Troubleshooting guide

8-6
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 8 Troubleshooting the Cisco Catalyst 6509
Make sure that the administrator has not shut down the ports involved (as mentioned earlier). The
administrator could have manually shut down the port on one side of the link or the other. This link will
not come up until you re-enable the port; check the port status.
Some switches, such as the Catalyst 4000/5000/6000, may shut down the port if software processes
inside the switch detect an error. When you look at the port status, it will read "errDisable". You must
fix the configuration problem and then manually take the port out of errDisable state. Some newer
software versions (CatOS 5.4(1) and later) have the ability to automatically re-enable a port after a
configurable amount of time spent in the errDisable state. Some of the causes for this errDisable state
are:
• EtherChannel Mis-configuration: If one side is configured for EtherChannel and the other is not,
it can cause the spanning tree process to shut down the port on the side configured for EtherChannel.
If you try to configure EtherChannel but the ports involved do not have the same settings (speed,
duplex, trunking mode, etc.) as their neighbor ports across the link, then it could cause the
errDisable state. It is best to set each side for the EtherChannel "desirable" mode if you want to use
EtherChannel. Sections later on talk in depth about configuring EtherChannel.
• Duplex Mismatch: If the switch port receives a lot of late collisions, this usually indicates a duplex
mismatch problem. There are other causes for late collisions: a bad NIC, cable segments that are too
long, but the most common reason today is a duplex mismatch. The full duplex side thinks it can
send whenever it wants to. The half duplex side is only expecting packets at certain times - not at
"any" time.
• BPDU Port-guard: Some newer versions of switch software can monitor if portfast is enabled on a
port. A port using portfast should be connected to an end-station, not to devices that generate
spanning tree packets called BPDUs. If the switch notices a BPDU coming in a port that has portfast
enabled, it will put the port in errDisable mode.
• UDLD: Unidirectional Link Detection is a protocol on some new versions of software that discovers
if communication over a link is one-way only. A broken fiber cable or other cabling/port issues could
cause this one-way only communication. These partially functional links can cause problems when
the switches involved do not know that link is partially broken. Spanning tree loops can occur with
this problem. UDLD can be configured to put a port in errDisable state when it detects a
unidirectional link.
• Native VLAN mismatch: Before a port has trunking turned on, it belongs to a single VLAN. When
trunking is turned on, the port can carry traffic for many VLANs. The port will still remember the
VLAN it was in before trunking was turned on, which is called the native VLAN. The native VLAN
is central to 802.1q trunking. If the native VLAN on each end of the link does not match,
a port will go into the errDisable state.
• Other: Any process within the switch that recognizes a problem with the port can place it in the
"errDisable" state.
Another cause of inactive ports is when the VLAN they belong to disappears. Each port in a switch
belongs to a VLAN. If that VLAN is deleted, then the port will become inactive. Some switches show a
steady orange light on each port where this has happened. If you come in to work one day and see
hundreds of orange lights don't panic; it could be that all the ports belonged to the same VLAN and
someone accidentally deleted the VLAN that the ports belonged to. When you add the VLAN back into
the VLAN table, the ports will become active again. A port remembers its assigned VLAN.