SMSC LAN9311 User Manual

Two Port 10/100 Managed Ethernet Switch with 16-Bit Non-PCI CPU Interface

Revision 1.4 (08-19-08)

76

SMSC LAN9311/LAN9311i

 

Note: When specifying Port 0 as the destination port, the VID will be set to 0. A VID of 0 is normally

considered a priority tagged packet. Such a packet will be filtered if Admit Only VLAN is set
on the host CPU port. Either avoid setting Admit Only VLAN on the host CPU port or set an
unused bit in the VID field.

Note: The maximum size tagged packet that can normally be sent into a switch port (from the Host

MAC) is 1522 bytes. Since the special tag consumes four bytes of the packet length, the
outgoing packet is limited to 1518 bytes, even if it contains a regular VLAN tag as part of the
packet data. If a larger outgoing packet is required, the Jumbo2K bit in the 

 of Port 0 should be set.

The 

 configures the switch to

add the special VLAN tag in packets to the host CPU as a source port indicator. A setting of 11b should
be used only on the port that is connected to the host CPU (typically Port 0). Other settings can be
used on the normal network ports as needed. 

The special VLAN tag is a normal VLAN tag where bits 0 and 1 of the VID field specify the source
port (0, 1, or 2).

Upon egress from the host CPU port, the special tag is added. If a regular VLAN tag already exists,
it is not deleted. Instead it will follow the special tag. 

Note: Since the special tag adds four bytes to the length of the packet, it is possible for a normally

tagged, maximum size, incoming packet to become 1526 bytes in length. In order for the Host
MAC to receive this length packet without indicating a length error, the 

 in the Host MAC should be set to 8100h and the 

 should be set to a value other than 8100h. This configuration

will allow frames up to 1538 bytes in length to be received.

Note: Since the special tag adds four bytes to the length of the packet, it is possible for a normally

tagged, maximum size, incoming jumbo packet to become 2052 bytes in length. This packet
will be received by the Host MAC with the following conditions:

„

The receive status will indicate Frame Too Long

„

Up to four bytes of the end of packet may be truncated (the maximum receive length at the 

Host MAC is 2048).

A counter is maintained per port that contains the number of MAC address that were not learned or
were overwritten by a different address due to MAC Address Table space limitations. These counters
are accessible via the following registers:

„

„

„

A counter is maintained per port that contains the number of packets filtered at ingress. This count
includes packets filtered due to broadcast throttling, but does not include packets dropped due to
ingress rate limiting. These counters are accessible via the following registers:

„

„

„