3Com 4007R Implementation Guide - Page 336

336 CHAPTER 15: PACKET FILTERING Port Group Filter See "Using Port Groups in Custom Packet Filters" for a port group filter example. Limits to Filter Size A packet filter program is stored in a preprocessed format to minimize the space that is required by the packet filter definition. Comments are stripped. When assigned to a port, the packet filter is converted from the stored format to a run-time format to optimize the performance of the filter. Each module is limited to a maximum of 16 packet filter programs. The maximum length of a packet filter definition is 4096 bytes. Storage Rules for Preprocessed Packet Filters Each module provides a maximum of 2048 bytes of nonvolatile storage for preprocessed packet filter programs. In the preprocessed stored format: s A single packet filter program is limited to 254 bytes. s Each instruction in the packet filter program requires 1 byte for the opcode and size, plus additional bytes for any explicit operands. s Module overhead is 22 bytes, plus a per-packet-filter overhead of 13 bytes. For example, assume a packet filter program requires 200 bytes for storing the instructions in the program. If this packet filter is the only one loaded, the nonvolatile memory required is 22 bytes (for module overhead) plus 13 bytes (for packet filter overhead) plus 200 bytes (for the program itself) - a total of 235 bytes. Run-time Storage of Packet Filters For run-time storage of packet filter programs, each module provides a maximum of 8192 bytes. There is no explicit system or per-packet-filter overhead; however, performance considerations can result in unused areas of the run-time storage. The run-time format is approximately eight times the size of the stored format. Thus a 200-byte packet filter program in stored format expands to approximately 1600 bytes in the run-time format. A single packet filter program cannot exceed 2048 bytes in the run-time format.