK

Mac-addresses are only used on the same vlan. So if the source hosts are on the same vlan then yes you could use src-mac to load-balance and different hosts would get sent across different links.

But if the source hosts are on a different vlan then remember that the mac-addresses of the source hosts are not carried across vlans ie.

H1 -> (fa0/0)R1(fa0/1) -> S1

H1 is a host on vlan 10

R1 is a router

S1 is the server

H1 sends a packet to S1.

src IP = H1

dst IP = S1

src mac = H1

dest mac = fa0/0 on R1

R1 forwards packet on to S1

src IP = H1

dst IP = S1

src mac = fa0/1 on R1

dest mac = S1

So you can see that for hosts on different vlans/subnets than the server the src mac-address would always be the fa0/1 interface of R1. So traffic load balanced on src/dst mac-address would always use the same link for all hosts.

Jon

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Jon Marshall Wed, 06/11/2008 - 10:16

Just to clarify

if you bundle 5 x 1Gps ports together you get a 5Gbps etherchannel. So no you will not get 25Gbps by bundling together 5 x 1Gbps.

Edit - Most switches support up to 8 separate connections in one etherchannel so with 1Gbps connections the max you would get is 8Gbps.

Jon

nick.franzen Wed, 06/11/2008 - 10:59

You would get 5 GB in each direction for a total bandwith of 10GB, I believe.

skhirbash Wed, 06/11/2008 - 11:08

Sorry, I madea a mistake when calculating the total of BW. It should have been 1Gbps x 5 =5Gbps.

So, how is the load distributed on this 5Gbps? To clarify the question, if I have a server that has 5 1Gbps interfaces aggregated to one 1 channel(interface) and on the switch I bundeled 5 1Gbps ports into 1 etherchannel and, for instance, 400MBps load comes into the etherchannel, would this load use one port to deliver packets to the server or load balance the load amongest the 5 ports (5 interfaces)?

Thanks,

sK

Jon Marshall Wed, 06/11/2008 - 11:13

sK

It would use one of the ports usually. It does depend on the load balancing algorithm used but assuming it is either src/dst mac-address or src/dst ip address and the 400Mbps is coming from the same source to the server then only one of the links will be used.

If another source then sent more data, the source was on a different subnet and you were load-balancing on src/dst IP then another link in the etherchannel could be used.

Jon

skhirbash Wed, 06/11/2008 - 11:32

Thanks Jon.

What about using src-mac or src-ip as load balancig, would this allow the use of different ports on the etherchannel? If this is a good solution, are there any requirements to implement src-mac or src-ip besides just specifying it at the global level?

Thanks,

sK

Jon Marshall Wed, 06/11/2008 - 11:36

sK

It depends on what traffic is going to the server.

For example if all the clients talking to the server are in the same vlan then it would make sense to load balance on src mac-address because that would be different per client.

But if most/all of the clients are on a different vlan then load-balancing on src mac-address would not benefit you because the src mac-address will always be the vlan interface mac-address that the server lives on. So you are better to use src ip load balancing.

Some switches can also load-balance based on L4 port numbers ie. the 6500.

You need to decide which load-balancing method will give you the most variation which means more links utilised in the etherchannel.

Jon

skhirbash Wed, 06/11/2008 - 12:10

I may be confused between the src-mac and src-ip, my understanding is that either one of those algorithmes would load balance to different ports/links based on the src. So, different source hosts would use different ports/links to send packets to the server. Is this correct?

The volume of data pushed to the server is over 200GB.

Thanks in advance Jon,

sK

Correct Answer
Jon Marshall Wed, 06/11/2008 - 12:18

sK

Mac-addresses are only used on the same vlan. So if the source hosts are on the same vlan then yes you could use src-mac to load-balance and different hosts would get sent across different links.

But if the source hosts are on a different vlan then remember that the mac-addresses of the source hosts are not carried across vlans ie.

H1 -> (fa0/0)R1(fa0/1) -> S1

H1 is a host on vlan 10

R1 is a router

S1 is the server

H1 sends a packet to S1.

src IP = H1

dst IP = S1

src mac = H1

dest mac = fa0/0 on R1

R1 forwards packet on to S1

src IP = H1

dst IP = S1

src mac = fa0/1 on R1

dest mac = S1

So you can see that for hosts on different vlans/subnets than the server the src mac-address would always be the fa0/1 interface of R1. So traffic load balanced on src/dst mac-address would always use the same link for all hosts.

Jon

rsohi Wed, 06/11/2008 - 11:43

Hello SK, I agree with Jon. Not sure what type of switch you are using, but to get an idea of how the load balancing is working try the following. This is for a 6509:

6509> (enable) show lacp traffic

ChanId Port Rx-Ucst Tx-Ucst

1678 5/25 17.16% 48.11%

1678 5/26 82.84% 51.89%

Raj

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