CCNA - ETHERCHANNEL - Understanding and Configuration

What is EtherChannel?

    EtherChannel combines certain individual ports from the switch to form a single logical link. They provide redundancy  and load balancing and also enable us to make the full use of bandwidth. 

To better understand EtherChannel look at the following STP:

           In the above network setup, one link is purposely made down by the STP protocol to avoid looping. Both the cables are FastEthernet. They have a total of 200 Mbps bandwidth (100 Mbps each). However with STP in place we will not be able to use the only 100 Mbps as only one link is active and all the other links will be in the blocking state due to STP. 

Etherchannel will help us to use the total available bandwidth and also make sure that no looping is formed.

• EtherChannel will combine the multiple physical link into one logical link.
• EtherChannel helps with load balancing traffic over the logical link
• It can use up to 8 links to create the logical link.

Requirements:

1. Duplex : Should be on the same duplex.
2. Speed : All the links should be on the same speed. (i.e., Either all should be on FastEthernet or GigabitEthernet)
3. Access /Trunk  : All the ports should be configured as Access port or trunk port. But it is suggested that they are configured as trunk port. This way it is easy to configure VLANS.

• Access ports : Should be on the same VLANs as all other ports
• Trunk ports : Should have same allowed VLANs on both sides and same Native VLAN.
• 
  

How to configure EtherChannel?

There are two ways to configure EtherChannel

• Manually
• Dynamically
• PAgP (Port Aggregation Protocol)
• It is a Cisco proprietary protocol.
• Can be implemented between two Cisco devices.
• LACP (Link Aggregation Control Protocol)
• It is an open standard protocol.
• Can be used to connect between Cisco/non-Cisco device to another Cisco/non-Cisco device. 

Different mode available for configuration

 Manual : We need to enable the EtherChannel manually using the keyword 'ON'

PAgP:

•  Desirable: Actively negotiate PAgP with the neighbour. They will ask/ negotiate with the neighbour to form EtherChannel. It will also reply to request initiated from other side to form Etherchannel.
• Auto : Passively listens for PAgP. They will respond to request initiated from other side to form Etherchannel.
• In order to form Etherchannel using PAgP, at least one side has to be on Desirable. If both sides are on 'Auto' then they will never for EtherChannel.

LACP:

• Active: Actively negotiate LACP with the neighbour. They will negotiate with the neighbour to form EtherChannel. They will also reply to request initiated from other side to form Etherchannel.
• Passive  : Passively listens for PAgP. They will respond to request initiated from other side to form Etherchannel.
• In order to form Etherchannel using LACP, at least one side has to be on Active. If both sides are on 'Passive' then they will never for EtherChannel.

Some Successful combination of EtherChannel are

• On - On
• Desirable - Desirable
• Desirable - Auto
• Active -Active
• Active Passive

Code to understand : Channel-group 12 mode ?

Channel-group 12 : This is the Port Channel number. The significance of the number is local to the router.

Mode : This is where the protocol is mentioned (Desirable / auto/Active/Passive)

Sample config:

Switch1(config-if-range)#channel-group 5 mode ?

active Enable LACP unconditionally

auto Enable PAgP only if a PAgP device is detected

desirable Enable PAgP unconditionally

on Enable Etherchannel only

passive Enable LACP only if a LACP device is detected

Configuration code :

Switch 1:

Switch1>enable

Switch1#Configure Terminal

Switch1(config)#Interface range fastethernet 0/23-24

Switch1(config-if-range)#channel-group 5 mode ?

  active     Enable LACP unconditionally

  auto       Enable PAgP only if a PAgP device is detected

  desirable  Enable PAgP unconditionally

  on         Enable Etherchannel only

  passive    Enable LACP only if a LACP device is detected

Switch1(config-if-range)#channel-group 5 mode desirable

Switch1(config-if-range)#exit

Switch1(config)#interface port-channel 5

Switch1(config-if)#switchport mode trunk

Switch1(config-if-range)#exit

Switch 2:

Switch2>enable

Switch2#configure terminal

Switch2(config)#interface range fastEthernet 0/23-24

Switch2(config-if-range)#channel-group 5 mode auto

Switch2(config-if-range)#exit
Switch2(config)#interface port-channel 5

Switch2(config-if)#switchport mode trunk

Switch2(config-if-range)#exit

Verification commands:

show etherchannel summary  : This command is used to see the status of the EtherChannel. This will show the protocol name and the ports involved.

 Switch1#show etherchannel summary 

Show Ip interface brief: This will show the Port-channel as a separate interface 

Switch1#show ip interface brief 

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