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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.
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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!
A Cisco CCNA certification is the very first step an IT professional can take in establishing a career in the field of networking. Not only do employers and clients demand a reliable networking professional to be holding a CISCO CCNA certification, but as a responsible professional, you should be aware of ccna course london the fact that attaining the certification is important to your career and profile as well, since a number of advanced networking certifications hold CCNA as a prerequisite.
There are a few basic steps you can take for preparing and taking the CCNA examination. You should first of all study the examinations you will have to pass and come up with a rough preparation plan in how much you need to cover in the curriculum and how long will it realistically take for you to prepare for them in a satisfactory manner. Once you are familiar with the structure of the examinations for this certification, you can proceed to register yourself for the exams.
You can register yourself and schedule the exams with Pearson VUE, the world renowned testing body, which administers the Cisco certification examination. The registration fee amounts to $150 and you will need to take the required exam of 640-802 CCNA or 640-822 ICND1 and 640-816 ICND2, concerning the understanding of the Interconnecting Cisco Networking Devices.
The passing of these examinations will qualify you to become a Cisco certified network administrator for a validity of three years, which will guarantee your employers or clients that you are capable of handling a network infrastructure effectively, at least on the basic level, and most organizations consider CCNA as a sufficient qualification for offering job positions. But of course, this is only the first step to further career growth for a networking specialist. You can always renew your certification before the expiration takes effect by passing the CCNA exam or the ICND2 exam again.
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