Understanding Y.1731
Function Overview
Fault management functions include continuity check (CC), loopback (LB), and linktrace (LT). Y.1731 fault management is similar to CFM fault management.
Performance monitoring functions include one- and two-way frame delay measurement and alarm indication signal (AIS) on virtual local area networks (VLANs).
Function |
Purpose |
Usage |
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Measures the network delay time on a unidirectional link between MEPs. |
To measure the link delay time, select either one- or two-way frame delay measurement:
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Measures the network delay time on a bidirectional link between MEPs. |
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Detects server-layer faults and suppresses alarms, minimizing the impact on network management systems (NMSs). |
AIS is used to suppress local alarms in the scenario where faults must be rapidly detected. When CFM detects connectivity faults, AIS suppresses generated alarms. |
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Identifies faults and discovers RMEPs. |
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ETH-DM
Delay measurement (DM) measures the delay time and delay variation. A MEP sends a message carrying ETH-DM information to its RMEP and then receives a response message carrying ETH-DM information from its RMEP.
One-way frame delay measurement
A MEP sends a 1DM message carrying one-way ETH-DM information to its RMEP. After receiving this message, the RMEP measures the one-way frame delay or delay variation.
One-way frame delay measurement can be implemented only after the MEP synchronizes the time with its RMEP. The delay variation can be measured regardless of whether the MEP synchronizes the time with its RMEP. If a MEP synchronizes its time with its RMEP, the one-way frame delay and delay variation can be measured. If the time is not synchronized, only the one-way delay variation can be measured.
One-way frame delay measurement can be implemented in either of the following modes:On-demand measurement calculates the one-way frame delay at a time or a specific number of times for diagnosis.
Proactive measurement calculates the one-way frame delay periodically.
In on-demand or proactive mode, a MEP sends 1DM frames to its RMEP. Figure 11-2 illustrates the procedure for one-way delay measurement.
One-way frame delay measurement is implemented on an end-to-end link between a local MEP and its RMEP by exchanging 1DM frames. After one-way frame delay measurement is configured, a MEP periodically sends 1DM frames carrying TxTimeStampf (the time when the 1DM frame was sent). After receiving a 1DM frame, the RMEP parses TxTimeStampf and compares this value with RxTimef (the time when the 1DM frame was received). The RMEP calculates the one-way frame delay based on these values using the following formula:
Frame delay = RxTimef - TxTimeStampf
The frame delay value can be used to measure the delay variation.
A delay variation is an absolute difference between two delays.
Service packets are prioritized based on 802.1p priorities and are transmitted using different policies. Traffic passing through a P on the network shown in Figure 11-3 carries 802.1p priority values of 1 and 2.
One-way delay measurement is enabled on PE1 to send traffic with the priority value of 1 to measure the frame delay on a link between PE1 and PE2. Traffic with the priority value of 2 is also sent. After receiving traffic with the priority values of 1 and 2, the P forwards traffic with a higher priority, delaying the arrival of traffic with the priority value of 1 at PE2. As a result, the frame delay calculated on PE2 is inaccurate.
802.1p priority-based one-way frame delay measurement can be enabled to obtain accurate results.
Two-way frame delay measurement
A MEP sends a delay measurement message (DMM) carrying an ETH-DM request to its RMEP. After receiving the DMM, the RMEP sends a delay measurement reply (DMR) carrying an ETH-DM response to the MEP.
Two-way frame delay measurement can be implemented in either of the following modes:The on-demand measurement calculates the two-way frame delay at a time for diagnosis.
The proactive measurement calculates the two-way frame delay periodically.
Figure 11-4 illustrates the procedure for two-way delay measurement.
Two-way frame delay measurement is performed by a local MEP to send a DMM to its RMEP and then receive a DMR from the RMEP. After the two-way frame delay measurement is configured, a MEP periodically sends DMMs carrying TxTimeStampf (the time when the DMM was sent). After receiving the DMM, the RMEP replies with a DMR. The DMR carries RxTimeStampf (the time when the DMM was received) and TxTimeStampb (the time when the DMR was sent). The value in every field of the DMM is copied to the DMR except that the source and destination MAC addresses were interchanged. Upon receiving the DMR, the MEP calculates the two-way frame delay by using the following formula:
Frame delay = (RxTimeb - TxTimeStampf) - (TxTimeStampb - RxTimeStampf)
The frame delay value can be used to measure the delay variation.
A delay variation is an absolute difference between two delays.
Service packets are prioritized based on 802.1p priorities and are transmitted using different policies. Traffic passing through a P on the network shown in Figure 11-5 carries 802.1p priority values of 1 and 2.
Two-way delay measurement is enabled on PE1 to send traffic with the priority value of 1 to measure the frame delay on a link between PE1 and PE2. Traffic with the priority value of 2 is also sent. After receiving traffic with the priority values of 1 and 2, the P forwards traffic with a higher priority, delaying the arrival of traffic with the priority value of 1 at PE2. As a result, the frame delay calculated on PE2 is inaccurate.
802.1p priority-based two-way frame delay measurement can be enabled to obtain accurate results.
AIS
Alarm indication signal (AIS) is used to transmit fault information.
If CFM detects a fault in the link between AIS-enabled PEs, CFM sends AIS packet data units (PDUs) to CEs. After receiving the AIS PDUs, the CEs suppress alarms, minimizing the impact of a lot of alarms on a network management system (NMS).
After the link between the PEs recovers, the PEs stop sending AIS PDUs. CEs do not receive AIS PDUs during a period of time 3.5 times as long as the interval at which AIS PDUs are sent. Therefore, the CEs cancel the alarm suppression function.
Multicast MAC Ping
Fault verification
Fault verification provided by multicast MAC ping is similar to that provided by 802.1ag MAC ping. They are both used to check the reachability between the local and destination devices by sending test packets. Unlike 802.1ag MAC ping, multicast MAC ping can detect faults in links between multiple MEPs at a time.
Multicast MAC ping is applied to the networking where multiple MEPs are deployed. Multicast MAC ping is initiated by the local MEP and responded to by the RMEP.
As shown in Figure 11-7, MEP1 on PE1 has two RMEPs, namely, MEP2 and MEP3. MEP1 initiates a multicast MAC ping probe. The probe process is as follows:In the case where the link between MEP1 and MEP2 or between MEP1 and MEP3 is normal:
MEP1 of the local device sends a multicast LBM.
After receiving the multicast LBM, MEP2 and MEP3 reply with an LBR.
The local device receives the LBRs and displays the contents of the LBRs.
- In the case where the link between MEP1 and MEP2 or between MEP1 and MEP3 is faulty:
MEP1 of the local device sends a multicast LBM.
MEP1 does not receive any LBR from the RMEPs within a specified timeout period. The output of the related command shows that a fault occurs on the link between MEP1 and MEP2 or the link between MEP1 and MEP3.
RMEP discovery
As shown in Figure 11-7, RMEPs need to be configured for MEP1 of PE1. The IDs and MAC addresses of the RMEPs may be unknown. Therefore, multicast MAC ping can be performed to discover the RMEPs of MEP1. The process of multicast MAC ping is as follows:MEP1 of the local device sends a multicast LBM.
After receiving the multicast LBM, MEP2 and MEP3 reply with an LBR.
The local device receives the LBRs and displays the contents of the LBRs. The LBRs contain the MAC addresses of the RMEPs, MEP IDs, and delay.
