1. Ethernet Ring Protection Switching (ERPS)
Technology Brief
Ethernet Ring Protection Switching
(ERPS)
April. 2010
D-Link HQ

2. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

3. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

4. What is ERPS ?
ERPS (Ethernet Ring Protection Switching) is the first industry standard (ITU-T G.8032) for Ethernet ring protection switching.
It is achieved by integrating mature Ethernet operations, administration, and maintenance (OAM) * functions and a simple automatic protection switching (APS) protocol for Ethernet ring networks.
It provides sub-50ms protection for Ethernet traffic in a ring topology.
It ensures that there are no loops formed at the Ethernet layer.
The first version supported a single ring architecture and the second version is expected to address multiple inter-connected rings.
Less than 1,200 km of ring fiber circumference, and fewer than 16 nodes, the switch completion time (transfer time) for a failure on a ring link shall be less than 50 ms. On rings under all other conditions, the switch completion time may exceed 50 ms (the specific interval is under study).
* Please refer to the document – “Technology Brief-Ethernet OAM”.

5. ERPS Scenario Aggregation Ring Link Down! RPL Access Ring
Use a ring topology for the Ethernet traffic.
One link within a ring will be blocked to avoid Loop (called RPL: Ring Protection Link) .
When the failure happens, protection switching blocks the failed link and unblocks the RPL.
When the failure clears, protection switching blocks the RPL again and unblocks the link on which the failure is cleared.
Aggregation Ring
Link Down!
RPL
Access Ring
Disable Blocking
RPL
Link Down!
* The slide is animated.

6. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

7. Drivers for ERPS Ethernet Technology Development
Ethernet as a carrier-class technology continues to make considerable progresses within carrier networks.
According to a recent market study report, over 75% of respondents have a strategy of using Ethernet instead of SDH or SONET for accessing and transmitting customer traffic.
The Ethernet bandwidth-cost merits increase rapidly with 1GbE, 10GbE, 100GbE, etc., due to its commoditization.

8. Drivers for ERPS Ring’s Application for Service Provider
The ring topology is considered the simplest redundant topologies and are often implemented in Metropolitan Ethernet Networks (Metro) to minimize optical fiber usage.
Effective means of aggregating DSLAMs or small Ethernet switches in multi-tenant or multi-dwelling environments in Metro access networks
To support video and voice services for their triple-play deployments, service providers require fast and predictable convergence time.

9. Drivers for ERPS Drawbacks of STP – The convergence time is too long!
STP (spanning-tree protocol) is built for the redundant links and loop avoidance network. When topology changes, STP takes about seconds to converge.
RSTP (Rapid STP) improves the speed of convergence for bridged network from seconds to about 4 seconds, by immediately transitioning root and designated ports to the forwarding state.
ERPS only spends ms converging but it uses a ring topology instead of tree topology.
STP
RSTP
ERPS
Topology Type
Any Topology
Ring Topology
Convergence Time
30-50 seconds
4 seconds
50-200ms*
* ITU requires sub-50ms convergence time for ring protection. To fulfill this requirement, it needs hardware support . Most current switches use software to develop ERPS, and they always take about ms to converge because of the hardware limitation.

10. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

11. Ring Topology ERPS must support Ethernet ring topology as below:
Single ring (a)
Two single-rings with a shared node (b)
Multi-ring: Rings share common link and nodes (c)
Nested multi-ring in a ladder topology (d)
Each ring node has two ring ports per ring and all nodes can be connected in a physically closed loop.
A physical ring will maintain a logical (non-looping) linear MAC topology with one link blocked.
The current G.8032 supports single rings in Figure (a) and (b). In the multi-ring cases of Figure (c) and (d), rings can be inter-connected via a 4 shared link. In the further development of G.8032, multi-rings and ladder networks consisting of conjoined Ethernet rings will provide the Ethernet ring protection.
APS (Automatic Protection Switching) protocol will be described in later slides.

