Ethernet OAM is a protocol for installing, monitoring, and troubleshooting metro Ethernet networks and Ethernet WANs. It relies on a new, optional sublayer in the data link layer of the OSI model. Ethernet OAM can be implemented on any full-duplex point-to-point or emulated point-to-point Ethernet link. A system-wide implementation is not required; OAM can be deployed for part of a system; that is, on particular interfaces.

Normal link operation does not require Ethernet OAM. OAM frames, called OAM protocol data units (PDUs), use the slow protocol destination MAC address 0180.c200.0002. They are intercepted by the MAC sublayer and cannot propagate beyond a single hop within an Ethernet network.

Ethernet OAM is a relatively slow protocol with modest bandwidth requirements. The frame transmission rate is limited to a maximum of 10 frames per second; therefore, the impact of OAM on normal operations is negligible. However, when link monitoring is enabled, the CPU must poll error counters frequently. In this case, the required CPU cycles will be proportional to the number of interfaces that have to be polled.

Two major components, the OAM client and the OAM sublayer, make up Ethernet OAM. The following two sections describe these components.

• OAM Client
  
• OAM Sublayer
  
• Benefits of Ethernet OAM
  

The Cisco IOS implementation of Ethernet OAM consists of the Ethernet OAM shim and the Ethernet OAM module.

The Ethernet OAM shim is a thin layer that connects the Ethernet OAM module and the platform code. It is implemented in the platform code (driver). The shim also communicates port state and error conditions to the Ethernet OAM module via control signals.

The Ethernet OAM module, implemented within the control plane, handles the OAM client as well as control block functionality of the OAM sublayer. This module interacts with the command-line interface (CLI) and Simple Network Management Protocol (SNMP)/programmatic interface via control signals. In addition, this module interacts with the Ethernet OAM shim through OAM PDU flows.

The OAM features as defined by IEEE 802.3ah, Ethernet in the First Mile , are discovery, Link Monitoring, Remote Fault Detection, Remote Loopback, and Cisco Proprietary Extensions.

Ethernet OAM messages or OAM PDUs are standard length, untagged Ethernet frames within the normal frame length bounds of 64 to 1518 bytes. The maximum OAM PDU frame size exchanged between two peers is negotiated during the discovery phase.

OAM PDUs always have the destination address of slow protocols (0180.c200.0002) and an Ethertype of 8809. OAM PDUs do not go beyond a single hop and have a hard-set maximum transmission rate of 10 OAM PDUs per second. Some OAM PDU types may be transmitted multiple times to increase the likelihood that they will be successfully received on a deteriorating link.

Four types of OAM messages are supported:

• Information OAM PDU--A variable-length OAM PDU that is used for discovery. This OAM PDU includes local, remote, and organization-specific information.
• Event notification OAM PDU--A variable-length OAM PDU that is used for link monitoring. This type of OAM PDU may be transmitted multiple times to increase the chance of a successful receipt; for example, in the case of high-bit errors. Event notification OAM PDUs also may include a time stamp when generated.
• Loopback control OAM PDU--An OAM PDU fixed at 64 bytes in length that is used to enable or disable the remote loopback command.
• Vendor-specific OAM PDU--A variable-length OAM PDU that allows the addition of vendor-specific extensions to OAM.

The IEEE 802.3ah Link Fault RFI Support feature provides a per-port configurable option that moves a port into a blocking state when an OAM PDU control request packet is received with the Link Fault Status flag set. In the blocking state, the port can continue to receive OAM PDUs, detect remote link status, and automatically recover when the remote link becomes operational. When an OAM PDU is received with the Link Fault Status flag set to zero or FALSE, the port is enabled and all VLANs configured on the port are set to "forwarding."

Note	If you configure the Ethernet OAM timeout period to be the minimum allowable value of 2 seconds, the Ethernet OAM session may be dropped briefly when the port transitions from blocked to unblocked. This action will not occur by default; the default timeout value is 5 seconds.

