Hello Abirami
Let’s take a look at our diagram from the lab:
The goal of the exercise is to make SW1 and SW2 think that they’re directly connected and that they have created an EtherChannel bundle between them. SW2 and SW3 which belong to the ISP appear to be “transparent” to the customer switches.
In order to make this work, we must create a transit VLAN within the ISP infrastructure for every physical link of the EtherChannel. In this case, we have two physical links, so we need two transit VLANs, specifically VLANs 100 and 200.
Why? Well, we want to make sure that:
- traffic that exits Fa0/23 on SW1 enters Fa0/23 on SW4.
- traffic that exits Fa0/24 on SW1 enters Fa0/24 on SW4
- traffic that exits Fa0/23 on SW4 enters Fa0/23 on SW1.
- traffic that exits Fa0/24 on SW4 enters Fa0/24 on SW1
Now in the lesson, Rene states that:
If we use a single transit VLAN for all traffic on the FastEthernet 0/23 and 24 interfaces, then we run into issues with our Etherchannel because FastEthernet 0/23 could talk to 0/24 or vice versa.
What he is saying essentially is that if we don’t segregate the traffic from each individual physical link onto a separate transit VLAN, traffic that arrives at the switch may enter from either port. Such an arrangement would cause EtherChannel to fail. Does that make sense?
I hope this has been helpful!
Laz