IEEE 802.3ah OAM

The EFM-OAM protocol provides the ability to monitor the link for error conditions that may indicate the link is starting to degrade or has reached an error rate that exceeds an acceptable threshold.

Link monitoring can be enabled for three types of frame errors; errored-frame, errored-frame-period, and errored-frame-seconds. The errored-frame monitor is the number of frame errors compared to the threshold over a window of time. The errored-frame-period monitor is the number of frame errors compared to the threshold over a window of number of received packets. This window is checked once per second to see if the window command option has been reached. The errored-frame-seconds monitor is the number of errored seconds compared to the threshold over a window of time. An errored second is any second with a single frame error.

An errored frame is counted when any frame is in error as determined by the Ethernet physical layer, including jabbers, fragments, FCS or CRC and runts. This excludes jumbo frames with a byte count higher than 9212, or any frame that is dropped by the physical layer before reaching the monitoring function.

Each frame error monitor functions independently of other monitors. Each of monitor configuration includes an optional signal degrade threshold , a signal failure threshold, a window and the ability to communicate failure events to the peer by setting a Flag field in the Information OAMPDU or the generation of the Event Notification OAMPDU, event-notification. The command options are uniquely configurable for each monitor.

Use the following commands to configure a signal degrade or signal failure threshold:

• MD-CLI

  configure port ethernet symbol-monitor signal-degrade threshold
  configure port ethernet symbol-monitor signal-failure threshold

• classic CLI

  configure port ethernet symbol-monitor sd-threshold
  configure port ethernet symbol-monitor sf-threshold
  

A degraded condition is raised when the configured signal degrade threshold is reached. This provides a first level log only action indicating a link could become unstable. This event does not affect the port state. The critical failure condition is raised when the configured signal failure threshold is reached. By default, reaching the signal failure threshold causes the port to enter the Link Up condition unless the local signal failure has been modified to a log-only action. Signal degrade conditions for a monitor in signal failed state is suppressed until the signal failure has been cleared.

Use the following command to configure a local signal failure action.

configure port ethernet efm-oam link-monitoring local-sf-action local-port-action

The initial configuration or the modification of either of the threshold values take effect in the current window. When a threshold value for a monitor is modified, all active local events for that specific monitor are cleared. The modification of the threshold acts the same as the clear command described later in this section.

Notification to the peer is required to ensure the action taken by the local port detecting the error and its peer are synchronized. If peers do not take the same action then one port may remain fully operational while the other enters a non-operational state. These threshold crossing events do not shutdown the physical link or cause the protocol to enter a non-operational state. The protocol and network element configuration is required to ensure these asymmetrical states do not occur. There are two options for exchanging link and event information between peers; Information OAMPDU and the Event Notification OAMPDU.

As discussed earlier, the Information OAMPDU conveys link information using the Flags field; dying gasp, critical link and link fault. This method of communication has a number of significant advantages over the Event Notification OAMPDU. The Information OAMPDU is sent at every configured transmit-interval. This allows the most recent information to be sent between peers, a critical requirement to avoid asymmetrical forwarding conditions. A second major advantage is interoperability with devices that do not support Link Monitoring and vendor interoperability. This is the lowest common denominator that offers a robust communication to convey link event information. Because the Information OAMPDU is already being sent to maintain the peering relationship this method of communication adds no additional overhead. The local-sf-action options allow the dying gasp and critical event flags to be set in the Information OAMPDU when a signal failure threshold is reached. It is suggested that this be used in place of or in conjunction with Event Notification OAMPDU.

Event Notification OAMPDU provides a method to convey very specific information to a peer about various Link Events using Link Event TLVs. A unique Event Notification OAMPDU is generated for each unique frame error event. The intention is to provide the peer with the Sequence Number, Event Type, Timestamp, and the local information that caused the generation of the OAMPDU; window, threshold, errors and error running total and event running total specific to the port.

• sequence number

  The unique identification indicating a new event.

• window

  The size of the unique measurement period for the error type. The window is only checked at the end. There is no mid-window checking.

• threshold

  The value of the configured sf-threshold.

• errors

  The errors counted in that specific window.

• error running total

  The number of errors accumulated for that event type since monitoring started and the protocol and port have been operational or a reset function has occurred.

• event running total

  The number of events accumulated for that event type since the monitoring started and the protocol and port have been operational.

By default, the Event Notification OAMPDU is generated by the network element detecting the signal failure event. The Event Notification OAMPDU is sent only when the initial frame event occurs. No Event Notification OAMPDU is sent when the condition clears. A port that has been operationally affected as a result of a Link Monitoring frame error event must be recovered manually. The typical recovery method is to administratively disable the port and administratively enable the port. This clears all events on the port. Any function that affects the port state, physical fiber pull, soft or hard reset functions, protocol restarts, and so on, also clears all local and remote events on the affected node experiencing the operation. None of these frame errors recovery actions cause the generation of the Event Notification OAMPDU. If the chosen recovery action is not otherwise recognized by the peer and the Information OAMPDU Flag fields have not been configured to maintain the current event state, there is a high probability that the ports have different forwarding states, notwithstanding any higher level protocol verification that may be in place.

