LAN9303 datasheet(68/366 Pages) SMSC

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Small Form Factor Three Port 10/100 Managed Ethernet Switch with Single MII/RMII/Turbo MII

Datasheet

Revision 1.3 (08-27-09)

SMSC LAN9303/LAN9303i

DATASHEET

6.4.1.1

Learning/Aging/Migration

The ALR adds new MAC addresses upon ingress along with the associated receive port.

If the source MAC address already exists, the entry is refreshed. This action serves two purposes.

First, if the source port has changed due to a network reconfiguration (migration), it is updated.

Second, each instance the entry is refreshed, the aging status bit is set, keeping the entry active.

Learning can be disabled per port via the Enable Learning on Ingress field of the Switch Engine Port

Ingress Configuration Register (SWE_PORT_INGRSS_CFG).

During each aging period, the ALR scans the learned MAC addresses. For entries which have the

aging status bit set, the ALR simply clears the bit. As mentioned above, if a MAC address is

subsequently refreshed, the aging bit will be set again and the process would repeat. If a learned entry

already had its aging status bit cleared (by a previous scan), the ALR will instead remove the learned

entry. Therefore, if two scans occur before a MAC address is refreshed, the entry will be aged and

removed. Each aging period is approximately 5 minutes. Therefore an entry will be aged and removed

at a minimum of 5 minutes, and a maximum of 10 minutes.

6.4.1.2

Static Entries

If a MAC address entry is manually added by the host CPU, it can be (and typically is) marked as

static. Static entries are not subjected to the aging process. Static entries also cannot be changed by

the learning process (including migration).

6.4.1.3

Multicast Pruning

The destination port that is returned as a result of a destination MAC address lookup may be a single

port or any combination of ports. The latter is used to setup multicast address groups. An entry with

a multicast MAC address would be entered manually by the host CPU with the appropriate destination

port(s). Typically, the Static bit should also be set to prevent automatic aging of the entry.

6.4.1.4

Address Filtering

Filtering can be performed on a destination MAC address. Such an entry would be entered manually

by the host CPU with the Filter bit active. Typically, the Static bit should also be set to prevent

automatic aging of the entry.

6.4.1.5

Spanning Tree Port State Override

A special spanning tree port state override setting can be applied to MAC address entries. When the

host CPU manually adds an entry with both the Static and Age bits set, packets with a matching

destination address will bypass the spanning tree port state (except the Disabled state) and will be

forwarded. This feature is typically used to allow the reception of the BPDU packets while a port is in

the non-forwarding state. Refer to Section 6.4.5, "Spanning Tree Support," on page 74 for additional

details.

6.4.1.6

MAC Destination Address Lookup Priority

If enabl ed glob ally in the Switch Engi ne Globa l In gre s s Config ura t i on Regi ster

(SWE_GLOBAL_INGRSS_CFG) and per entry with the Priority Enable bit, the transmit priority for MAC

address entries is taken from the associated data of that entry.

6.4.1.7

Host Access

The ALR contains a learning engine that is used by the host CPU to add, delete, and modify the MAC

Address Table. This engine is accessed by using the Switch Engine ALR Command Register

(SWE_ALR_CMD), Switch Engine ALR Command Status Register (SWE_ALR_CMD_STS), Switch

Engine ALR Write Data 0 Register (SWE_ALR_WR_DAT_0), and Switch Engine ALR Write Data 1

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