Before the release of the IEEE 802.3ah Link Fault RFI Support feature, when an OAM PDU control request packet was received with the Link Fault Status flag set, one of three actions was taken:

• The port was put in the error-disable state, meaning that the port did not send or receive packets, including Bridge Protocol Data Units (BPDU) packets. In the error-disable state, a link can automatically recover after the error-disable timeout period but cannot recover automatically when the remote link becomes operational.
• A warning message was displayed or logged, and the port remained operational.
• The Link Fault Status flag was ignored.

A new keyword, error-block-interface, for the CLI command ethernet oam remote-failure action is introduced with the IEEE 802.3ah Link Fault RFI Support feature. For detailed information about this command, see the Cisco IOS Carrier Ethernet Command Reference.

Ethernet connectivity fault management (CFM) is an end-to-end per-service-instance Ethernet layer OAM protocol that includes proactive connectivity monitoring, fault verification, and fault isolation. End to end can be provider edge (PE) to PE or customer edge (CE) to CE. Per service instance means per VLAN.

For more information about Ethernet CFM, see Ethernet Connectivity Fault Management .

In access and service provider networks using Ethernet technology, High Availability (HA) is a requirement, especially on Ethernet OAM components that manage Ethernet virtual circuit (EVC) connectivity. End-to-end connectivity status information is critical and must be maintained on a hot standby Route Processor (RP) (a standby RP that has the same software image as the active RP and supports synchronization of line card, protocol, and application state information between RPs for supported features and protocols). End-to-end connectivity status is maintained on the CE, PE, and access aggregation PE (uPE) network nodes based on information received by protocols such as Ethernet LMI, CFM, and 802.3ah. This status information is used to either stop traffic or switch to backup paths when an EVC is down. The Non-Stop Forwarding/Stateful Switchover (NSF/SSO) and In Service Software Upgrade (ISSU) support enhancements are introduced and enabled automatically during configuration of the Cisco 7600 router. Metro Ethernet clients (E-LMI, CFM, 802.3ah) maintain configuration data and dynamic data, which is learned through protocols. Every transaction involves either accessing or updating data among the various databases. If the databases are synchronized across active and standby modules, the RPs are transparent to clients.

Cisco IOS infrastructure provides various component application program interfaces (APIs) for clients that are helpful in maintaining a hot standby RP. Metro Ethernet HA clients (Ethernet LMI, HA/ISSU, CFM HA/ISSU, 802.3ah HA/ISSU) interact with these components, update the databases, and trigger necessary events to other components.

• Benefits of 802.3ah HA
  

The redundancy configurations SSO and NSF are both supported in Ethernet OAM and are automatically enabled. A switchover from an active to a standby RP occurs when the active RP fails, is removed from the networking device, or is manually taken down for maintenance. NSF interoperates with the SSO feature to minimize network downtime following a switchover. The primary function of Cisco NSF is to continue forwarding IP packets following an RP switchover.

For detailed information about the SSO feature, see the "Stateful Switchover" chapter of the Cisco IOS High Availability Configuration Guide. For detailed information about the NSF feature, see the "Cisco Nonstop Forwarding" chapter of the Cisco IOS High Availability Configuration Guide.

ISSU allows you to perform a Cisco IOS software upgrade or downgrade without disrupting packet flow. ISSU is automatically enabled in 802.3ah. OAM performs a bulk update and a runtime update of the continuity check database to the standby RP, including adding, deleting, or updating a row. This checkpoint data requires ISSU capability to transform messages from one release to another. All the components that perform active RP to standby RP updates using messages require ISSU support.

ISSU lowers the impact that planned maintenance activities have on network availability by allowing software changes while the system is in service. For detailed information about ISSU, see the "Cisco OS In Service Software Upgrade Process" chapter of the Cisco IOS High Availability Configuration Guide.

