About 40-Gbps Interface Speed; Unidirectional Link Detection Parameter - Cisco Nexus 3548 Configuration Manual
Nx-os interfaces release 9x
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About 40-Gbps Interface Speed
• The chassis ID is an optional entry that you can use to address the ports of a connected Fabric Extender.
About 40-Gbps Interface Speed
You can enable 40-Gigabits per second (Gbps) speed on up to 12 interfaces. You enable 40-Gbps speed on
the first port of a group of four adjacent ports. For example, you enable 40-Gbps speed on port 1 of port group
1-4, port 5 of port group 5-8, and port 9 of port group 9-12, and so on. The 40-Gbps port numbering is Ethernet
interface 1/1, 1/5, 1/9, 1/13, 1/17, and so on.
The configuration is applied to the first port, not on the remaining three ports in the group. The remaining
ports act like the ports without an enhanced small form-factor pluggable (SFP+) transceiver inserted. The
configuration takes effect immediately. You do not need to reload the switch.
An SFP+ transceiver security check is performed only on the first port of the group.
Unidirectional Link Detection Parameter
The Cisco-proprietary Unidirectional Link Detection (UDLD) protocol allows ports that are connected through
fiber optics or copper (for example, Category 5 cabling) Ethernet cables to monitor the physical configuration
of the cables and detect when a unidirectional link exists. When the switch detects a unidirectional link, UDLD
shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems,
including spanning tree topology loops.
UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link.
At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that
autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected
LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together
to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
A unidirectional link occurs whenever traffic transmitted by the local device over a link is received by the
neighbor but traffic transmitted from the neighbor is not received by the local device. If one of the fiber strands
in a pair is disconnected, and if autonegotiation is active, the link does not stay up. In this case, the logical
link is undetermined, and UDLD does not take any action. If both fibers are working normally at Layer 1,
then UDLD at Layer 2 determines whether those fibers are connected correctly and whether traffic is flowing
bidirectionally between the correct neighbors. This check cannot be performed by autonegotiation, because
autonegotiation operates at Layer 1.
A Cisco Nexus device periodically transmits UDLD frames to neighbor devices on LAN ports with UDLD
enabled. If the frames are echoed back within a specific time frame and they lack a specific acknowledgment
(echo), the link is flagged as unidirectional and the LAN port is shut down. Devices on both ends of the link
must support UDLD in order for the protocol to successfully identify and disable unidirectional links.
The following figure shows an example of a unidirectional link condition. Device B successfully receives
traffic from Device A on the port. However, Device A does not receive traffic from Device B on the same
port. UDLD detects the problem and disables the port.
Cisco Nexus 3548 Switch NX-OS Interfaces Configuration Guide, Release 9x
4
The chassis ID is configured on a physical Ethernet or EtherChannel interface on the switch to identify
the Fabric Extender discovered through the interface. The chassis ID ranges from 100 to 199.
Configuring Layer 2 Interfaces
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