How to Configure IEEE 802.3ah-compliant LFM in Junos

Ethernet troubleshooting tools include a complete set of standards for operations, administration, and maintenance (OAM). One group of OAM tools is simple link fault management (LFM).

LFM operates at the Ethernet frame level and sets thresholds for frame error counts and other link-level events. Once triggered, LFM can create a syslog entry, declare the link down (and unusable), send a critical event trap to a management system, or all of the above. LFM operates on a link-by-link basis and is suitable for networks with many point-to-point Ethernet links.

LFM provides the following functions on a point-to-point Ethernet link:

  • Unidirectional or bidirectional detection of physical link failures

  • Loopback for remote port diagnostics

  • Error reporting and recording for link layer frame and symbol errors

In this example, you configure LFM to run on a point-to-point Gigabit Ethernet link between a customer edge (CE) router and a provider edge (PE) router. If the link fails, LFM detects the fault, and the interfaces on both ends are marked Link-Layer-Down. Other subsystems, such as routing, will react to the outage.

The LFM PDU interval is set to 1000 milliseconds (the default: the range is from 100 to 1000 milliseconds). Higher values mean faster reaction times, but they add traffic to the link. The LFM lost PDU threshold is set to 3 (also the default: the range is from 3 to 10 PDUs). Lower values react faster but do not tolerate transient conditions well.

The configuration also allows both sides to put each other into loopback if the other end requests this action. Both sides must be configured to allow this negotiation and acceptance to take place.

Configuring Ethernet LFM with remote loopback on a span.
Configuring Ethernet LFM with remote loopback on a span.

Configuration for the CE router

user@CE_router>set interfaces ge-1/0/1 unit 0 family inet address 10.11.11.1/24;
user@CE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/1 pdu-interval  1000
user@CE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/1 pdu-threshold 3
user@CE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/1 negotiation-options allow-remote-loopback
user@CE_router>show protocols oam etherent link-fault-management
interface ge-1/0/1 {
              pdu-interval 1000;
              pdu-threshold 3;
              negotiation-options {
                 allow-remote-loopback;
              }
}

Configuration for the PE router

user@PE_router>set interfaces ge-1/0/2 unit 0 family inet address 10.11.11.2/24;
user@PE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/2 pdu-interval  1000
user@PE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/1 pdu-threshold 3
user@PE_router>set protocols oam ethernet link-fault-management interface 
       ge-1/0/2 negotiation-options allow-remote-loopback
user@CE_router>show protocols oam etherent link-fault-management
interface ge-1/0/2 {
    pdu-interval 1000;
    pdu-threshold 3;
    negotiation-options {
        allow-remote-loopback;
    }
}

Note: The interval and threshold parameters do not have to match in both directions, but this practice results in more predictable bidirectional behavior.

If the link fails, the status of the interface changes to declare the link failed:

user@router> show oam ethernet link-fault-management brief
Interface: ge-1/0/1
Status: Fail, Discovery state: Send Any
Peer address: 00:90:69:72:2c:83
Flags: Link-Layer-Down
Remote loopback status: Disabled on local port, Enabled on peer port
Remote entity information:
Remote MUX action: discarding, Remote parser action: loopback
Discovery mode: active, Unidirectional mode: unsupported
Remote loopback mode: supported, Link events: supported
Variable requests: unsupported
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