admin- Posted on
The Spanning-Tree Protocol is well-known among CCNA and CCNP candidates, and one of the best things about STP is that it fits well with little or no additional configuration. STP operates against us in one case when preventing switching loops, and that is where two switches have multiple physical links.
You’d imagine that since two switches had two different physical links, twice as much data might be sent from one to the other than if only one link existed. STP does not make this by nature, but one of the pathways would be blocked to avoid switching loops from forming.
On ports fast0/11 and fast0/12, SW1 and SW2 are linked by two different physical links. Just port 0/11 is currently forwarding traffic, as we can see on SW1. The other port has been blocked by STP (BLK).
SW1#show spanning vlan 10
(some output removed for clarity)
Interface Role Sts Cost Prio.Nbr Type
Fa0/11 Root FWD 19 128.11 P2p
Fa0/12 Altn BLK 19 128.12 P2p
Although STP is helpful in that it avoids switching loops, it is also counterproductive in that it prevents us from using a perfectly acceptable route between SW1 and SW2. If we could use the route that is actually blocked, we could practically twice the bandwidth available between the two switches.
Configuring an Etherchannel is the key to using the currently blocked route. A logical bundling of 2 – 8 physical links between two Cisco switches is known as an Etherchannel.
It’s really very easy to set up an Etherchannel. On each port you want to be put into the Etherchannel, use the command “channel-group 1 mode on.” This must be done on both switches, of course. The line protocol will go down and remain down if you configure an Etherchannel on one switch but not on the right ports on the other switch.
The Etherchannel’s uniqueness is that STP considers it as a single link. STP does not notice whether all of the physical links within the Etherchannel go down, and it will not recalculate. Although traffic between the two switches will certainly be delayed, a STP recalculation will not create a transmission delay. We may also use several physical links at the same time with an Etherchannel.
Here’s how to put these ports into an Etherchannel:
SW1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface fast 0/11
SW1(config-if)#channel-group 1 mode on
Creating a port-channel interface Port-channel 1
SW1(config-if)#interface fast 0/12
SW1(config-if)#channel-group 1 mode on
SW2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#int fast 0/11
SW2(config-if)#channel-group 1 mode on
SW2(config-if)#int fast 0/12
SW2(config-if)#channel-group 1 mode on
The command “show interface trunk” and “show spanning-tree vlan 10” will be used to verify the Etherchannel configuration.
SW2#show interface trunk (some output removed for clarity)
Port Mode Encapsulation Status Native vlan
Po1 desirable 802.1q trunking 1
SW2#show spanning vlan 10 (some output removed for clarity)
Interface Role Sts Cost Prio.Nbr Type
Po1 Desg FWD 12 128.65 P2p
Individual ports were visible here before configuring the Etherchannel. Now we see “Po1,” which stands for “port-channel1” interface. When an Etherchannel is installed, this is the logical interface that is generated. Both physical routes between the two switches are still being used at the same time!
Let’s look at another big profit in motion. When the single open route between two trunking switches fails, there is usually a substantial delay until another valid path is found – up to a minute in some cases. We’ll now disable port 0/11 on SW2 and see if it impacts the etherchannel.
SW2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#int fast 0/11
SW2(config-if)#shutdown
3w0d: %LINK-5-CHANGED: Interface FastEthernet0/11, changed
state to administratively down
SW2#show spanning vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Interface Role Sts Cost Prio.Nbr Type
Po1 Desg FWD 19 128.65 P2p
SW2#show interface trunk
Port Mode Encapsulation Status Native vlan
Po1 desirable 802.1q trunking 1
The Etherchannel remained operating! Since STP considers the Etherchannel to be a single thread, nothing happened in STP’s eyes.
Building an Etherchannel and recognizing how it will help the network is a prerequisite for CCNA and CCNP registration, and it also comes in handy on the job. Before taking Cisco’s tests, make sure you’re happy designing one.