Configuring Etherchannel between layer 2 and 3 switches

[MK31]

Hello!

 Anyone to help me out with the following question.

1. Why do the two cables on the Etherchannel not constitute a loop, either on layer 2 or on layer 3? In other words, a) What prevents the access switch from receiving a frame on one interface of the Etherchannel, and sending it out on the other one (layer–2 loop)?                                                                                                          b)What prevents the MLS from receiving a packet on the Etherchannel, and sending it back out again (layer–3 loop)?

[Richard Burts · Accepted solution]

On a regular (non EtherChannel) switch interface there is processing for sending and receiving frames on the physical interface. When you configure an interface as part of EtherChannel then the sending and receiving logic is performed on the virtual interface. So if a port in an EtherChannel receives a frame on Fa0/0/1 which is part of an EtherChannel with Fa0/0/2 the logic for forwarding the frame is not executed on Fa0/0/1 but is executed on the virtual interface and it will not forward the frame out Fa0/0/2. Similar logic applies to layer 3 forwarding. If the L3 switch receives a packet on a physical interface which is part of an EtherChannel then the forwarding logic operates on the virtual interface and not the physical interface and the virtual interface knows not to forward it back out the same interface. This statement assumes that there is appropriate/correct routing logic.

But perhaps you want to consider this particular situation. Think of 2 switches connected by EtherChannel and an upstream router connected to one of the switches. SwitchA has 3 connected subnets 172.16.1.0/24, 172.16.2.0/24, 172.16.3.0/24 and has a static default route with SwitchB as the next hop. SwitchB has several connected subnets, has a static default route with the upstream router as the next hop, and has these static routes with SwitchA as the next hop 172.16.1.0/24, 172.16.2.0/24, 172.16.3.0/24, 172.16.4.0/24. So if SwitchB has a packet with destination address 172.16.4.23 it will forward the packet to SwitchA over the EtherChannel. SwitchA looks at the destination, determines that it is not a local subnet and uses the default route to forward the packet back to SwitchB. And yes in this case a L3 loop does form on the EtherChannel. And it is due to a serious configuration mistake on SwitchB.

[Richard Burts]

I am glad that my explanation was helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.