1.    enable

2.    configure terminal

3.    interface type number

4.    ethernet oam [max-rate oampdus | min-rate num-seconds| mode {active | passive} | timeout seconds]

5.    exit

Link monitoring is enabled by default when you enable Ethernet OAM. Perform these tasks to disable and enable link monitoring sessions:

• Disabling a Link Monitoring Session
  
• Enabling a Link Monitoring Session
  

Link monitoring operations start automatically when Ethernet OAM is enabled on an interface. When link monitoring operations are stopped, the interface does not actively send or receive event notification OAM PDUs. The tasks in this section describe how to stop and start link monitoring operations.

• Stopping Link Monitoring Operations
  
• Starting Link Monitoring Operations
  

1.    enable

2.    configure terminal

3.    interface type number

4.    ethernet oam [max-rate oampdus | min-rate num-seconds| mode {active | passive} | timeout seconds]

5.    ethernet oam link-monitor high-threshold action error-disable-interface

6.    ethernet oam link-monitor frame {threshold {high {none | high-frames} | low low-frames} | window milliseconds}

7.    ethernet oam link-monitor frame-period {threshold {high {none | high-frames} | low low-frames} | window frames}

8.    ethernet oam link-monitor frame-seconds {threshold {high {none | high-frames} | low low-frames} | window milliseconds}

9.    ethernet oam link-monitor receive-crc {threshold {high {high-frames | none} | low low-frames} | window milliseconds}

10.    ethernet oam link-monitor transmit-crc {threshold {high {high-frames | none} | low low-frames} | window milliseconds}

11.    ethernet oam link-monitor symbol-period {threshold {high {none | high-symbols} | low low-symbols} | window symbols}

12.    exit

Router# configure terminal
 
Enter configuration commands, one per line.  End with CNTL/Z.
 
Router(config)# interface gigabitEthernet 3/8
Router(config-if)#
Router(config-if)# ethernet oam
 
Router(config-if)# ethernet oam link-monitor high-threshold action error-disable-interface
Router(config-if)# ethernet oam link-monitor frame window 399
Router(config-if)# ethernet oam link-monitor frame-period threshold high 599
Router(config-if)# ethernet oam link-monitor frame-seconds window 699
Router(config-if)# ethernet oam link-monitor receive-crc window 99
Router(config-if)# ethernet oam link-monitor transmit-crc threshold low 199
Router(config-if)# ethernet oam link-monitor symbol-period threshold high 299
Router(config-if)# exit
Router# show running-config
 
Building configuration...
Current configuration : 5613 bytes
!
!
version 12.2
!
!
.
.
.
!
!
interface GigabitEthernet3/8
 no ip address
 ethernet oam link-monitor high-threshold action error-disable-interface
 ethernet oam link-monitor frame window 399
 ethernet oam link-monitor frame-period threshold high 599
 ethernet oam link-monitor frame-seconds window 699
 ethernet oam link-monitor receive-crc window 99
 ethernet oam link-monitor transmit-crc threshold low 199
 ethernet oam link-monitor symbol-period threshold high 299
 ethernet oam

1.    enable

2.    configure terminal

3.    template template-name

4.    ethernet oam link-monitor receive-crc {threshold {high {high-frames | none} | low low-frames} | window milliseconds}

5.    ethernet oam link-monitor transmit-crc {threshold {high {high-frames | none} | low low-frames} | window milliseconds}

6.    ethernet oam link-monitor symbol-period {threshold {high {none | high-symbols} | low low-symbols} | window symbols}

7.    ethernet oam link-monitor high-threshold action error-disable-interface

8.    ethernet oam link-monitor frame {threshold {high {none | high-frames} | low low-frames} | window milliseconds}

9.    ethernet oam link-monitor frame-period {threshold {high {none | high-frames} | low low-frames} | window frames}

10.    ethernet oam link-monitor frame-seconds {threshold {high {none | high-frames} | low low-frames} | window milliseconds}

11.    exit

12.    interface type number

13.    source template template-name

14.    exit

15.    exit

16.    show running-config

1.    enable

2.    configure terminal

3.    interface type number

4.    ethernet oam remote-failure {critical-event | dying-gasp | link-fault} action {error-block-interface | error-disable-interface}

5.    exit