12. G.8032 Objectives and Principles
Use of standard 802 MAC and OAM frames around the ring. Uses standard Q (and amended Q bridges), but with xSTP disabled.
Ring nodes support the standard FDB (forwarding database) MAC learning, forwarding, flushing behavior and port blocking/unblocking mechanisms.
A ring node with a blocked port (either a pre-determined link or a failed link) prevents R-APS massages received at one port from being forwarded to the other ring port.
The ring protection link(RPL) is the ring link which under normal conditions is blocked (at one end or both ends) for traffic channel, to prevent loops.
Monitoring of the ETH layer for discovery and identification of Signal Failure (SF) conditions.
Protection and recovery switching is within 50 ms for typical rings.
OAM : Operation, Administration and Maintenance
xSTP means STP, RSTP, or MSTP etc.
A filtering database (FDB) flush consists of removing the learned MAC addresses of the ring ports from the node's filtering database.

13. G.8032 Terms and Concepts
RPL (Ring Protection Link) – Link designated by mechanism that is blocked during Idle state to prevent loop on Bridged ring
RPL Owner – Node connected to RPL that blocks traffic on RPL during Idle state and unblocks during Protected state
R-APS (Ring – Automatic Protection Switching) - Protocol messages defined in Y.1731 and G.8032 used to coordinate the protection actions over the ring through RAPS VLAN (R-APS Channel).
R-APS Message
Signal Fail (SF) – Signal Fail is declared when ETH trail signal fail condition is detected
No Request (NR) – No Request is declared when there are no outstanding conditions (e.g., SF, etc.) on the node
RPL Blocked (RB) – RPL Blocked is declared by RPL Owner; it always comes with NR.
ETH :Ethernet Layer Network
CC :Continuity Check
CFM: Connectivity Fault Management
* Please refer to Appendix for R-APS packet format and R-APS process.

14. R-APS Channel & ERP Control
RAPS VLAN (R-APS Channel) – A separate ring-wide VLAN for transmission of R-APS messages
Protected VLANs – The service traffic VLANs for transmission of normal network traffic
The ERP (Ethernet Ring Protection) Control process controls forwarding of service traffic and R-APS protocol messages.
Model of a ring node with its port blocked
Blocking by ERP Control
R-APS forwarding (RAPS VLAN)
ERP Control
Service traffic forwarding
(Protected VLANs)
Exterior of ring
Node
The figure presents a model of a ring node and its port blocking. The ERP control process controls forwarding of service traffic and R-APS protocol messages, and also processes R-APS protocol messages and local signal failure events from the server layer. R-APS messages received at each port is terminated but can be forwarded to the ERP control process.
RPL
WEST LINK
EAST LINK
* The slide is animated.

15. Node States and Switching Triggers
Three node states –
Initialization – when first defining the ring
Idle – the ‘normal’ state of the ring nodes with RPL blocked and all nodes/ports working
Protecting – when protection switching is in effect, RPL unblocked, other (usually fault) link is blocked.
State Machine describes the actions to be taken by the node dependent on current state and input message.*
Protection switching triggered by
Detection/clearing of Signal Failure (SF) by ETH CC (Continuity Check) OAM messages
Remote requests over R-APS channel (using Y.1731 protocol)
Expiration of G.8032 timers (WTR or hold-off timer)
* Please refer to Appendix.

16. The Protection Mechanism
R-APS control the communication and states of the ring nodes
Two basic R-APS messages specified - R-APS(SF) and R-APS(NR)
RPL Owner may modify the R-APS(NR) to R-APS(NR,RB) indicating the RPL is blocked
Different actions include – transmission of R-APS message, blocking/unblocking a port, flushing the FDB, switching current state of node, and starting/stopping timers.
FDB flushing will clear all of the learned MAC addresses of the ring ports from the forwarding database (FDB).

17. Timers
G.8032 specifies the use of different timers to avoid race conditions and unnecessary switching operations
Hold-off Timers – Used by underlying ETH layer to filter out intermittent link faults ; Faults will only be reported to the ring protection mechanism if this timer expires
WTR (Wait to Restore) Timer –When the failed link recovered, this is Used by the RPL Owner to verify that the ring has stabilized before restoring the ring (blocking the RPL).
Guard Timers – Used to prevent ring nodes from receiving outdated R-APS messages. During the duration of the guard timer, all received R-APS messages are ignored by the ring protection control process. *
* Please refer to Appendix to see the guard timers function in
R-APS process.

