Broadcast traffic in etherchannel

[whizkidraj]

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Hello everyone, I understand STP and Etherchannel. I understand why STP is used i.e. to avoid broadcast storms i.e. switching loops by blocking certain ports, and etherchannel is used so that all the links bundled in an etherchannel appear as one logical link to STP and it won't block any port and we get full bandwidth.

But I am still confused about how etherchannel prevents switching loops. Because STP is not blocking any ports, and although etherchannel makes it look like there's just one logical link connection, but in real there's still multiple physical links connected in etherchannel so why doesn't a broadcast storm i.e. switching loop occur in etherchannel.

How does a broadcast traffic travel on an etherchannel bundled link.

I know about the load-balancing algorithm of etherchannel i.e. traffic is not load-balanced in round-robin but per-flow basis. So how does this apply for broadcast traffic on an etherchannel bundled link.

[MHM Cisco World · Accepted solution]

There are two plane to prevent loop,

Data plane and control plane

Stp is control plane

Etherchannel is data plane.

For etherent since it logical one link then,

When one sw send broadcast the other sw will recive this broadcast but data plane never resend this broadcast to link it recive from it and hence the loop is prevent.

[Joseph W. Doherty · Accepted solution]

STP is not, per se, used to stop broadcast storms (which you might still have even with STP), STP is to avoid having L2 loops.  (BTW, although a broadcast storm can create havoc within a L2 broadcast domain, again a L2 loop not needed, looping unicast L2 traffic often does too create havoc.)

The reason why Etherchannel does not create L2 loops is for the same reason a single link does not create one.  L2 network devices do not resend traffic received on a port back to the same port.  In the case of Etherchannel, even though there are multiple physical links, logically the network device treats all of them as one port.  I.e. a frame received on physical link 1, of a Etherchannel bundle, would not be transmitted (from the receiving device) to any physical link of the same Etherchannel bundle, but if they were not Etherchannel bundled links, the network device might.  (BTW, you can have L2 loops using Etherchannel if there are multiple Etherchannel ports, for example, if you had port-channel 1 and port-channel 2 between two L2 devices, you would have a L2 loop [which can also be blocked by STP].)

As to how broadcasts transit an Etherchannel, the broadcast frame would be transmitted on one member link, which would depend on the attributes of the broadcast frame and the hashing algorithm being used.