SMSC LAN1198 5.5 Configuring Address Filtering Options

LAN9118 Family Programmer Reference Guide

Revision 1.0 (12-14-09) 18 SMSC AN 12.12

APPLICATION NOTE

5.5 Configuring Address Filtering Options

Table5.2, "Address Filtering Modes" shows the relation s between the control flags which affect the

address filter mode.

Multicast applications act very much like broadcast radio, where all receivers attuned to one station

receive the same programming. Multicast packets are a type of broadcast packet, but require filtering

to be received by the intended audience.

The hash filtering works by identifying incoming packets with a 48-bit destination multicast MAC

addresses, those addresses having a leading bit pattern of 01:00:5e:xx:yy:zz, running only the

destination MAC address through a hash (actually it’s the CRC32 algorithm, taking the result of the

first 6 bytes of the incoming packet and extracting from that the high 6-bits), resulting in a 6-bit (1-of-

64) value, which is used to index a 64 x 1-bit array contained across the MAC HASHH:HASHL register

pair (from bit 0 of HASHL:0 on up through bit 31 of HASHH:63). If the bit indexed by the hash is a

‘1’, it is considered to be a match, and the entire packet is brought into the device for perfect multicast

address filtering in software.

At startup the hash filter is cleared. As the driver accepts requests to join multicast sessions, the driver

must add the multicast address to its perfect filter table in software. Then it must calculate the

appropriate bit to set in the hash array and update the devices’ HASHH:HASHL register pair. When

the application leaves the multicast session, the driver must update the filter table and the

HASHH:HASHL register pair appropriately.

Some applications need to examine every single packet traversing the LAN, whether the packet

destination MAC address is broadcast or not, and whether the packet data regards the host in question

or not. Communally these programs are referred to as “packet sniffers”. An example is the Ethereal

program, available for a wide range of operating systems (http://www.ethereal.com). At the heart of

these programs is the requirement for the LAN device driver to receive every packet on the LAN.

The term for receiving every packet on the LAN is called Promiscuous Mode, and it is supported by

all LAN9118 Family members. To enable Promiscuous Mode, set the MAC_CR:PRMS bit to a ‘1’ and

reset the MAC_CR:INVFLT bit to’0’.

Table5.2 Address Filtering Modes

MCPAS PRMS INVFILT HFILT HPFILT DESCRIPTION

0 0 0 0 0 MAC Address Perfect filtering only for

all addresses.

0 0 0 0 1 MAC Address Perfect filtering for

physical address and Hash filtering for

multicast addresses

0 0 0 1 1 Hash Filtering for physical and

multicast addresses

0 0 1 0 0 Inverse Filtering

X 1 0 X X Promiscuous

1 0 0 0 X Pass all multicast frames. Frames with

physical addresses are perfect-filtered

1 0 0 1 1 Pass all multicast frames. Frames with

physical addresses are hash-filtered