Operations, Administration, and Management 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 E Series router 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:

ATM interfaces always generate and validate CRC-10 checksums on OAM cells.

For information about configuring OAM on the router, see the following sections:

End-to-End and Segment Endpoints

An ATM connection consists of a group of points. This OAM implementation provides management for the following points:

Fault Management

ATM uses two types of fault management cells to convey defect information to the endpoints of a 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 router 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. Because 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:

  1. When an ATM interface receives a configurable number of F4 or F5 AIS cells, it enters the AIS state.
  2. While in the AIS state, the ATM interface sends F4 or F5 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.

    For all RDI cells sent, the defect type and defect location fields contain the values from the received AIS cells.

  3. RDI cell generation stops when one of the following conditions occurs:
    • The interface receives an F4 or F5 loopback cell or an F4 or F5 CC cell.
    • The interface does not receive an AIS cell for a configurable time period.
    • The OAM VC status field of show atm vc atm shows that 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 RDI cells for the duration of the AIS condition. On receipt of a configurable number of F4 or F5 RDI cells, 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 a configurable time period.

The OAM VC status field of show atm vc atm shows whether the circuit is in RDI state.

Continuity Verification

CC cells provide continual monitoring of a connection on a segment or end-to-end basis. To verify 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 signaling circuits.

Activation and Deactivation Cells

To enable and disable CC cell flows, ATM OAM uses activation and deactivation cells:

Activating CC Cell Flow

When the router sends a CC activation cell to the peer, one of the following occurs:

If the VC or VP is the source point, CC cell generation starts as soon as the router 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

The process of sending a deactivation request is the same as for activation cells except that deactivation cells are sent instead.

Also, the atm oam flush command causes the router to send a deactivation request to the peer and suspend all CC operations. Therefore, we recommend that you disable CC cell generation and transmission on all VCs before issuing atm oam flush.

After 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:

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 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. VC integrity provides continuous ATM VC-layer connectivity verification by periodically sending F5 end-to-end loopback cells on individual PVCs to verify end-to-end connectivity. You can set the frequency with which loopback cells are transmitted for an individual VC.

If VC integrity is enabled, the peer ATM host must respond to the router’s loopback cells, or the circuit will be disabled. The ATM interface does not reenable the circuit until it receives loopback responses or until local VC integrity is disabled.

You can set the following VC integrity parameters for an individual VC with the oam retry command. For more information, see oam retry.

VC integrity is a best-effort mechanism that tries to adhere to the loopback cell transmission frequency and retry frequency values configured for each VC without consuming excessive processing time on the line module. When you configure VC integrity for a large number of circuits on the line module, delays in transmitting OAM loopback cells might occur so new subscribers can connect and to maintain existing subscriber connections.

To set up the ATM interface to transmit F5 end-to-end loopback cells over a VC, use the oam keyword and an optional frequency with the atm pvc command. To send F5 segment loopback cells, use the ATM ping mechanism, described in ATM Ping .

F5 loopback receive and transmit statistics are available with show atm vc atm.

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

With ATM ping you can verify whether a connection endpoint or segment point can be reached on a VC or VP. ATM ping uses F4 and F5 loopback cells and is supported only for data circuits and not control circuits (ILMI, signaling circuits). To generate:

You can specify the number of loopback cells that are sent, the location ID, and the timer value. After 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 and transmission of OAM cells on the ATM interface (by using the atm oam flush command) stops all outstanding ping operations on the ATM interface. You need to manually restart the ping operation after you enable receipt and transmission of OAM cells for the interface.

How the ATM Interface Handles Loopback Cells Received

The ATM interface responds to received F4 and F5 loopback cells as indicated in Table 6.

Table 6: Handling of F4 and F5 Loopback Cells Received

Loopback Cell Received

ATM Interface Response

F4 and F5 end-to-end loopback cells and segment loopback cells with the loopback location field set to all 1s (ones) and the loopback indication set.

Clears the loopback indication (sets it to all zeros) and loops back the received cell.

F4 and F5 segment loopback cells with the loopback location field set to all 0s (zeros) and the loopback indication set.

Resets the loopback indication and the location ID to all 1s (ones) and loops back the received cells.

F4 and F5 end-to-end loopback cells and segment loopback cells with the loopback location field set to the loopback location ID of the ATM interface and the loopback indication set.

Clears the loopback indication and loops back the received cell without resetting the location ID.

F5 end-to-end loopback cells with the loopback location field set to a value other than all 1s and the loopback location ID of the ATM interface.

Discards the cell.

F5 segment loopback cells with the loopback location field set to other than all 1s (ones), set to all 0s (zeros), or set to the loopback location ID of the ATM interface.

Discards the cell.

Automatic Disabling of F5 OAM Services

The router automatically disables all F5 OAM fault management and VC integrity services configured on a VC when you change the administrative status of the corresponding ATM interface, ATM AAL5 interface, or ATM 1483 subinterface from enabled to disabled.

To set the administrative status of an interface to disabled, use the atm shutdown command (for an ATM interface), the atm aal5 shutdown command (for an ATM AAL5 interface), or the atm atm1483 shutdown command (for an ATM 1483 subinterface). You can also use the shutdown command to disable the interface.

When F5 OAM is disabled, the OAM VC status field in the show atm vc atm command display indicates that the VC is not managed. The VC does not receive or transmit F5 OAM cells while F5 OAM is disabled. For examples of the show atm vc atm command display, see show atm vc atm.

When the corresponding ATM interface, ATM AAL5 interface, or ATM 1483 subinterface is reenabled, the router automatically restores F5 OAM services on the associated VCs.

Note: If you administratively issue the shutdown command on an ATM major interface in which the ATM PVC is configured over a dynamic ATM 1483 subinterface column, or if subscribers on the ATM 1483 subinterfaces log out, the ATM PVC is deleted immediately.

Rate Limiting for F5 OAM Cells

The router implements rate limiting for ATM F5 OAM cells to protect the corresponding ATM interface from denial-of-service (DoS) attacks. The interface discards control packets when the rate of control packets received exceeds the rate limit for ATM interfaces.

An ATM interface has a rate limit control that is non-configurable and always in effect; the rate limit is the same for all ATM interfaces. In addition, each ATM VC maintains its own state and statistics counters for tracking the rate. The rate limit for ATM OAM cells is approximately 5 packets per second.

For an ATM VC, the router increments the InOamCellDiscards statistics counter in the show atm vc atm command display to track the number of OAM cells received on this circuit that were discarded. The InOamCellDiscards counter operates on a per-circuit basis, not on a per-interface basis.

For examples of the show atm vc atm command display, see show atm vc atm.