18. Ring Idle State (Normal Condition)
ETH-CC
Physical topology has all nodes connected in a ring
ERP guarantees lack of loop by blocking the RPL (link between 6 & 1 in figure)
Logical topology has all nodes connected without a loop.
Each link is monitored by its two adjacent nodes using ETH CC OAM messages
Signal Failure as defined in Y.1731, is triggered to ring protection
Loss of Continuity
Server /Physical layer failure
ETH-CC
RPL Owner
RPL
ETH-CC
ETH-CC
ETH-CC
ETH-CC
ETH-CC
ETH-CC
ETH-CC 1 2 6 4 3 5
RPL
Physical topology
APS Automatic Protection Switching
CC Continuity Check
CI Characteristic Information
ERP Ethernet Ring Protection
ETH Ethernet Layer Network
ETH_CI ETH characteristic information
ETH_FF Ethernet flow forwarding function
NR No Request is declared when there are no outstanding
conditions on the nodes
OAM Operations, Administration, and Maintenance
R-APS Ring APS
RB RPL Blocked
RPL Ring Protection Link 1 2 6 4 3 5
Logical topology

19. Link Failure
Link/node failure is detected by the nodes adjacent to the failure.
The nodes adjacent to the failure block the failed link and report this failure to the ring using R-APS (SF) message
R-APS (SF) message triggers
RPL Owner unblocks the RPL
All nodes perform FDB flushing
Ring is in protection state
All nodes remain connected in the logical topology.
RPL Owner
RPL
R-APS(SF)
R-APS(SF) 1 2 6 4 3 5
RPL 1 2 6 4 3 5
RPL
Physical topology 1 2 6 4 3 5 1 2 6 4 3 5
Logical topology
* The slide is animated. * Please refer to Appendix for detailed process.

20. R-APS(NR, RB)
Failure Recovery
When the failed link recovers, the traffic is kept blocked on the nodes adjacent to the recovered link
The nodes adjacent to the recovered link transmit R-APS(NR) message indicating they have no local request present
When the RPL Owner receives R- APS(NR) message it Starts WTR timer
Once WTR timer expires, RPL Owner blocks RPL and transmits R-APS (NR, RB) message
Nodes receiving the message – perform a FDB Flush and unblock their previously blocked ports
Ring is now returned to Idle state
RPL Owner
R-APS(NR)
RPL
R-APS(NR) 1 2 6 4 3 5
RPL 1 2 6 4 3 5
RPL
Physical topology 2 1 6 1 2 6 4 3 5 3 4 5
Logical topology
* The slide is animated. * Please refer to Appendix for detailed process.

21. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

22. Benchmarking Company D-Link Cisco 3 COM Foundry HP Extreme
Allied Telesis
Protocol
ERPS
(ITU-T G.8032)
REP
RRPP
MRP/ MRPII
EAPS (RFC-3619)
EPSR
Name
Ethernet Ring Protection Switching
Resilient Ethernet Protocol
Rapid Ring Protection Protocol
Metro Ring Protocol
Ethernet Automatic Protection Switching
Ethernet Protection Switching Ring
Ring Topologies
Single ring/ Multiple rings
Single ring/ More complex rings
Single ring/
Two or more rings
Overlapping rings
Single rings
Single ring
Convergence Time
50-200ms
50-250ms
< 200ms
50ms
Faster than RSTP

23. D-Link Products support ERPS
DES-3028/52 Series R3
DES-3528/52 Series R3
DES-3810 Series R1
DGS-3400 Series R3
DGS-3600 Series R2.8
DGS-3700 Series R2

24. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

25. ERPS CLI Command enable erps
Enable the global ERPS function on a switch.
disable erps
Disable the global ERPS function on a switch.
create erps raps_vlan <vlanid>
Create ring automatic protection switching (R-APS) vlan. The switch uses the R-APS protocol to coordinate the switching behavior through this vlan ( APS Channel ) with other ring switches.
config erps raps_vlan <vlanid> ring_mel <value 0-7>
The ring MEL value is the maintenance entity group (MEG) level providing a communication channel for ring automatic protection switching (R-APS) information.
Please refer to Appendix to see the R-APS packet format.
For detailed description of maintenance entity, please refer to Technology_Brief-Ethernet_OAM.ppt
Configure the MEL value of the ERPS ring for a specific R-APS VLAN

