R-APS PDU Format

Ethernet Ring Protection Switching (ERPS) protocol packets are called R-APS PDUs. Ring Auto Protection Switching (R-APS) Protocol Data Units (PDUs) are transmitted on ERPS rings to convey ERPS ring information. Figure 1 shows the basic R-APS PDU format.

Figure 1 Basic R-APS PDU format

Table 1 describes the fields in an R-APS PDU.

**Table 1** R-APS PDU field description
Field Name	Length	Description
MEL	3 bits	Identifies the maintenance entity group (MEG) level of the R-APS PDU.
Version	5 bits	0x00: used in ERPSv1. 0x01: used in ERPSv2.
OpCode	8 bits	Indicates an R-APS PDU. The value of this field is 0x28.
Flags	8 bits	Is reserved. The value of this field is fixed at 0x00.
TLV Offset	8 bits	Indicates that the TLV starts after an offset of 32 bytes. The value of this field is fixed at 0x20.
R-APS Specific Information	32 x 8 bits	Carries R-APS ring information and is the core in an R-APS PDU. This field has different meanings for some of its 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 R-APS Specific Information field format in ERPSv1

Figure 3 R-APS Specific Information field format in ERPSv2
Click to enlarge

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 Name	Length	Description
Request/State	4 bits	Indicates that this R-APS 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 with its value determined by the Request/State field value: If the Request/State field value is 1110, the Sub-code value is 0000, meaning Flush Request. If the Request/State field value is any value other than 1110, the Sub-code value is 0000 and ignored upon reception.
Sub-code	4 bits
Status	8 bits	Includes the following status information: RPL Blocked (RB) (1 bit): If the value is 1, the RPL owner port is blocked; if the value is 0, the RPL owner port is unblocked. The nodes without the RPL owner port set this sub-field to 0 when sending an R-APS PDU. Do Not Flush (DNF) (1 bit): If the value is 1, an FDB flush should not be triggered by the reception of the R-APS PDU; if the value is 0, an FDB flush may be triggered by the reception of the R-APS PDU. Blocked port reference (BPR) (1 bit): If the value is 0, ring link 0 is blocked; if the value is 1, ring link 1 is blocked. BPR is valid only in ERPSv2. Status Reserved (5 bits): This sub-field is reserved for future specification and should be ignored upon reception. This sub-field should be encoded as all 0s in transmission.
Node ID	6 x 8 bits	Identifies the MAC address of a node on the ERPS ring. It is informational and does not affect protection switching on the ERPS ring.
Reserved 2	24 x 8 bits	Reserved for future extension and should be ignored upon reception. Currently, this sub-field should be encoded as all 0s in transmission.