KSZ8563R datasheet(26/221 Pages) MICROCHIP

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KSZ8563R

DS00002418D-page 26

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4.4

Switch

4.4.1

SWITCHING ENGINE

A high-performance switching engine is used to move data to and from the MAC's packet buffers. It operates in store

and forward mode, while an efficient switching mechanism reduces overall latency. The switching engine has a

256KByte internal frame buffer that is shared between all the ports.

For the majority of switch functions, all of the data ports are treated equally. However, a few functions such as IGMP

snooping, 802.1X, forwarding invalid VLAN packets, etc., give special recognition to the host port. Any port (but most

commonly port 3) may be assigned as the host port by enabling tail tagging mode for that port. Only one port may be a

host port.

When a switch receives a non-error packet, it checks the packet's destination MAC address. If the address is known,

the packet is forwarded to the output port that is associated with the destination MAC address. The following paragraphs

describe the key functions of destination address lookup and source address learning. These processes may be com-

bined with VLAN support and other features, which are described in the subsequent sub-sections.

4.4.2

ADDRESS LOOKUP

Destination address lookup is performed in three separate internal address tables in the device:

Address Lookup (ALU) Table: 4K dynamic + static entries

Static Address Table: 16 static entries

Reserved Multicast Address Table: 8 pre-configured static entries

4.4.2.1

Address Lookup (ALU) Table

The Address Lookup (ALU) Table stores MAC addresses and their associated information. This table holds both

dynamic and static entries. Dynamic entries are created automatically in hardware, as described in Section 4.4.2.4,

"Learning". Static entries are created by management software.

This table is a 4-way associative memory, with 1K buckets, for a total of 4K entries. A hash function translates the

received packet's MAC address (and tthe FID) into a 10-bit index for accessing the table. At each bucket are four fully-

associative address entries. All four entries are simultaneously compared to the MAC address (plus FID) for a possible

match.

Three options are available for the hashing function, as described in Table 4-5. If VLAN is enabled (802.1Q VLAN

Enable bit in the Switch Lookup Engine Control 0 Register), the VLAN group (FID) is included in the hashing function

along with the MAC address. If VLAN is not enabled, the hashing function is applied to the MAC address and FID in the

default VLAN (VID=1), which is 0.

4.4.2.2

Static Address Table

The 16-entry Static Address Table is typically used to hold multicast addresses, but is not limited to this. As with static

entries in the ALU table, entries in the Static Address Table are created by management software. It serves the same

function as static entries that are created in the ALU table, so its use is optional.

TABLE 4-5:

ADDRESS LOOKUP TABLE HASHING OPTIONS

HASH_OPTION

(Switch Lookup Engine

Control 0 Register

)

Description

01b (Default)

A hash algorithm based on the CRC of the MAC address plus FID. The hash algorithm uses

the CRC-CCITT polynomial. The input to the hash is reduced to a 16-bit CRC hash value.

Bits [9:0] of the hash value plus (binary addition) 7-bit FID (zero extended on the left) are

used as an index to the table. The CRC-CCITT polynomial is: X16+X12+X5+1.

10b

An XOR algorithm based on 16 bits of the XOR of the triple-folded MAC address. Bits [9:0]

of the XOR value plus 7-bit FID (left-extended) are used to index the table.

00b or 11b

A direct algorithm. The 10 least significant bits of the MAC address plus 7 bit FID are used

to index the table.

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