A burst of between one and five Event Notification OAMPDU packets may be sent. By default, only a single Event Notification OAMPDU is generated, but this value can be changed under the local-sf-action context. An Event Notification OAMPDU is only processed if the peer had previously advertised the EV capability. The EV capability is an indication the remote peer supports link monitoring and may send the Event Notification OAMPDU.

The network element receiving the Event Notification OAMPDU uses the values contained in the Link event TLVs to determine if the remote node has exceeded the failure threshold. The locally configured action determines how and if the local port is affected. By default, processing of the Event Notification OAMPDU is log only and does not affect the port state. By default, processing of the Information OAMPDU Flag fields is port affecting. When Event Notification OAMPDU has been configured as port affecting on the receiving node, action is only taken when errors are equal to or above the threshold and the threshold value is not zero. No action is taken when the errors value is less than the threshold or the threshold is zero.

Symbol error monitoring is also supported but requires specific hardware revisions and the appropriate code release.

Use the following command to configure symbol error handling.

configure port ethernet efm-oam link-monitoring errored-symbols

The symbol monitor differs from the frame error monitors. Symbols represent a constant load on the Ethernet wire whether service frames are present or not. This means the optional signal degrade threshold has an additional purpose when configured as part of the symbol error monitor. When the signal degrade threshold is not configured, the symbol monitor acts similar to the frame error monitors, requiring manual intervention to clear a port that has been operationally affected by the monitor. When the optional signal degrade threshold is configured, it again represents the first level warning. However, it has an additional function as part of the symbol monitor. If a signal failure event has been raised, the configured signal degrade threshold becomes the equivalent to a lowering threshold. If a subsequent window does not reach the configured signal degrade threshold then the previous event is cleared and the previously affected port is returned to service without user intervention. This return to service automatically clears any previously set Information OAMPDU Flags fields set as a result of the signal failure threshold. The Event Notification OAMPDU is generated with the symbol error Link TLV that contains an error count less than the threshold. This indicates to the peer that initial problem has been resolved and the port should be returned to service.

The errored-symbol window is a measure of time that is automatically converted into the number of symbols for that specific medium for that period of time. The standard MIB entries ‟dot3OamErrSymPeriodWindowHi” and ‟dot3OamErrSymPeriodWindowLo” are marked as read-only instead of read-write. These values cannot be configured directly. The configuration of the window converts the time and programs the two MIB values in an appropriate manner.

Use the following command to configure the symbol error window.

configure port ethernet efm-oam link-monitoring errored-symbols window

Use the following command to display both the configured window and the number of symbols.

show port 1/1/1 ethernet efm-oam

===============================================================================
Ethernet Oam (802.3ah)
===============================================================================
Admin State        : up
Oper State         : operational
Mode               : active
Pdu Size           : 1518
Config Revision    : 0
Function Support   : LB
Transmit Interval  : 1000 ms
Multiplier         : 5
Hold Time          : 0
Tunneling          : false
Loop Detected      : false
Grace Tx Enable    : true (inactive)
Grace Vendor OUI   : 00:16:4d
Dying Gasp on Reset: true (inactive)
Soft Reset Tx Act  : none
Trigger Fault      : none
Vendor OUI         : 00:16:4d (alu)
Vendor Info        : 00:01:00:02
Peer Mac Address   : d8:1c:01:02:00:01
Peer Vendor OUI    : 00:16:4d (alu)
Peer Vendor Info   : 00:01:00:02
Peer Mode          : active
Peer Pdu Size      : 1518
Peer Cfg Revision  : 0
Peer Support       : LB
Peer Grace Rx      : false
Loopback State     : None
Loopback Ignore Rx : Ignore
Ignore Efm State   : false
Link Monitoring    : disabled
Peer RDI Rx
  Critical Event   : out-of-service
  Dying Gasp       : out-of-service
  Link Fault       : out-of-service
  Event Notify     : log-only
Local SF Action                         Discovery
  Event Burst      : 1                    Ad Link Mon Cap  : yes
  Port Action      : out-of-service
  Dying Gasp       : disabled
  Critical Event   : disabled
Errored Frame                           Errored Frame Period
  Enabled          : no                   Enabled          : no
  Event Notify     : enabled              Event Notify     : enabled
  SF Threshold     : 1                    SF Threshold     : 1
  SD Threshold     : disabled (0)         SD Threshold     : disabled (0)
  Window           : 10 ds                Window           : 1488095 frames
Errored Symbol Period                   Errored Frame Seconds Summary
  Enabled          : no                   Enabled          : no
  Event Notify     : enabled              Event Notify     : enabled
  SF Threshold     : 1                    SF Threshold     : 1
  SD Threshold     : disabled (0)         SD Threshold     : disabled (0)
  Window (time)    : 10 ds                Window           : 600 ds
  Window (symbols) : 125000000
===============================================================================
Active Failure Ethernet OAM Event Logs
===============================================================================
Number of Logs : 0
===============================================================================
===============================================================================
Ethernet Oam Statistics
===============================================================================
                                                   Input                 Output
-------------------------------------------------------------------------------
Information                                       238522                 238522
Loopback Control                                       0                      0
Unique Event Notify                                    0                      0
Duplicate Event Notify                                 0                      0
Unsupported Codes                                      0                      0
Frames Lost                                                                   0
===============================================================================

clear port ethernet efm-oam events local
clear port ethernet efm-oam events remote

show port ethernet efm-oam event-log

configure port ethernet efm-oam link-monitoring local-sf-action info-notification

