Cisco CCNA Troubleshooting VLANs

Problems with VLANs can be physical or logical.  The problem could be in any of the first three layers of the OSI reference model (physical, data link, network).  A physical problem is typically a duplex or cabling problem.  A logical problem is typically a configuration problem either in the router/switch configuration or with the VLAN configuration.

Some good troubleshooting commands are as follows:

show vlan

show mac address-table

show interfaces switchport

show interfaces trunk

Cisco CCNA Troubleshooting VLAN Question

Hosts A, B and C are all configured on different subnets.  Everything from the hosts perspective looks to be configured correctly.  Note that host A is on VLAN1 while host B and C are on VLANs 32 and 33 respectively.  Since host A can ping the switch, the only possible answer could be that the switch has an IP address define on VLAN 1 but does not have an “ip default-gateway” assigned.

Cisco CCNA Troubleshooting Trunk Lines

When devices cannot establish a connection across a trunk link, both sides of the link need to be looked at.  Verify settings on both sides of the link as they need to be in synch, otherwise the trunk may not come up.

 

•Make sure:
•The Layer 2 interface mode configured on both ends of the link is valid.
•The trunk encapsulation type configured on both ends of the link is valid.
•The native VLAN is the same on both ends of the trunk (802.1Q trunks).
•Allowed VLANs are the same on both ends of the trunk.

 

Cisco CCNA Troubleshooting Trunk Interfaces

To verify trunk configuration use one of the following two commands:

To display the administrative and operational status of a switching (nonrouting) port, use the show interfaces switchport command.

show interfaces

[interface-id] switchport [module mod]

interface-id – (Optional) Interface ID for the physical port.

module mod – (Optional) Limits the display to interfaces on the specified module; valid values are from 1 to 6.

To display port and module interface-trunk information, use the show interfaces trunk command.

show interfaces trunk [module mod]

module mod – (Optional) Limits the display to interfaces on the specified module; valid values are from 1 to 6.

Cisco CCNA Troubleshooting Switchport

When the configuration is different on each side, the status will show up or connected and traffic on the native VLAN will work fine but other VLANs on the trunk will not pass traffic.

An incorrect trunking configuration (different on each side) results in an operational state of trunk on one switch while the other switch has an operational state of static access