High Performance Two Port 10/100 Managed Ethernet Switch with
Datasheet
6.4.1.1Learning/Aging/MigrationThe 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.
6.4.1.2Static EntriesIf 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).
6.4.1.3Multicast PruningThe 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.
6.4.1.4Address FilteringFiltering 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.
6.4.1.5Spanning Tree Port State OverrideA 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 and will be forwarded. This feature is typically used to allow the reception of the BPDU packets while a port is in the
If enabled in the Switch Engine Global Ingress Configuration Register (SWE_GLOBAL_INGRSS_CFG), the transmit priority for static MAC address entries is taken from the associated data of that entry.
6.4.1.7Host AccessThe 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 Register (SWE_ALR_WR_DAT_1).
Revision 1.4 | 64 | SMSC LAN9312 |
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