Multicast Address Table — Offset 0x0002_0000-0x0003_FFFF - 7.2 English

AXI 1G/2.5G Ethernet Subsystem Product Guide (PG138)
Document ID
PG138
Release Date
2023-11-15
Version
7.2 English

The Multicast Address Table entry is shown in Figure 2. The multicast address table is only present when extended multicast address filtering is selected at build-time (C_MCAST_EXTEND = 1). The purpose of the table is to allow the subsystem to support reception of frames addressed to many multicast addresses while providing some of the filtering in hardware to offload some of the overhead required for filtering in software.

While an Ethernet MAC multicast address is defined as any 48-bit Ethernet MAC address that has bit 0 (LSB) set to 1 (for example 01:00:00:00:00:00), in most cases the Ethernet MAC multicast address is created from a IP multicast address, as shown in the following figure.

Figure 1. Mapping IP Multicast Addresses to MAC Multicast Addresses AXI Ethernet Page-1 Sheet.5 Sheet.17 224.0.0.0 to 239.255.255.255 224.0.0.0 to 239.255.255.255 Sheet.18 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff Sheet.19 multicast bit (bit 0 byte 0) multicast bit(bit 0 byte 0) Sheet.20 32-bit IP multicast address (typically shown as four8-bit val... 32-bit IP multicast address(typically shown as four8-bit values in decimal) Sheet.21 48-bit MAC destination multicast address (typically shown as ... 48-bit MAC destination multicast address(typically shown as six8-bit values in decimal) Sheet.22 Constant Constant Sheet.23 lower 23 bits lower 23 bits Sheet.24 32K x 1 Table 32K x 1Table Sheet.25 Hardware look-up upper 15 bits of 23 bits for 1 or 0 Hardware look-up upper 15 bitsof 23 bits for 1 or 0 Sheet.26 if 0, drop this frame. if 0, drop this frame. Sheet.27 If 1, pass up to software to make decision. If 1, pass up tosoftware to make decision. Side brace 1-D single.31 Side brace.2 Side brace.1 Side brace.4 Sheet.3 1-D single.6 1-D single.7 1-D single.8 Sheet.9 Sheet.10 X14065 X14065

When a multicast address frame is received while this extended multicast filtering is enabled, the subsystem first verifies that the initial 24 bits are 01:00:5E and then uses the upper 15 bits of the unique 23 bit Ethernet MAC multicast address to index this memory. If the associated memory location contains a 1 then the frame is accepted and passed up to software for a comparison on the full 23-bit address. If the memory location is a 0 or the upper 24 bits are not 01:00:5E then the frame is not accepted and it is dropped.

The memory is 1-bit wide but is addressed on 32-bit word boundaries. The memory is 32K deep. This table must be initialized by software through the AXI4-Lite interface.

Important: When using the extended multicast address filtering, the TEMAC must be set to promiscuous mode so that all frames are available for filtering. When doing this the TEMAC no longer checks for a unicast address match. Additional registers (UAWL and UAWU) are available to provide unicast address filtering while in this mode.

For builds that have the extended multicast address filtering enabled, promiscuous mode can be achieved by making sure that the TEMAC is in promiscuous mode and by clearing the EMultiFltrEnbl bit (bit 19) in the Reset and Address Filter register (RAF). See Extended Multicast Address Filtering Mode.

Figure 2. Multicast Address Table Entry (0x0002_0000-0x0003_FFFF) AXI Ethernet Page-1 Sheet.1 Sheet.2 Sheet.3 Sheet.4 31 31 Sheet.7 Sheet.12 0 0 Sheet.13 1 1 Sheet.14 Reserved Reserved Sheet.15 McastAdrEnbl McastAdrEnbl Sheet.18 MSB MSB Sheet.19 LSB LSB Sheet.21 X14066 X14066 Standard Arrow.4 Standard Arrow.6

The following table shows the Multicast Address Table bit definitions.

Table 1. Multicast Address Table (0x0002_0000-0x0003_FFFF)
Bits Name Access Reset Value Description
31:1 Reserved RO 0x0 Reserved: These bits are reserved for future use and always return zero.
0 McastAdrEnbl R/W 0 Multicast Address Enable: This bit indicates that the received multicast frame with this upper 15 bits of the unique 23-bit Ethernet MAC multicast address field should be accepted or rejected.

0 – Drop this frame

1 – Accept this frame