UDLD, UniDirectional Link Detection

Description Glossary RFCs Publications Obsolete RFCs

Description:

Protocol suite: TCP/IP.
Type:Data link layer protocol.
Destination MAC:01-00-0C-CC-CC-CC.
MIME subtype:
SNMP MIBs:
Working groups:
Links:

A proprietary protocol developed by Cisco, Inc.

RFC 5171:

UDLD is meant to be a Layer 2 detection protocol that works on top of the existing Layer 1 detection mechanisms defined by the IEEE standards. For example, the Far End Fault Indication (FEFI) function for 100BaseFX interfaces and the Auto-Negotiation function for 100BaseTX/1000BaseX interfaces represent standard physical-layer mechanisms to determine if the transmission media is bidirectional. The typical case of a Layer 1 "fault" indication is the "loss of light" indication.


MAC header 802.2 LLC header UDLD header Data :::

IEEE 802.2 header:

0001020304050607 0809101112131415 1617181920212223
DSAP SSAP Control
Organization
Protocol  

DSAP, Destination Service Access Point. 8 bits.
Always set to 0xAA.

SSAP, Source Service Access Point. 8 bits.
Always set to 0xAA.

Control. 8 bits.
Always set to 0x03.

Organization. 24 bits.
Always set to 0x00000C.

Protocol. 16 bits.
Always set to 0x0111.

UDLD header:

0001020304050607 0809101112131415 1617181920212223 2425262728293031
Version Opcode Flags Checksum
Data :::

Version. 3 bits.
UDLD version number.

Opcode. 5 bits.

OpcodeDescription
0reserved.
1Probe.
2Echo.
3Flush.
4
-
255
reserved.

Flags. 8 bits.

0001020304050607
RT RSY reserved

RT, Recommended timeout flag. 1 bit.

RSY, ReSynch flag. 1 bit.

reserved. 6 bits.

Checksum. 16 bits.
A one's complement of the one's complement sum. The odd byte at the end of an odd length message is used as the low 8 bits of an extra word, rather than as the high 8 bits.


Glossary:


RFCs:

[RFC 5171] Cisco Systems UniDirectional Link Detection (UDLD) Protocol.


Publications:


Obsolete RFCs:


Description Glossary RFCs Publications Obsolete RFCs