Datasheet

All contents are Copyright © 1992--2002 Cisco Systems, Inc. All rights reserved. Important Notices and Privacy Statement.
Page 5 of 10
QoS 802.1p Classification
QoS evaluates received frames as either classified or unclassified:
• Frames with CoS values are called classified frames.
• Frames without CoS values are called unclassified frames.
On each port, QoS applies the port’s CoS value (the default is zero) to unclassified frames or, if the port is untrusted, QoS applies
the port's CoS value to all frames.
Before entering the receive queue, all frames have either a received CoS value or the applied port CoS value.
Unclassified frames received on trusted ports and all frames received on untrusted ports may be assigned a CoS value.
In addition, a CoS value may be assigned to all frames, from both trusted and untrusted ports, destined for a particular host
destination MAC address and VLAN number value pair.
Mapping 802.1Q VLAN IDs to ISL VLAN IDs
IEEE 802.1Q VLAN trunkssupport VLANs 1 through 4095.ISL VLAN trunks support VLANs1 through 1024 (1005 to1024 are
reserved).The switch automaticallymaps 802.1Q VLANs1000 and lower toISL VLANs withthe same number.This feature allows
the administrator to define mappings of 802.1Q VLANs above 1000 to ISL VLANs.
Note: You can map up to eight VLANs. Only one 802.1Q VLAN can be mapped to a specific ISL VLAN. For example, if 802.1Q
VLAN 800 has been automatically mapped to ISL VLAN 800, do not manually map any other 802.1Q VLANs to ISL VLAN 800.
Uni-Directional Link Detection for Fiber Links
UDLD is a new-link layer protocol developed by Cisco to detect unidirectional connectivity on network links. The UDLD protocol
allows devices connected through fiber-optic Ethernet,Fast Ethernet, and Gigabit Ethernet to monitorthe physicalconfiguration of
the cables and detect when a unidirectional link or a self loop exists. When a link error is detected, UDLD shuts down the affected
port and alerts the user.
A unidirectional link occurs when traffic transmitted by one device over a link is received by the neighbor, but traffic from the
neighbor is not received. Unidirectional links may cause a variety of problems difficult to troubleshoot , including spanning-tree
topology loops.
UDLD Hello packets are sent periodically to neighbor devices connected through fiber-optic links to keep each device informed
about its neighbors. When a Hello message is received, it is cached and kept in memory for a defined time interval, called holdtime,
after which the cache entry is considered stale and is aged out. If a new Hello message is received when a correspondent old cache
entry has not been aged out yet, then the old entry is dropped and replaced by the new one with a reset time-to-live timer.
If a switch running the UDLD protocol receives UDLD hello packets from a neighbor switch, but these packets fail to contain
the proper UDLD neighbor information, the link is flagged as unidirectional and the port is shut down. Note that for UDLD to
work properly, both switches need to be configured for UDLD. If both switches on either side of the fiber optic link are not
configured for UDLD, the protocol will not shut down the port. UDLD is configured on either a global or per-port basis. It is
supported on all Catalyst 6000 Family switches and will be supported on the Catalyst 5000 Family switches in an upcoming
software release. UDLD is disabled globally on the switch by default. Once enabled globally, it will then be enabled on all Ethernet
line cards that use fiber media (for example, 100BaseFX, 1000BaseSX, and so on.).
Figures 1-1, 1-2, and 1-3 show examples of link-error conditions UDLD discovers.
In Figure 1-1, Switch A successfully receives traffic from Switch B on the fiber-optic port. However, due to either a faulty fiber
optic cable, a faulty transmitter in Switch A, or receive port failure in Switch B, Switch B does not receive traffic from Switch A on
the same port. The UDLD protocol running on Switch A will signal an error condition and disable the port.