High Performance Two Port 10/100 Managed Ethernet Switch with 32-Bit Non-PCI CPU Interface

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6.4.4VLAN Support

The switch engine supports 16 active VLANs out of a possible 4096. The VLAN table contains the 16 active VLAN entries, each consisting of the VID, the port membership, and un-tagging instructions.

17

16

15

14

13

12 11

...

0

Member

Un-tag

Member

Un-tag

Member

Un-tag

 

VID

 

Port 2

Port 2

Port 1

Port 1

MII

MII

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.6 VLAN Table Entry Structure

On ingress, if a packet has a VLAN tag containing a valid VID (not 000h or FFFh), the VID table is searched. If the VID is found, the VLAN is considered active and the membership and un-tag instruction is used. If the VID is not found, the VLAN is considered foreign and the membership result is NULL. A NULL membership will result in the packet being filtered if Enable Membership Checking is set. A NULL membership will also result in the packet being filtered if the destination address is not found in the ALR table (since the packet would have no destinations).

On ingress, if a packet does not have a VLAN tag or if the VLAN tag contains VID with a value of 0 (priority tag), the packet is assigned a VLAN based on the Port Default VID (PVID) and Priority. The PVID is then used to access the above VLAN table.

The VLAN membership of the packet is used for ingress and egress checking and for VLAN broadcast domain containment. The un-tag instructions are used at egress on ports defined as hybrid ports.

Refer to Section 14.5.3.8, on page 375 through Section 14.5.3.11, on page 378 for detailed VLAN register descriptions.

6.4.5Spanning Tree Support

Hardware support for the Spanning Tree Protocol (STP) and the Rapid Spanning Tree Protocol (RSTP) includes a per port state register as well as the override bit in the MAC Address Table entries (Section 6.4.1.5, on page 64) and the host CPU port special tagging (Section 6.4.10, on page 75).

The Switch Engine Port State Register (SWE_PORT_STATE) is used to place a port into one of the modes as shown in Table 6.2. Normally only Port 1 and Port 2 are placed into modes other than forwarding. Port 0 should normally be left in forwarding mode.

Table 6.2 Spanning Tree States

Port State

Hardware Action

Software Action

 

 

 

01 - Blocking

Received packets on the port are

The MAC Address Table should be programmed

(also used for

discarded.

with entries that the host CPU needs to receive

disabled)

 

(e.g. the BPDU address). The static and override

 

Transmissions to the port are blocked.

bits should be set.

 

Learning on the port is disabled.

The host CPU should not send any packets to the

 

 

port in this state.

 

 

The host CPU should discard received packets

 

 

from this port when in the Disabled state.

 

 

Note: There is no hardware distinction between

 

 

the Blocking and Disabled states.

 

 

 

Revision 1.4 (08-19-08)

70

SMSC LAN9312

 

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