Etherchannel and Cisco Portfast

Another day for more review, another day for more blogs. I'm getting the hang of this haha. Anyway, today I'll be discussing about Etherchannel and Cisco Portfast technology and how the work and benefit in out network.

First, people believed that if they had two connections to a switch, they would have more bandwidth, thus improving their downstream & upstream speed. To tell you honestly my friends, I too was mislead. What happens if you do this is just like having two separate doors in your house. Both could let people in and out, but it could only allow as much at the same time due to the size of the door. Why so? Because of STP, since every port is treated as a separate connection, STP will always make sure that there would be a loop free network connection, thus blocking one of the two connections to a switch. If we had two connection doesn't make the two doors combine to let more people pass at the same time, unless you reconstructed your house.

So to make the two connections combine and make it possible to increase your upstream downstream bandwidth, we configure etherchannels on your switch. By having etherchannels, you tell ports (from 2 to 8 ports only) to group as one logical port and STP will see these ports as one -no more blocking of the other ports.

To enable etherchannel on switches, you must configure them on both switches first. To configure you must go to the interface-configuration mode of the switch (the switch ports that you want to group). Here it goes:
S1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
S1(config)#interface fast 0/11
S1(config-if)#channel-group 1 mode on
Creating a port-channel interface Port-channel 1
S1(config-if)#interface fast 0/12
S1(config-if)#channel-group 1 mode on

S2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
S2(config)#int fast 0/11
S2(config-if)#channel-group 1 mode on
S2(config-if)#int fast 0/12
S2(config-if)#channel-group 1 mode on

After this configuration, both ports fa0/11 and fa0/12 of both switches will group logically as one. For us to verify, we use the commands 'show interface trunk' and 'show spanning-tree' in the privelage-user mode. You'll notice that the name of the port changed. Instead of having individual entries for each port, only one port name appears for the grouped ports which is 'Po1'. Po1 stands for 'port-channel1' which is created when etherchannel is configured. For the ouput of the 'show spanning-tree', instead of seeing two individual ports (fa0/11 & fa0/12) with one in blocking state, we only see one port entered which is Po1.

Just a note: since etherchannel is a logical group of ports, even if one port goes down, as long as there are ports in that etherchannel that are still up, the etherchannel logical port will be up.

Now we know how to use etherchannels in our network, now we discuss about Cisco Portfast technology.

Cisco PortFast Technology applies to networks that has STP running, and it applies to ports that we consider to be directly connected to end-nodes (computers/printers). Cisco PortFast allows ports to transition from blocking state to forwarding state immediately. With normal operations, switchports transitions from blocking to listening to learning then to forwarding or blocking again (the listening & learning states are 15 secs long by default). This is to ensure that we have a loop-free network. Come to think, switchports that are connected to other switches only need all four transitions to ensure a loop-free network. Having them on access ports seems useless and it takes much time for a PC to get connected. So applying Cisco PortFast on access ports enables servers, workstations or any end-user devices to get hooked up immediately.

To enable Cisco PortFast on your switch, you must go to the interface-configuration mode of the switch (the switch ports that you want to enable PortFast on). Here it goes:
S1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
S1(config)#interface fast 0/11
S2(config-if)#spanning-tree portfast

Just a note: only enable PortFast on access ports, if you enable them on ports that connects to other switches your in for some switching loop trouble! Use it with care.

Alright friends, another topic reviewed, and if you guys have any questions regarding this topic just ask, I'll be more than happy to find the answer. Till next time.