ATM OAM Traffic Reduction

The OAM management portion of a PVC sends OAM loopback cells at periodic intervals. When OAM management is enabled at both ends of the PVC, the cells are transmitted and looped back at both ends. This transmission is redundant, because the OAM cells travel through the same physical circuit twice.

In the figure below, assume PVCs are configured between router R1 and router R2 , and that OAM management is enabled on both ends of the PVC. Router R1, upon receiving OAM command cells from router R2, can stop its own OAM command cell transmission and can manage the link on the basis of incoming OAM command cells. Router R1 can reinitiate OAM command cell transmission upon discovering the absence of command cells from router R2.

Figure 1

ATM OAM Traffic Flow

When router R1 detects the first OAM command cell from router R2, time stamp T1 is noted. When the next OAM command cell is detected, time stamp T2 is noted. The interval T1 minus T2 provides the OAM the loopback frequency of router R2. The average value of this interval is taken by measuring it a random number of times. (The interval needs to be taken a random number of times to avoid a race condition that might happen when routers R1 and R2 implement this algorithm and the frequency is the same.)

At the end of the random time period, router R1 stops sending OAM command cells and starts the OAM traffic monitoring timer. This timer in router R1 checks for a change in interval frequency in router R2. If there is a change, the traffic monitoring timer is stopped and the VC goes into Retry mode and checks whether the link is still up. In Retry mode, OAM command loopback cells are transmitted at an interval of one per second for 3 seconds. If router R1 does not receive a response to the command cell, the link is changed to the Down state.

Note

If ATM OAM traffic reduction is enabled on routers R1 and R2, then because of the random nature of the traffic reduction algorithm, either router can initiate OAM command cells and the other router will have to listen to the incoming OAM command cells. If this feature is enabled on only one router (R1, for example), then the frequency of that router must be greater than or equal to the interval frequency set in the other router (R2) in order for router R1 to stop sending OAM command cells. The ATM OAM Traffic Reduction feature is enabled by the oam-pvc command. When the optimum keyword is enabled, and when a change in the interval frequency of router R2 is detected, the VC initiates an OAM command cell from router R1 and does not go into the Retry mode immediately. If no response is obtained, the VC goes into the Retry mode and follows the OAM Retry procedure.

1.    enable

2.    configure terminal

3.    interface atm interface-number . subinterface-number

4.    pvc vpi / vci

5.    oam-pvc [frequency | manage [frequency [auto-detect [optimum] | keep-vc-up [seg aisrdi failure] | loop-detection]]]

6.    exit

1.    enable

2.    configure terminal

3.    vc-class atm vc-class-name

4.    oam-pvc [frequency | manage [frequency [auto-detect [optimum] | loop-detection]]]

5.    exit

6.    interface atm interface-number . subinterface-number

7.    class-int vc-class-name

8.    pvc vpi / vci

9.    exit

1.    enable

2.    show atm oam auto-detect [atm interface-number . subinterface-number]

3.    show atm pvc [interface atm type number[vpi / vci | vci | word] vpi / vci | vci | word]

The following example enables ATM OAM traffic reduction on an ATM interface:

interface ATM1/1/1.100 point-to-point 
 ip address 60.1.2.1 255.255.255.0 
 no atm enable-ilmi-trap 
 pvc 11/111 
  oam-pvc manage auto-detect optimum 
  encapsulation aal5snap 

The following example enables ATM OAM traffic reduction using a VC class:

vc-class atm oam-tests 
 oam-pvc manage auto-detect optimum 
interface ATM1/1/2.100 point-to-point 
 ip address 60.1.3.1 255.255.255.0 
 class-int oam-tests 
 no atm enable-ilmi-trap 
 pvc 12/222 
  encapsulation aal5snap 

In the following examples, the output is displayed for each command in the task.

Router# show atm oam auto-detect atm 1/1/2.100
 
ATM OAM Auto Detect statistics on ATM1/1/2 
Auto Detection statistics: 
ATM OAM AUTO DETECT INIT : 1 
ATM OAM SENDING MONITORING : 0 
ATM OAM MONITORING : 0 
OAM Loopback statistics: 
 DownRetry : 0 
 UpRetry : 0 
 Verified : 1 
 Not Verified : 0 
 Verified and Not Monitoring: 1 
Router# 

Router# show atm pvc 12/222
 
ATM1/1/2.100: VCD: 1, VPI: 12, VCI: 222 
UBR, PeakRate: 149760 (353208 cps) 
AAL5-LLC/SNAP, etype:0x0, Flags: 0x1840, VCmode: 0x0, Encapsize: 12 
OAM frequency: 10 second(s), OAM retry frequency: 1 second(s) 
OAM up retry count: 3, OAM down retry count: 5 
OAM Loopback status: OAM Received 
Last cell looped No 
Loop detect state 0, Last cell looped 0, OAM Retries 0, Loop Retries 0 
OAM VC Status: Verified 
OAM Auto Detect state: ATM OAM AUTO DETECT INIT 
OAM PEER frequency: 0 second(s)