Extern Table - 2025.1 English - UG1308

Extern Tables, unlike other tables, are not implemented inside VitisNetP4 IP, but outside instead, similarly to how User Externs are integrated. Any table can be made an ‘extern table’ as long as the attribute user_extern is used. This attribute takes one or two inputs, depending on the mode required, similarly to how User Externs are configured:

Fixed latency mode

Only one value is used to specify the fixed latency value in clock cycles. Example:

table forwardIPv4 {
     key = { hdr.ipv4.tos : exact; }
     actions = { forwardPacket; NoAction; }
     size = 64;
     default_action = NoAction;
     user_extern = 15; // fixed latency mode, latency = 15 cycles
}

Variable latency mode

Two values are used. The first value is used to specify the minimum latency value in clock cycles. The second value is used to specify the maximum pipeline capacity of the extern table in clock cycles. Example:

table forwardIPv4 {
     key = { hdr.ipv4.tos : ternary; }
     actions = { forwardPacket; NoAction; }
     size = 1024;
     default_action = NoAction;
     user_extern = {5, 30}; // variable latency mode. Min latency = 5 cycles & max pipe capacity = 30 cycles
}

Note: In variable latency mode, it is important to connect the ready signals to the provided user extern interface ports. See Table 1.