26. ERPS CLI Command
config erps raps_vlan <vlanid> ring_port [west <port> | east <port>]
Configure the ports connecting to the ERPS ring for a specific R-APS VLAN
There are “west” and “east” ring ports of the switch to the ring.
config erps raps_vlan <vlanid> rpl_port [west|east|none]
Configure the RPL port for a specific R-APS VLAN.
config erps raps_vlan <vlanid> rpl_owner [enable|disable]
Configure the RPL owner for a specific R-APS VLAN.
config erps raps_vlan <vlanid> protected_vlan [add|delete] vlanid <vidlist>
Configure the protected VLAN for a specific R-APS VLAN.
“protected_vlan” is used to configure the VLANs of the (service) traffic channel that are protected by the ERPS function.

27. ERPS CLI Command
config erps raps_vlan <vlanid> timer { holdoff_time <value > | guard_time <value > | wtr_time <value >}]
Configure the ERPS timers for a specific R-APS VLAN.
config erps trap [enable | disable]
Configure the trap state of the ERPS.
config erps raps_vlan <vlanid> state [enable | disable]
Configure the state of the specified ring.
show erps {raps_vlan <vlanid> {sub_ring}}
“show erps” display ERPS configuration and operation information:
The port state of the ring port may be as "Forwarding", "Blocking", "Signal Fail". "Forwarding" indicates that traffic is able to be forwarded. "Blocking" indicates that traffic is blocked by ERPS and a signal failure is not detected on the port. "Signal Fail" indicates that a signal failure is detected on the port and traffic is blocked by ERPS.
The RPL owner administrative state could be configured to "Enabled" or "Disabled". But the RPL owner operational state may be different from the RPL owner administrative state, for example, the RPL owner conflict occurs. "Active" is used to indicate that the RPL owner administrative state is enabled and the device is operated as the active RPL owner. "Inactive" is used to indicate that the RPL owner administrative state is enabled, but the device is operated as the inactive RPL owner.
Display ERPS configuration and operation information.

28. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

29. Testing Scenario I Topology
Hands-on Configuration Sample
Internet
DPH-125MS
UPLINK
DVX-2000MS
RPL Owner
P4: East P3: West
(A) DES 3052
P3: West P4: East
(B) DES 3052P
Ring connection port 4(A) to port 3(B) 1 2
Ring connection port 3(A) to port 4(B)
DPH-125MS

30. Testing Scenario I Topology
Hands-on Configuration Sample
Set Switch A to be RPL Owner
config serial_port auto_logout never
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port west
config erps raps_vlan 3 rpl_owner enable
enable erps
Set Switch B to be a regular switch
config serial_port auto_logout never
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port none
enable erps

31. Testing Scenario I Topology
Hands-on Configuration Sample
Switch A
DES-3052:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Signal Fail Blocked)
East Port : 4 (Forwarding)
RPL port : west port
RPL owner : Yes
Protection VLANs : 1
Ring MEL : 1
Holdoff time : 0 ms
Guard time : 500 ms
WTR time : 5 minutes
Log : Disabled
Current Ring State : Protection
Switch B
DES-3052P:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Forwarding)
East Port : 4 (Signal Fail)
RPL Port : None
Owner : Disabled
Protection VLANs : 1
Ring MEL : 1
Holdoff Time : 0 ms
Guard Time : 500 ms
WTR Time : 5 minutes
Log : Disabled
Current Ring State : Protection

32. Testing Scenario II Topology
Internet
DPH-125MS
P3: West
P4: East
DVX-2000MS
RPL Owner
(A) DES 3052P
P4: East
(B) DES 3052
(D) DES 3052
P3: West
P3: West
P4: East
Ether Ring Connection
(C) DES 3052P
P3: West
P4: East
DPH-125MS