The SR OS implementation of the EFM-OAM protocol supports vendor-specific soft reset graceful recovery. This feature is not enabled by default.

A vendor-specific Grace TLV is included in the Information PDU generated as part of the 802.3ah OAM protocol when a network element undergoes an ISSU function. Nodes that support the Soft Reset messaging functions allow the local node to generate the Grace TLV.

The Grace TLV informs a remote peer that the negotiated interval and multiplier should be ignored and the new 900 s timeout interval should be used to timeout the session. The peer receiving the Grace TLV must be able to parse and process the vendor-specific messaging.

This command exists at two levels of the hierarchy: system level and port level. By default, this feature is enabled on the port. At the system level, this command defaults to disabled. To enable this feature, both the port and the system commands must be enabled. If either is not enabled, the combination does not allow those ports to generate the vendor specific Grace TLV. This feature must be enabled at both the system and port level before the ISSU or soft reset function. If this feature is enabled during a soft reset or after the ISSU function is already in progress, it has no affect during that window. Both Passive and Active 802.3ah OAM peers can generate the Grace TLV as part of the informational PDU.

There is no CLI command to enable this feature on the receiving node. As long as the receiver understands and can parse the Grace TLV, it enters the grace mode of operation.

The following basic protocol flows demonstrate the interaction between passive/active and active/active peer combinations supporting the Grace TLV. In the following figure, the passive node is entering an ISSU on a node that supports soft reset capabilities.

In the following figure, the active node is experiencing the ISSU function on a node that supports soft reset capabilities.

The difference between Grace TLV passive node with soft reset and Grace TLV active node with soft reset is subtle but important. When an active node performs this function, it generates an Informational TLV with the Local TLV following the successful soft reset. When the active node receives the Information PDU with the Grace Ack, it sends its own Information PDU with both Local and Remote TLV completed, which completes the protocol restart. When a passive node is reset, the passive port waits to receive the 802.3ah OAM protocol before sending its own Information PDU with both the Local and Remote TLV, completing the protocol restart.

The renegotiation process allows the node, which experienced the soft reset, to rebuild the session without restarting the session from the discovery phase. This significantly reduces the native protocol impact on data forwarding.

Any situation that could cause the renegotiation to fail forces the protocol to revert to the discovery phase and fail the graceful restart. During an ISSU when the EFM-OAM session is held operational by the Grace function, if the peer MAC address of the session changes, there is no log event raised for the MAC address change.

The vendor-specific grace function benefits are realized when both peers support the transmitting, receiving, and processing of the vendor-specific Grace TLV. In the case of mixed code versions, products, or vendor environments, a standard EFM-OAM message to the peer can be used to instruct the peer to treat the session as failed.

configure port ethernet efm-oam dying-gasp-tx-on-reset

An initial burst of three Informational OAM PDUs is sent using a 1-second spacing, regardless of the protocol interval. The peer may process these flags to affect its port state and take the appropriate action. The control of the local port state where the soft reset is occurring is left to the soft reset function. This EFM-OAM function does not affect local port state. If the peer has acted on the exception flags and affected its port state, the local node must take an action to inform the upstream nodes that a condition has occurred and forwarding is no longer possible. Routing protocols like ISIS and OSPF overload bits are typically used in routed environments to accomplish this notification.

configure port ethernet efm-oam grace-vendor-oui

configure port ethernet efm-oam trigger-fault

This sets the appropriate flag fields in the Information OAM PDU and transitions a previously operational local port to Link Up. Removing this command from the configuration stops the flags from being set and allows the port to return to service, assuming no other faults would prevent this resumption of service. In cases where a port must be administratively shut down, this command can be used to signal a peer using the EFM-OAM protocol, and the session should be considered failed.

These features do not support clearing an IOM that does not trigger a soft reset. IOM clearing is a forceful event that does not trigger graceful protocol renegotiation.

The show commands are enhanced to help users determine the state of the 802.3ah OAM Grace function and whether the peer is generating or receiving the Grace TLV.

Use the following command to view system level information.

show system information

===============================================================================
System Information
===============================================================================
System Name            : system-name
System Type            : 7750 SR-12
System Version         : 11.0r4
System Contact         :
System Location        :
System Coordinates     :
System Active Slot     : A
System Up Time         : 62 days, 20:29:48.96 (hr:min:sec)

…snip… 

EFM OAM Grace Tx Enable: False
===============================================================================