IEEE 802.3ah OAM

IEEE 802.3ah Clause 57 (EFM OAM) defines the OAM sublayer, which provides useful mechanisms for monitoring link operation, such as remote fault indication and remote loopback control. In general, OAM provides network operators the ability to monitor the network health and determine the location of failing links or fault conditions. EFM OAM described in this clause provides data link layer mechanisms that complement applications that may reside in higher layers.

OAM information is conveyed in slow protocol frames called OAM PDUs. OAM PDUs contain the control and status information used to monitor, test, and troubleshoot OAM-enabled links. OAM PDUs traverse a single link, passed between peer OAM entities. As a result, they are not forwarded by MAC clients (like bridges or switches).

The following EFM OAM functions are supported:

  • EFM OAM capability discovery – allows the port to discover and advertise their OAM capabilities to the peer

  • operational mode – supports both active and passive modes

  • Remote Failure Indication (RFI) - enables the port to signal to its peer that a critical fault (for example, link fault, dying gasp) has occurred on its local receive path

  • loopback – allows for a data link layer frame-level loopback mode. Both remote and local loopback modes are supported.

  • EFM OAM PDU tunneling – allows PDUs to pass transparently through service provider network

  • high-resolution timer for EFM OAM in 100 ms interval (minimum) – allows a more precise detection of connectivity failures

  • non-zero Vendor Specific Information Field – adds extra device-specific detail to the standard Ethernet OAM protocol. The 32-bit field is encoded using the format 00:PP:CC:CC and references TIMETRA-CHASSIS-MIB:

    • 00 – must be zeros

    • PP – represents the platform type based on the installed IOM from tmnxHwEquippedPlatform.

    • CC:CC – represents the chassis type index value from tmnxChassisType which is indexed in tmnxChassisTypeTable. The table identifies the specific chassis backplane.

The 00:00:00:00 value is sent for all software releases that do not support the non-zero value or are unable to identify the required elements. Peer or local vendor information fields are not decoded on the network element. The hexadecimal value is included in the output of the following command.

show port ethernet efm-oam

When the EFM-OAM protocol fails to negotiate a peer session or encounters a protocol failure following an established session, the Port State enters the Link Up condition. This port state is used by many protocols to indicate the port is administratively UP, has physical connectivity, but the ports operational state is in a DOWN state due to a protocol, such as EFM-OAM. A reason code is added to help discern if the EFM-OAM protocol is the underlying reason for the Link Up condition.

Use the following command to display port information.

show port
===============================================================================
Ports on Slot 1
===============================================================================
Port        Admin Link Port    Cfg  Oper LAG/ Port Port Port   C/QS/S/XFP/
Id          State      State   MTU  MTU  Bndl Mode Encp Type   MDIMDX
-------------------------------------------------------------------------------
1/1/1       Down  No   Down    1578 1578    - netw null xcme
1/1/2       Down  No   Down    1578 1578    - netw null xcme
1/1/3       Up    Yes  Link Up 1522 1522    - accs qinq xcme
1/1/4       Down  No   Down    1578 1578    - netw null xcme
1/1/5       Down  No   Down    1578 1578    - netw null xcme
1/1/6       Down  No   Down    1578 1578    - netw null xcme

Use the following command to display information about a specific port.

show port 1/1/3
===============================================================================
Ethernet Interface
===============================================================================
Description        : 10/100/Gig Ethernet SFP
Interface          : 1/1/3                      Oper Speed       : N/A
Link-level         : Ethernet                   Config Speed     : 1 Gbps
Admin State        : up                         Oper Duplex      : N/A
Oper State         : down                       Config Duplex    : full
Reason Down        : efmOamDown
Physical Link      : Yes                        MTU              : 1522
Single Fiber Mode  : No                         Min Frame Length : 64 Bytes
IfIndex            : 35749888                   Hold time up     : 0 seconds
Last State Change  : 12/18/2012 15:58:29        Hold time down   : 0 seconds
Last Cleared Time  : N/A                        DDM Events       : Enabled
Phys State Chng Cnt: 1

Configured Mode    : access                     Encap Type       : QinQ
Dot1Q Ethertype    : 0x8100                     QinQ Ethertype   : 0x8100
PBB Ethertype      : 0x88e7
Ing. Pool % Rate   : 100                        Egr. Pool % Rate : 100
Ing. Pool Policy   : n/a
Egr. Pool Policy   : n/a
Net. Egr. Queue Pol: default
Egr. Sched. Pol    : n/a
Auto-negotiate     : true                       MDI/MDX          : unknown
Oper Phy-tx-clock  : not-applicable
Accounting Policy  : None                       Collect-stats    : Disabled
Acct Plcy Eth Phys : None                       Collect Eth Phys : Disabled
Egress Rate        : Default                    Ingress Rate     : Default
Load-balance-algo  : Default                    LACP Tunnel      : Disabled