33. Testing Scenario II Topology
Set Switch A to be RPL Owner
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port west
config erps raps_vlan 3 rpl_owner enable
enable erps
Set Switch B to be a regular switch
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port none
enable erps
Set Switch C to be a regular switch
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port none
enable erps
Set Switch D to be a regular switch
create vlan vlanid 3
config vlan vlanid 3 add tagged 3-4
create erps raps_vlan 3
config erps raps_vlan 3 ring_port west 3
config erps raps_vlan 3 ring_port east 4
config erps raps_vlan 3 protected_vlan add vlanid 1
config erps raps_vlan 3 rpl_port none
enable erps

34. Testing Scenario I Topology
Hands-on Configuration Sample
Switch A
DES-3052P:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Signal Fail Blocked)
East Port : 4 (Forwarding)
RPL port : west port
RPL owner : Yes
Protection VLANs : 1
Ring MEL : 1
Holdoff time : 0 ms
Guard time : 500 ms
WTR time : 5 minutes
Log : Disabled
Current Ring State : Protection
Switch B
DES-3052:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Forwarding)
East Port : 4 (Signal Fail)
RPL Port : None
Owner : Disabled
Protection VLANs : 1
Ring MEL : 1
Holdoff Time : 0 ms
Guard Time : 500 ms
WTR Time : 5 minutes
Log : Disabled
Current Ring State : Protection

35. Testing Scenario I Topology
Hands-on Configuration Sample
Switch C
DES-3052P:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Forwarding)
East Port : 4 (Forwarding)
RPL Port : None
Owner : Disabled
Protection VLANs : 1
Ring MEL : 1
Holdoff Time : 0 ms
Guard Time : 500 ms
WTR Time : 5 minutes
Log : Disabled
Current Ring State : Protection
Switch D
DES-3052:4#show erps
Command: show erps
ERPS Information
ERPS Status : Enabled
R-APS VLAN : 3
West Port : 3 (Forwarding)
East Port : 4 (Forwarding)
RPL Port : None
Owner : Disabled
Protection VLANs : 1
Ring MEL : 1
Holdoff Time : 0 ms
Guard Time : 500 ms
WTR Time : 5 minutes
Log : Disabled
Current Ring State : Protection

36. Agenda What is ERPS ? Drivers for ERPS ERPS Technology Benchmarking
ERPS CLI commands
Hands-on configuration
Application Examples

37. ETTx Solution Core Aggregation Access Internet
IPTV Partner
Future
Internet
Core
DGS-8000
VoIP Partner
Aggregation
New!
DGS G
DGS-3612/27G/50
Access
Coming Soon!
DES-3526/50
DES-3028/52
DES /18/28
Core Level Ring Protection
Aggregation Level Ring Protection
Access Level Ring Protection 37

38. Campus Solution Core Aggregation Access Campus Core Ring Protection
Future
Core
DGS-8000/
DGS-6600
Aggregation
DGS-3612/27G
DGS-3612/27G
Access
DES-3528/52
Campus Core Ring Protection
Aggregation devices connect to the nodes in core Ring
(Optional) Access Ring Protection. Usually for long distance, sparse deployment 38

39. Appendix

40. Ring Protection Requests Priority
Protection switching algorithm is based on priorities assigned to all triggers. When a ring node has multiple outstanding requests, it only responds to the one with highest priority.
Request
Type
Priority
Local SF
Local
highest
Local Clear SF
R-APS (SF)
Remote
WTR Expires
WTR Running
R-APS (NR, RB)
R-APS (NR)
lowest
WTR (Wait to Restore)– Used by the RPL Owner to verify that the ring has stabilized before blocking the RPL after SF Recovery

