214 C HAPTER 10: PACKET FILTERINGPlacing a filter on the receive path confines the packet to the segmentthat it originated from if it does not meet the forwarding criteria. Placinga filter on the transmit path prohibits a packet from accessing certainsegments unless it meets the forwarding criteria. The system discards anypacket that does not meet the forwarding criteria on the transmit path.If you want to filter packets destined for the switch itself (for example,ping packets or Telnet packets), you must use the receive internal path.They are not filtered on the receive all path.Custom Packet Filters You create custom packet filters by writing a packet filter definition.Software implements custom filters. Consequently, use custom filters onlyon ports and paths that need them. Processing too many frames insoftware can affect performance on the ports where custom filters areassigned.If you are trying to filter a certain type of broadcast or multicast packetassign the filter to either the txM or the rxM paths, allowing only unicasttraffic to bypass the filter.Each packet-processing path on a port may have a unique custom packetfilter definition or may share a definition with other ports on the system.Custom packet filter definitions are written in the packet filter language,which allows you to construct complex logical expressions.After you write a packet filter definition, you load it onto a system; thecorresponding port assignments are preserved in the nonvolatile memory(NVRAM) of the system, thus ensuring that the packet filter configurationfor each system is saved across system reboots and power failures.