Down-when-looped   : Disabled                   Keep-alive       : 10
Loop Detected      : False                      Retry            : 120
Use Broadcast Addr : False

Sync. Status Msg.  : Disabled                   Rx Quality Level : N/A
Tx DUS/DNU         : Disabled                   Tx Quality Level : N/A
SSM Code Type      : sdh                        ESMC Tunnel      : Disabled

Down On Int. Error : Disabled

CRC Mon SD Thresh  : Disabled                   CRC Mon Window   : 10 seconds
CRC Mon SF Thresh  : Disabled

Configured Address : d8:ef:01:01:00:03
Hardware Address   : d8:ef:01:01:00:03
The user can also decouple the EFM-OAM protocol from the port state and operational state. To remove the protocol, monitor the protocol only, migrate, or make changes, the user should configure the following command. 
configure port ethernet efm-oam ignore-efm-state
When the ignore-efm-state command is configured on a port, the protocol continues as normal. However, any failure in the protocol state machine (discovery, configuration, time-out, loops, and so on) does not impact the port on which the protocol is active and the optional ignore-efm-state command is configured. A protocol warning message is generated only if there are protocol issues. When this optional command is not configured, the default behavior is that the port state is affected by any EFM-OAM protocol fault or clear conditions. Adding and removing this optional ignore-efm-state command immediately represents the Port State and Oper State based on the active configuration. For example, if the ignore-efm-state command is configured on a port that is exhibiting a protocol error, this error does not affect the port state or operational state and there is no Reason Down code. If the ignore-efm-state is removed from a port with an existing EFM-OAM protocol error, the port transitions to Link UP, Oper Down with the reason code efmOamDown.

OAM events

The Information OAMPDU is transmitted by each peer at the configured intervals. This OAMPDU performs keepalive and critical notification functions. Various local conditions are conveyed through the setting of the Flags field. The following Critical Link Events, defined in IEEE 802.3 Section 57.2.10.1, are supported:

  • link fault – the PHY has determined a fault has occurred in the receive direction of the local DTE

  • dying gasp – an unrecoverable local failure condition has occurred

  • critical event – an unspecified critical event has occurred

The local node can set and unset the various Flag fields based on the operational state of the port, shutdown or activation of the EFM-OAM protocol, or locally raised events. These Flag fields maintain the setting for a specific event. Changing port conditions, protocol state, or user intervention may impact the setting of these fields in the Information OAMPDU.

A peer processing the Information OAMPDU can take a configured action when one or more of these Flag fields are set. By default, receiving a set value for any of the Flag fields causes the local port to enter the Link Up port state and an event is logged. To bypass the default behavior and log the event without affecting the local port, use options under the following context, configurable per Flag field.
configure port ethernet efm-oam peer-rdi-rx

Remote loopback

EFM OAM provides a link-layer frame loopback mode that can be remotely controlled.

To initiate remote loopback, the local EFM OAM client sends a loopback control OAM PDU by enabling the OAM remote-loopback command. After receiving the loopback control OAM PDU, the remote OAM client places the remote port in local loopback mode.

To exit remote loopback, the local EFM OAM client sends a loopback control OAM PDU by disabling the OAM remote-loopback command. After receiving the loopback control OAM PDU, the remote OAM client places the port back in normal forwarding mode.

During a remote loopback test operation, all frames except EFM OAM PDUs are dropped at the local port for the receive direction, where remote loopback is enabled. To forward the received looped back non-EFM frames must be forwarded, use the following command to enable forwarding:

  • MD-CLI
    configure port ethernet efm-oam remote-loopback-forward-non-efm-frames true
  • classic CLI
    configure port ethernet efm-oam remote-lb-fwd-non-efm-frames true

If local loopback is enabled, all frames received on the port are looped back, and any frames generated or forwarded by the node are dropped in the transmit direction. This behavior may result in some protocols (for example, STP or LAG) resetting the state machines.

When a port is in loopback mode, service mirroring does not work if the port is a mirror-source or a mirror-destination.

802.3ah OAM PDU tunneling for Epipe service

Nokia routers support 802.3ah. Customers who subscribe to Epipe service treat the Epipe as a wire, and expect the ability to run 802.3ah between their devices located at either end of the Epipe.

This feature applies only to port-based Epipe SAPs because 802.3ah runs at the port level, not at the VLAN level. These ports must be configured as null encapsulated SAPs.

When OAM PDU tunneling is enabled, 802.3ah OAM PDUs received at one end of an Epipe are forwarded through the Epipe. 802.3ah can run between devices that are located at either end of the Epipe. When OAM PDU tunneling is disabled (the default setting), OAM PDUs are either dropped or processed locally, according to the EFM-OAM configuration state.

Enabling 802.3ah and OAM PDU tunneling on the same port are mutually exclusive operations.