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802.3ah OAM Link fault management feature and simulating Cisco "UDLD" with EX-series Switches

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Article ID: KB13314 KB Last Updated: 03 Mar 2017Version: 2.0
Summary:
The EX-series Switches (EX 3200 and EX 4200) support 802.3ah starting with JUNOS 9.4 software release. This feature provides OAM link fault management for EX-series switches.

The purpose of this article is to provide a workaround to UDLD using 802.3ah feature or LACP in a single member LAG.

Unidirectional link detection is a feature on Cisco catalyst switches which helps detect unidirectional links at layer 2 and allows actions like disabling the port or sending alarms when such a condition is detected. The partners on both ends on the link are configured for udld and they exchange protocol packets at layer 2 by looping each others packets back to the partner. Thus the link partners are sure about the existence of the other end being active and transmitting packets. It helps detect prevent miswiring with fiber cabling, and also catches faulty hardware.


Symptoms:
If the feature is not configured, then the link partners have to rely on the link to go down at layer 1 to determine that the link partner is down. However if the link remains up, but due to a miswiring or the transmit circuitry / hardware failing on one end, that device fails to transmit packets and only receives them, then the other end makes mistakes with spanning-tree calculations. It would only be sending BPDUs and not receiving any and can then incorrectly put its port into forwarding state and cause a bridging loop.

The cisco UDLD feature is documented in rfc 5171. Currently, many switch vendors have their own propreitary implementation of a "UDLD" like feature. However there is no interoperability between such protocols when different vendor switches are inter-connected.
Solution:
 An easy way to acheive a UDLD-like check between switches of any vendors is to use LACP protocol in a single member LAG. The LACP protocol assures that device-id and port-id are exchanged between the switches whose links are inter-connected. This assures that the link stays up and functioning only if the other end is transmitting LACP packets at layer 2, else the local side takes down the LAG link.

NOTE:
9.4 JUNOS Software Release for EX-series Switches Release Note for IEEE 802.3ah Feature:

Ethernet OAM link fault management for EX-series switches-JUNOS software for EX-series switches allows the Ethernet interfaces on these switches to support the IEEE 802.3ah standard for the Operation, Administration, and Maintenance (OAM) of Ethernet in access networks. The standard defines OAM link fault management (LFM). You can configure IEEE 802.3ah OAM LFM on point-to-point Ethernet links that are connected either directly or through Ethernet repeaters. Ethernet OAM provides the tools that network management software and network managers can use to determine how a network of Ethernet links is functioning.

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