41. State Machine
The R-APS request processing logic is defined in the format of a state machine.
Inputs
Outputs
Node State
High Priority request
Row#
Actions
Next state *
Initialization
Stop guard timer
Stop WTR timer
If RPL Owner:
Block RPL port
Unblock non-RPL port
Tx R-APS (NR, RB)
Else:
Block both ports
Stop Tx R-APS B
A (Idle)
Local SF 1
Block failed port; Unblock non Failed port;
Tx R-APS(SF);
Flush FDB;
Local Clear SF 2
No action A
R-APS(SF) 3
Unblock non Failed port;
If not “DNF” Flush FDB;
WTR Expires 4
WTR Running 5
R-APS(NR, RB) 6
R-APS(NR) 7
Inputs
Outputs
Node State
High Priority request
Row#
Actions
Next state B
(Pro-tecting)
Local SF 8
Block failed port; Unblock non Failed port; Stop WTR;
Tx R-APS(SF);
Local Clear SF 9
Start guard timer, Tx R-APS(NR),
R-APS(SF) 10
Stop WTR, Unblock non Failed port, Stop Tx R-APS
WTR Expires 11
Block RPL port, Unblock non-RPL port
Tx R-APS (NR,RB)
Flush FDB, A
WTR Running 12
No action
R-APS(NR, RB) 13
If not RPL Owner:
Unblock both ports,
Stop Tx R-APS,
If not “DNF” Flush FDB
R-APS(NR) 14
If RPL Owner:
Start WTR
The possible actions triggered by this process are:
-Block failed port – Blocks traffic channel and R-APS channel on ring ports which have an SF condition. If a port is already blocked it remains blocked.
-Unblock non-failed port – Unblock traffic channel and R-APS channel on both ring ports if they do not have an SF condition. If a port is already unblocked it remains unblocked.
-Block RPL port – Block traffic channel and R-APS channel on the port which is defined as the RPL. If the RPL port is already blocked it remains blocked.
-Block both ports – Blocks traffic channel and R-APS channel on both ring ports. If a port is already blocked it remains blocked.
-Unblock non-RPL port – Unblock traffic channel and R-APS channel on both ring ports if they are not the RPL port. If a port is already unblocked it remains unblocked.
-Unblock both ports – Unblock traffic channel and R-APS channel on both ring ports. If a port is already unblocked it remains unblocked.
-Start WTR – Starts the WTR timer if it is stopped. If the WTR timer is already running then no action.
-Stop WTR – Stop the WTR timer if it is running.
-Start guard timer – Starts the guard timer. While the guard timer is running received R-APS messages are not forwarded to the priority logic.
-Stop guard timer – Stops the guard timer if it is running.
- Stop Tx R-APS – Stop transmission of any R-APS messages.
-Tx R-APS (msgtype, status_bits) – Start transmission of R-APS message.
-Flush FDB – Flush the FDB.

42. R-APS Specific Information Format
R-APS information is carried within the R-APS PDU, which is one of a suite of Ethernet OAM defined in Y.1731
Version – ‘00000’ – for this version of Recommendation
OpCode – defined to be 40 in Y.1731
Flags – ‘ ’ – should be ignored by ERP 1 2 3 4 8 7 6 5
MEL
Version (0)
OpCode (R-APS = 40)
Flags (0)
TLV Offset (32)
R-APS Specific Information (32 octets) .. … 37
[optional TLV starts here; otherwise End TLV]
last
End TLV (0)
32 octets in the R-APS PDU are used to carry R-APS specific information. In addition, the TLV Offset field is required to be set to 32.
For other fields such as Version, OpCode, Flags, and End TLV, the following values shall be used as defined in [ITU-T Y.1731].
- Version: 0x00 shall be transmitted in the current version of this Recommendation. This field
should be ignored upon reception.
- OpCode: 40 shall be transmitted as defined within [ITU-T Y.1731].
- Flags: 0x00 shall be transmitted in the current version of this Recommendation. This field
End TLV: 0x00 shall be transmitted.
In the MEL field, the MEG level of the R-APS PDU is inserted.
The ring MEL is the maintenance entity group (MEG) level providing a communication channel for ring automatic protection switching (R-APS) information.
Defined by Y.1731
Defined by G.8032
Non-specified content

