RAPS PDUs

ERPS protocol packets are called Ring Auto Protection Switching (RAPS) Protocol Data Units (PDUs), which are transmitted in ERPS rings to convey ERPS ring information. Figure 1 shows the RAPS PDU format.

Figure 1 RAPS PDU format

Table 1 describes the fields in an RAPS PDU.

**Table 1** Fields in an RAPS PDU
Field	Length	Description
MEL	3 bits	Identifies the maintenance entity group (MEG) level of the RAPS PDU.
Version	5 bits	0x00: ERPSv1 0x01: ERPSv2
OpCode	8 bits	Indicates an RAPS PDU. The value of this field is 0x28.
Flags	8 bits	Is ignored upon RAPS PDU receiving. The value of this field is 0x00.
TLV Offset	8 bits	Indicates that the TLV starts after an offset of 32 bytes. The value of this field is 0x20.
R-APS Specific Information	32x8 bits	Is the core field in an RAPS PDU and carries ERPS ring information. There are differences between sub-fields in ERPSv1 and ERPSv2. Figure 2 shows the R-APS Specific Information field format in ERPSv1. Figure 3 shows the R-APS Specific Information field format in ERPSv2.
TLV	Not limited	Describes information to be loaded. The end TLV value is 0x00.

Figure 2 Format of the R-APS Specific Information field in ERPSv1

Figure 3 Format of the R-APS Specific Information field in ERPSv2

Table 2 describes sub-fields in the R-APS Specific Information field.

**Table 2** Sub-fields in the R-APS Specific Information field
Sub-Field	Length	Description
Request/State	4 bits	Indicates that this RAPS PDU is a request or state PDU. The value can be: 1101: forced switch (FS) 1110: Event 1011: signal failed (SF) 0111: manual switch (MS) 0000: no request (NR) Others: reserved
Reserved 1	4 bits	Reserved 1 is used in ERPSv1 for message reply or protection identifier. Sub-code is used in ERPSv2. The value depends on the Request/State field value: If the Request/State field value is 1110, the Sub-code value is 0000, indicating FDB entry update. If the Request/State field value is any other value than 1110, the Sub-code value is 0000 and ignored upon RAPS PDU receiving.
Sub-code	4 bits
Status	8 bits	Includes the following status information: RPL Blocked (RB) (1 bit): The value 1 indicates that the RPL owner port is blocked; the value 0 indicates that the RPL owner port is unblocked. The nodes where the RPL owner port is not configured set this sub-field to 0 in outgoing RAPS PDUs. Do Not Flush (DNF) (1 bit): The value 1 indicates that FDB entries are not updated when RAPS PDUs are received; the value 0 indicates that FDB entries may be updated when RAPS PDUs are received. Blocked port reference (BPR) (1 bit): The value 0 indicates that ring link 0 is blocked; The value 1 indicates that ring link 1 is blocked. BPR is valid only in ERPSv2. Status Reserved: This sub-field is reserved. This sub-field is all 0s during RAPS PDU transmission, and is ignored upon RAPS PDU receiving. In ERPSv1, this sub-field has 6 bits. In ERPSv2, this sub-field has 5 bits.
Node ID	6 x 8 bits	Identifies the MAC address of a node in an ERPS ring. It is informational and does not affect protection switching in the ERPS ring.
Reserved 2	24 x 8 bits	Is reserved and ignored upon RAPS PDU receiving. The value is all 0 during RAPS PDU transmission.