Troubleshooting etherchannel

Troubleshooting Etherchannel

We need to understand that most of the issues with Etherchannels (Link Aggregation) are because of misconfiguration and load balancing etherchannel issues.
So in this article we will  first  check how to gather information  of etherchannel, how to prevent issues  , commands of how to troubleshoot etherchannel , and load balancing etherchannel (XOR operations)

Remember that  the configuration of all physical interfaces has to match. In this article  we’ll take a look at regular  issues you will find with Etherchannels.


We will try to do that on a layer 2 etherchannel..

Same graphic than before..

If you want to do that with the configuration of the previous exercise (etherchannel layer 3) you will have some errors:

It's telling you that Fa0/9 and Fa0/10 are auto mode (Cisco PAGP) and Port Channel 20 is dot1q (open standard).This kind of message, that show up when you have this kind of mismatches  is called Etherchannel Guard, and we are going to talk about it later.


Now let's fix Switch A that mismatch ..

Port-channel 20 is now in desirable mode (Pagp) :a Cisco layer 2 protocol.
Now we will issue a show ip int brief command

We could see port channel 20 , but it's down because we need to configure Switch C..
let's do it

Switch C

In switch C the mode is Auto of PAGP, channel group 30: local significant (not in the network)

We will issue a show ip int brief

Now Port channel of Switch C is up, because we configure it properly and there is a connection with Switch A.

Let's issue some verification commands on Switch A


1st Verification  Command

Port channel 20 is up with 200 mbp/s link

2nd  Verification command

We see the port channel 20, layer 2, in use , port 9 and 10 bundle up together to form the PO 20 , using PAGP

3rd Verification Command

We see that it's in desirable mode on Switch A

Now let's go to Switch C and issue the same command

There is a difference..

We can use a feature called Etherchannel Guard and its purpose is to detect mismatches channels parameters between switches , generate errors messages  and set a port into error disable state.

 It's enable by default..
Let's confirm it..

 


This was the error messages that we saw at the beginning of this exercise!  We saw Etherchannel Guard  working!
As we mention it comes enable by default, but in case you disable it and wanted to enable one more time this is the command:  (config)#spanning-tree etherchannel guard misconfig .As a result ,ports will be place in a err-disable state  if a condition exist that might result in a switching loop


We could see here that we have a bundle of two ports, but it can be four or eight ports bundle.These ports are actively participating in the etherchannel but maybe with a bandwidth that is not what we expect.
Ethernet give us load balancing but not pure load balancing .

In our lab we have two parallels links , but that doesn't mean they are carrying 50% of the load.
Basically , a Cisco-propietary algorithm  is run and those values are assigned to links in the Etherchannel.The values  that are used to determine which link will handle which traffic flow.The values could be:

1. Source IP address
2. Destination IP address
3. Both source and destination IP address
4. Both source and destination Mac Address
5. TCP/UDP Port numbers

 Let's check our current situation..

 We can see here that the algorithm that is using is Source Mac address!

We can check the values that we mention before:

So, it's a good practice to consider not only the source but the destination as well ( a client for example).

One of our prefered values that we personally choose is source-destination IP address instead of Mac address, because Mac address is re-written in each router hop , and we will like to reassure that the Mac Address would be the same for a lot of the traffic.

The switch may use  the hash of the last  low-order bits to choose the link that will carry the traffic flow, or it may get the Exclusive- OR  operation ('XOR')  involved. The only time the XOR operation is used  is when one of the combination of load-balancing method is used -the source and destination ip  or mac  address or port numbers-)
A Xor  (Exclusive OR) is a Boolean operation that compares two binaries values and result in 1 if the values are different and result in 0 if the values are the same..
And that is only with two paths! If we have four paths we would be looking at the last two bits.


In this case ,we are looking at the last bit of the source and destination ip address , and just by doing that we are making our etherchannel more efficient in the bandwidth usage

Topics to take into consideration using Etherchannel

1. As you saw at the beginning of the exercise, if you change one of the attributes and the EC comes down , you need to change it back.It applies  with speed, duplex and native VLAN
2. A SPAN source port can be part of an Etherchannel, but not a SPAN destination  port.
3. Ports in a Etherchannel cannot be configured with port security
4. Ports in Etherchannel  should have the same native VLAN set 
5. If one end of the Etherchannel is in running in ON running mode, the other end has to as well
6. Learn  VERY WELL your PAGP and LACP modes.You need to have the same mode negotiation on BOTH sides.
7. Choose the load-balancing method that best fit your situation