1. Configuring EtherChannels and Switch Troubleshooting
Computer Networking
Configuring EtherChannels and Switch Troubleshooting

2. EtherChannels
STP is a great solution to avoid a broadcast storm in our network, but it effectively removes bandwidth we could use
If we have two gigabit connections connecting two switches together, using STP I will be limited to only using one port and blocking the other
EtherChannel technology allows us to bundle these connections together so that they are seen as one 2 gigabit per second connection
If we do this, they will even be seen by STP as one connection and not be blocked
This is a Cisco proprietary technology, and two protocols exist for it: PAgp (Port Aggregation Protocol) and LACP (Link Aggregation Control Protocol

3. PAgP VS LACP
PAgP is used between two Cisco switches. It will create EtherChannels as well as prevents loops, limits packet loss due to misconfigured channels, and aid in network reliability. Each port will operate in two modes -
Auto - the port is in a passive negotiating state and will form an EtherChannel if the port receives PAgP packets. The port will not initiate the negotiation
Desirable - the port is placed in a negotiating state to form an EtherChannel by sending PAgP packets. A channel is formed with another port group in either the auto or desirable mode
LACP is used between a Cisco switch and a non-Cisco switch that follows the IEEE 802.3ad standard. Ports can be in one of two modes as well -
Passive - the port is put into a passive negotiating state and forms an EtherChannel if the port receives LACP packets. The port does not initiate the negotiation. This is equivalent to auto
Active - the port is placed in a negotiating state to form an EtherChannel by sending LACP packets. A channel is formed with another port group in either the active or passive mode. This is equivalent to desirable

4. EtherChannel Commands
The following commands will control our EtherChannel configuration at the port level
#channel-protocol [lacp or pagp] - Selects the EtherChannel protocol on the interface
#channel-group[1-8] mode [auto, desirable, active or passive] - Selects the PAgP or LACP mode on the interface
#no channel-group[1-8] - disables the EtherChannel on the device
We can also use show etherchannel at the switch level to get details about the etherchannels on the switch

5. EtherChannel troubleshooting
Check that all ports are enabled
Make sure both ports are using the same protocol, and that one side is either active or desirable
Verify that both ports have the same speed and duplex. Not that LACP requires full duplex
Check that the ports have the correct and matching channel numbers
Verify that all ports are part of the same VLAN
Check that none of the ports are blocked by STP before configuring the EtherChannel
Make sure you are connecting the right number of ports. LACP EtherChannels can use up to 16 ethernet ports

6. Other Troubleshooting for Switches - Collisions
Collisions can only occur if there are multiple devices sharing transmission media
Each switch port is going to represent its own collision domain (meaning that we can’t have network collisions from outside this connection)
This means that so long as the port is connected to an end point device like a workstation or server, or another switch we should never see a collision
This means that there are only two possibilities if we are getting collisions
We have a hardware failure causing transmissions when it shouldn’t
We have a hub connected to a switch port. Since a hub broadcasts every frame it receives instead of selectively forwarding them our collision domain now includes every port on the hub and every device it is connected to
This means that in the case of a collision, we should either isolate the damaged hardware and replace it or change out a hub with a switch

7. Duplexing
Duplexing is whether we can receive and transmit at the same time on the same network medium. Full duplex means we can, half duplex means that we are limited to transmitting or receiving. This will affect our bandwidth
When full duplex is used, the devices assume there can be no collisions since only two devices are connected. Therefore, not only is half duplex slower, but it will also cause the devices to check for collisions, slowing down the connection further
By default most devices are going to be set to automatically configure their duplex mode when they are connected to another device

8. Mismatched Duplex Settings
If one of the devices is in half duplex mode, and the other device is set to auto, the auto device will not get a response to it’s duplexing negotiation query and will default to half duplex, since it can’t be sure it is only connected to one device
The problem is that it will also do this if one device is set to FULL duplex, which will create a duplex mismatch
The half duplex device will attempt to detect collisions, and the full duplex device will not, and continuously send information. This will drastically hurt bandwidth
To prevent this, keep all devices on auto if at all possible, but if you must manually set a duplex mode, set it for both sides of the connection

9. Link Speed Settings
Most devices will try to automatically detect the appropriate link speed for their connection
The connection will drop down to the lowest port setting of the two devices. For example, if I connect a NIC capable of 1 gigabit per second transmission to a 100 megabit switch port the connection will run at 100 megabit per second
If a link is running at a speed slower than you expect, first check that the connection is not manually configured to run at a lower speed. If the ports are auto-configured, try setting the speed on both to manually be at the high speed you expect
If the speed is set correctly and the link is still running slow, it is likely a hardware problem

10. Broadcast Storms
Broadcast storms caused by network loops are very dangerous as they can eat up all of the bandwidth on a connection and take down a network
We can prevent this from happening by using the Spanning Tree Protocol, or STP
Many high-end switches will also include special software that will detect broadcast storms, and drop some broadcast traffic to allow normal communications to occur

11. Misconfigured VLAN assignments
Remember that each separate VLAN on a switch will act as its own network, and that VLANs are assigned by switch port
This means that if we were to change the port a device is connected to, it is possible that we would be changing its VLAN, and no longer be able to connect to its proper network
In fact, it is commonly considered best-practice to set unconnected ports to their own separate VLAN so that people connecting rogue devices won’t be connected to other devices
If a device is unable to see the devices it should be connected to, check to make sure its switch port is on the correct VLAN

12. Frame Errors
Frame errors are when a switch sees a frame it cannot process, and it drops it
In general we will see one of three frame issues
Oversized - this usually means that the NIC that generated this frame is gibbering, sending out junk data that can’t be processed, and not sending a frame with a proper end signal
Undersized - this usually means there was a collision and the frame lost part of its data
CRC - The CRC check does not match the data. This could occur for a number of reasons, but definitely indicates the message has been corrupted
No matter what the error, the frame will be dropped. If this keeps happening it will drastically slow down our connection since the dropped frames will need to be resubmitted
The switch will record these dropped frames, so if there is a slow connection we can examine the switch and see if this is the issue