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Cisco CCNA / CCNP Certification: How And Why To Build An Etherchannel |
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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!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNA and CCNP tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, visit the website and download
your free copies. You can also get FREE CCNA and CCNP exam
questions every day! Pass the CCNA exam with The Bryant Advantage!
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