43. R-APS Specific Information Format
Specific information (32octets) defined by G.8032
Request/State(4bits) – ‘1011’ = SF | ’0000’ = NR | Other = Future
Status – RB (1bit) – Set when RPL is blocked (used by RPL Owner in NR)
Status – DNF (1bit) – Set when FDB Flush is not necessary (Future)
NodeID (6octets) – MAC address of message source node (Informational)
Reserved1(4bits), Status Reserved(6bits), Reserved2(24octets) - Future development  1 2 3 4 8 7 6 5 1
Request /State
Reserved 1
Status
Node ID (6 octets) RB
DNF
Status Reserved
(Node ID)
Reserved 2 (24 octets)
The fields of R-APS specific information:
- Request/State (4 bits) – This field represents a request or state, and is encoded as described
o 1011: Signal Fail (SF)
o 0000: No Request (NR)
o others: Reserved for future international standardization
- Reserved 1 (4 bits) – This field is reserved for future extension of requests or for indication of protection type. In the current version of this Recommendation, this field shall be encoded as “0000”. This field should be ignored upon reception.
- Status field – This includes the following status information.
o RB – RPL Blocked
􀂃 RB = 1 – represents that the RPL is blocked
􀂃 RB = 0 – represents that the RPL is unblocked.
This bit should be 0 when transmitted by non-RPL Owner nodes.
o DNF – Do Not Flush
􀂃 DNF = 1 - represents that an FDB Flush should not be triggered by the reception of this message.
􀂃 DNF = 0 - represents that an FDB Flush may be triggered by the reception of this message.
o Status Reserved (6 bits) – For future specification. This field shall be transmitted encoded all zeroes. This field should be ignored upon reception.
- Node ID (6 octets) – A MAC address unique to the ring node. This field is informational; it does not impact protection switching operation in this Recommendation.
- Reserved 2 (24 octets) - This field is reserved for future extensions of the R-APS Protocol.
In the current version of this Recommendation, this field shall be transmitted encoded all zeroes. This field should be ignored upon reception.

44. Link Failure (Normal to Protection)
RPL 1 2 3 4 5 6
RPL Owner
Normal A B
failure C
Protection
state D SF SF
50 ms SF SF
Flush
Flush E
Flush
Flush
Flush
Flush F SF SF SF SF SF SF G
A. Normal condition
B. Failure Occurs
C. Node 4 and 5 detect local Signal Failure condition and after respecting hold-off time, block failed port and perform FDB flush
D. Node 4 and 5 periodically send SF message, on both ring ports, while SF condition persists
E. All node receiving SF message flush FDB. RPL Owner receives SF message, unblocks its end of RPL link and flush FDB
F. Stable State – SF messages on the ring
G. Further SF messages trigger no further action
Message source
R-APS channel block
Client channel block

45. Failure Recovery 1 2 3 4 5 6 A B C D E F G H A. Stable SF condition
RPL 1 2 3 4 5 6
RPL Owner
Protection
state
failure A SF SF SF SF SF SF B
recovery C NR
Confirmation time NR NR NR NR NR D E F
NR, RPL Blocked
NR, RPL Blocked
50 ms
NR, RPL Blocked
Flush G
Normal
Flush
Flush
Flush
A. Stable SF condition
B. Recovery of Link failure
C. Node 4 and 5 detect clearing of SF condition , start Guard timer and initiate periodical transmission of NR message on both ring ports. ( Guard timer prevents reception of R-APS messages)
D. When the RPL Owner receives NR message, it starts the WTR timer
E. When node 4 and 5 expire the guard timer, they may accept the new R-APS messages that they receive.
F. At expiration of WTR timer, RPL owner blocks its end of the RPL, sends, NR RB message and Flushes FDB
G. When Node 4 and 5 receive NR RB message, they remove block on their blocked ports and stop sending NR message. In addition to this nodes 1 to 5 will Flush FDB when receiving NR RB message H
Flush
Flush
NR, RPL Blocked NR
NR, RPL Blocked
NR, RPL Blocked
Message source
R-APS channel block
Client channel block

46. Reference
Ethernet Ring Protection Switching by the ITU Telecommunication Standardization Sector (ITU-T) ITU-T Rec. G.8032, 2008.
Ethernet Ring Protection for Carrier Ethernet Networks by JEONG-DONG RYOO in Electronics and Telecommunications Research Institute (ETRI), South Korea