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How And Why To Build an Etherchannel
CCNA and CCNP candidates are well-versed in Spanning-Tree Protocol,
and one of the great things about STP is that it works well with
little or no additional configuration. There is one situation where
STP works against us just a bit while it prevents switching loops,
and that is the situation where two switches have multiple physical
connections.
You would think that if you have two separate physical connections
between two switches, twice as much data could be sent from one
switch to the other than if there was only one connection. STP
doesn't allow this by default, however in an effort to prevent
switching loops from forming, one of the paths will be blocked.
SW1 and SW2 are connected via two separate physical connections, on
ports fast0/11 and fast 0/12. As we can see here on SW1, only port
0/11 is actually forwarding traffic. STP has put the other port into
blocking mode (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
While STP is helping us by preventing switching loops, STP is also
hurting us by preventing us from using a perfectly valid path
between SW1 and SW2. We could literally double the bandwidth
available between the two switches if we could use that path that is
currently being blocked.
The secret to using the currently blocked path is configuring an
Etherchannel. An Etherchannel is simply a logical bundling of 2 - 8
physical connections between two Cisco switches.
Configuring an Etherchannel is actually quite simple. Use the
command "channel-group 1 mode on" on every port you want to be
placed into the Etherchannel. Of course, this must be done on both
switches if you configure an Etherchannel on one switch and don't do
so on the correct ports on the other switch, the line protocol will
go down and stay there.
The beauty of an Etherchannel is that STP sees the Etherchannel as
one connection. If any of the physical connections inside the
Etherchannel go down, STP does not see this, and STP will not
recalculate. While traffic flow between the two switches will
obviously be slowed, the delay in transmission caused by an STP
recalculation is avoided. An Etherchannel also allows us to use
multiple physical connections at one time.
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
Before configuring the Etherchannel, we saw individual ports here.
Now we see "Po1", which stands for the interface "port-channel1".
This is the logical interface created when an Etherchannel is built.
We are now using both physical paths between the two switches at one
time!
That's one major benefit in action let's see another. Ordinarily, if
the single open path between two trunking switches goes down, there
is a significant delay while another valid path is opened - close to
a minute in some situations. We will now shut down port 0/11 on SW2
and see the effect on 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 did not go down! STP sees the Etherchannel as a
single link therefore, as far as STP is concerned, nothing happened.
Building an Etherchannel and knowing how it can benefit your network
is an essential skill for CCNA and CCNP success, and it comes in
very handy on the job as well. Make sure you are comfortable with
building one before taking Cisco's exams!