Operations, Administration, and Management (OAM) of ATM Interfaces
ATM interfaces support the OAM standards of the ITU, per recommendation I.610. OAM provides VC/VP integrity and fault and performance management.
The system supports F4 and F5 ATM OAM fault management, loopback, and continuity check (CC) cells. These cells perform fault detection and notification, loopback testing, and link integrity. ATM uses F4 and F5 cell flows as follows:
Note that ATM interfaces always generate and validate CRC-10 checksums on OAM cells.
End-to-End and Segment Endpoints
An ATM connection consists of a group of points. This OAM implementation provides management for the following points:
- Connection endpoint—the end of a VC/VP connection where the ATM cells are terminated
- Segment endpoint—the end of a connection segment
Fault Management
ATM uses fault management cells to convey defect information to the endpoints of a VP/VC. There are two types of fault management cells:
- Alarm indication signal (AIS) cells, which are used to indicate a fault to the downstream endpoint. AIS cells contain defect type and defect location fields, which may optionally convey information about the type of defect detected and the location of the defect.
- Remote defect indication (RDI) cells, which are received from the remote endpoint of the VP/VC and indicate an interruption in the cell transfer capability of the VP/VC.
Connecting points in the VP/VC that detect a fault send AIS cells in the downstream direction to the endpoint of the VP/VC. Upon receipt of AIS cells, the downstream endpoint generates RDI cells in the upstream direction to alert all connecting points and the remote endpoint of an interruption in the cell transfer capability of the VP/VC.
If fault management detects a failure condition (because of arrival of AIS or RDI cells), the system disables the corresponding VC until the fault condition is no longer detected.
How the ATM Interface Handles AIS Cells
Nodes that detect a failure send AIS cells to the downstream endpoint. Since the ATM interface is an endpoint and there is no downstream neighbor to an ATM endpoint, the ATM interface never generates AIS cells. The ATM interface responds to the receipt of AIS cells as follows:
- When an ATM interface receives an F4 or F5 end-to-end AIS cell, it enters the AIS state.
- While in the AIS state, the ATM interface sends F4 or F5 end-to-end RDI cells to the remote endpoint. It sends the RDI cells at the rate of one cell per second for as long as the AIS condition exists.
Note that for all RDI cells sent, the defect type and defect location fields contain the values from the received AIS cells.
- The interface receives an F4 or F5 end-to-end loopback cell or an F4 or F5 CC cell.
- The interface does not receive an AIS cell for three seconds.
- The OAM VC status field of the show atm vc atm command shows whether the circuit is in AIS state.
How the ATM Interface Handles RDI Cells
RDI cells received from the remote endpoint of the VP/VC indicate an interruption in the cell transfer capability of the VP/VC. For example, the remote endpoint of a VC receives an F5 AIS cell, enters the AIS state, and transmits F5 end-to-end RDI cells for the duration of the AIS condition. On receipt of an F4 or F5 end-to-end RDI cell, the ATM interface declares an RDI state but does not generate OAM fault management cells in response to the condition. The ATM interface leaves the RDI condition when no RDI cells have been received for three seconds.
The OAM VC status field of the show atm vc atm command shows whether the circuit is in RDI state.
Continuity Check
CC cells provide continual monitoring of a connection on a segment or end-to-end basis. To check the integrity of the link, you can set up a VP or VC to regularly send or receive CC cells at either the segment level or at the end-to-end level.
The CC cell source generates the CC cells, and the sink receives and processes the cells. You can set up a VP or VC as the source, the sink, or both the source and the sink. If you enable a VP or VC as a CC cell source, it generates CC cells. The VP or VC counts CC cells whether or not CC cell flow is enabled. You can enable CC cells only on data circuits, not on control circuits, such as ILMI or signalling circuits.
Activation and Deactivation Cells
To enable and disable CC cell flows, ATM OAM uses activation and deactivation cells:
- To enable a CC cell flow, the system sends activation OAM cells to the peer. The peer replies with a confirmation or denial. If the CC sink point is not activated, all received CC cells are dropped. (See the next section for more details.)
