Network issues can be incredibly easy to sort out (replacing a network cable, for example) to ridiculously complicated and expensive (replacing a server, maybe.) In this article, I'm hopefully trying to provide a few pointers to solving the most common issues you'll come up against.
Configure Duplex and speed
Troubleshooting Note: Both, the host and the switch port must have the same duplex configuration. Otherwise, the switch will have the port set to 100 Mbits / Full duplex and the server will get the card network set to 100 Mbits / Half duplex. When a device set to autonegotiation is connected to a device that is not using autonegotiation, the autonegotiation process fails. The autonegotiating end of the connection is still able to correctly detect the speed of the other end, but cannot correct the duplex mode.
Symptoms: Increasing collision rate in the switch port, is not accepted in full duplex environments.
Spanning tree Root
Root Bridge Election Process
Each switch has a bridge ID (BID) of priority value followed by MAC address.
Switches exchange Bridge Protocol Data Unit (BDPU) to compare bridge IDs.
The switch with the lowest bridge ID becomes the root bridge.
Eventually, all switches agree that the switch with the lowest BID is the root bridge.
Troubleshooting Note: It is strongly recommended that the designated root bridge is the switch with high MTBF. If the root bridge becomes unavailable, there will be a deadlock in the network during the new election process and convergence topology.
Configuring Spanning Tree PortFast
During the execution of the Spanning-Tree Algorithm, Spanning-Tree will force the ports to go into five different states:
Troubleshooting Note: It is strongly recommended that the NICs of the servers have a shorter transition between the up and down status. However, be aware of possible loops that are mitigated when not using the spanning tree on that port.
Preventing Loops with Spanning Tree
Solution: Enabling BPDU guard
Bridge Protocol Data Unit = BPDU
Configuring Errdisable Port State Recovery --> Optional
Default recovery interval is 300
Troubleshooting Note: It is strongly recommended to use the bpduguard when portfast is configured. The bpduguard put the port in errdisable state when "listening" on port BPDU frames that had been set to portfast.
Troubleshooting Note: It is strongly recommended that new switches that are used in the production network are initially configured in "transparent" mode. Before you create VLANs, you must decide whether to use VTP in your network. Using VTP, you can make configuration changes centrally on one or more switches and have those changes automatically communicated to all the other switches in the network. Without VTP, you cannot send information about VLANs to other switches.
Configuring Switchport mode
Troubleshooting Note: It is strongly recommended not to leave any port configured as dynamic, this will prevent future unwanted occurrence of Trunks. Always configure all ports as "access" and then change to "trunk" ports that need to be configured as trunk. For the trunk to run smoothly, necessarily, both devices must be configured in the same VTP domain.
NOTE: When setting the mode of the port, the DTP protocol is disabled.
Troubleshooting Note: There are switches that do not have "switchport trunk encapsulation" command, these switches utilize dot1q protocol by default and do not support ISL protocol.
Using PAgP Protocol to configure EtherChannel
auto - passive negotiation of the channel
desirable - active negotiation of the channel
on - no protocols are used: it assumes the other side has also enabled link aggregation
Troubleshooting Note: Before configuring EtherChannel between switches, you must first put the ports in "administrative down" by using the "shutdown" command. Otherwise, the other Switch (which has not been configured) will detect loop, since the first device configured in etherchannel disclose the MAC ADDRESS of the first port on all ports that comprise the channel group. All ports that belong to a channel group must have the same settings regarding the speed and transmission mode.
It cannot be stressed too highly that you need to document your troubleshooting. Write down what you do all the way along – it will help you in the future with similar problems, and can also help other people help you.
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