- To disable a CC cell flow, the system sends deactivation OAM cells to the peer. The peer replies with a confirmation or denial.
Activating CC Cell Flow
When the system sends a CC activation cell to the peer, one of the following occurs:
- If the system receives a positive response (Activation Confirmed), the VC or VP goes to CC active state, and CC is enabled on the VC or VP.
- If the system receives a negative response (Activation Req. Denied), the VC or VP goes to CC failed state, and CC is not enabled on the VC or VP.
- If the system does not receive a response within 5 seconds, it sends another activation cell. This process is repeated three times. If the system does not receive a response, it stops the activation process.
If the VC or VP is the source point, CC cell generation starts as soon as the system sends the activation request to the peer. CC cell generation stops if the CC fails, when the maximum number of retries is reached, or when the deactivation process is complete.
Deactivating CC Cell Flow
In the case of a deactivation request, the process is the same except that deactivation cells are sent instead of activation cells.
Also, the atm oam flush command causes the system to send a deactivation request to the peer and suspend all CC operations. Therefore, it is recommended that you disable CC cell generation on all VCs before entering atm oam flush.
Once CC Cell Flow is Enabled
If the VC or VP is set up as the source point, the ATM interface sends one CC cell per second. CC cell generation stops if one of the following conditions occur:
- The ATM interface goes down.
- OAM CC is disabled on the interface with the atm pvc command.
- The peer deactivates the OAM CC cell flow.
- OAM cell reception is disabled on the interface with the atm oam flush command.
If the VP is set up as a CC sink point and no CC cell is received for 4 seconds, the VP goes to AIS state and sends one RDI cell per second.
To view the current state of the activation or deactivation process including statistics, use the show atm oam command for VPs and use the show atm vc atm interface command for VCs.
Loopback
You can use loopback cells to verify connectivity between VP/VC endpoints, as well as segment endpoints within the VP/VC. You can use these tests to perform fault isolation over the VP/VC.
The ATM interface supports VC integrity, which generates F5 end-to-end loopback cells. It also supports ATM ping, which generates F4 and F5 segment and end-to-end loopback cells to test the reachability of an endpoint or a segment endpoint.
VC Integrity
VC integrity is used to monitor the operational status of an individual VC, which provides continuous ATM VC-layer connectivity verification. It involves periodically sending F5 end-to-end loopback cells on individual PVCs to verify end-to-end connectivity.
If enabled, the peer ATM host must respond to the system's loopback cells, or the circuit will be disabled. The ATM interface will not reenable the circuit until it receives loopback responses or until local VC integrity is disabled.
To set up the ATM interface to transmit F5 end-to-end loopback cells over a VC, use the oam keyword with the atm pvc command. To send F5 segment loopback cells, use the ATM ping mechanism described later in this chapter.
F5 loopback receive and transmit statistics are available with the show atm vc atm command.
F4 OAM Cells
You can generate F4 loopback cells using the atm oam command or the ATM ping mechanism. F4 loopback receive and transmit statistics are available with the show atm oam command and include statistics on incoming and outgoing F4 end-to-end and segment loopback cells.
ATM Ping
ATM ping lets you check whether a connection endpoint or segment point can be reached on a VC or VP. ATM ping uses F4 and F5 loopback cells and as such is supported only for data circuits and not control circuits (ILMI, signalling circuits). To generate:
- F5 segment or end-to-end loopback cells, issue the ping atm command on a VC.
- F4 segment or end-to-end loopback cells, issue the ping atm command on a VP.
You can specify the number of loopback cells that are sent, the location ID, and the timer value. Once the interface sends the loopback cells, the timer is started and the interface waits for a response. On receiving the loopback response (or when the timer expires) the ATM interface sends the next cell. This operation is repeated for the number of cells specified.
Because F4 and F5 are OAM cells, disabling receipt of OAM cells (using the atm oam flush command) stops all outstanding ping operations. You need to manually restart the ping operation once receipt of OAM cells for the interface is enabled.
How the ATM Interface Handles Loopback Cells Received
The ATM interface responds to received F4 and F5 loopback cells as shown in Table 1-7.