IP1726 LF
Preliminary Data Sheet
12/33
Jun 15, 2005
Copyright © 2003, IC Plus Corp.
IP1726 LF-DS-R06
4.2
Switch Engine and Queue Management
4.2.1 Store & Forward Mechanism
The IP1726 LF utilizes the “store & forward” method to handle the switch engine. Each data packet will
not be forwarded to the destination port until the entire packet is received. The internal mechanism will
check whether the packet is correct. The data packet will be forwarded to the destination port if the
packet is correct; otherwise it will be discarded.
Besides the erroneous packet and the 802.3x pause packet are not forwarded. The 802.1D Reserved
Group packet with 01-80-c2-00-00-03 ~ 01-80-c2-00-00-0F MAC address can be optionally dropped by
setting pin 54 or register 01H.
4.2.2 Address Learning and aging time
The address learning function for each port can be either disabled or enabled by setting the
corresponding bit in registers 31H and 32H. The overall MAC address capacity is 4096. The aging time
can be programmed according to the system requirement, ranging from 30sec to 8400sec +-6.7%.
The IP1726 LF provides two kinks of hash method to store the MAC address table. One is the direct
mapping and the other is the CRC-12 algorithm. When the direct mapping method is selected, the
IP1726 LF recognizes the least significant 12 bits of the source MAC address. When the CRC-12
algorithm is used, the IP1726 LF executes the following equation to decide the address of the MAC
address table.
CRC-12 equation: X^12+X^11+X^3+X^2+X+1
Some source MAC address will be excluded in MAC address table. These data packets are:
Erroneous packet
802.3x pause packet
802.1D Reserved Group packet
Multicast source MAC address
4.2.3 Class of Service
The IP1726 LF implements two levels of priority queues, high priority and low priority. The priority for
each data packet is based on one of the following schemes: 802.1Q VLAN tag, IP TOS/DS or physical
port. Each incoming data packet can be mapped to either one priority. When no CoS function is enabled,
the first-in first-out forwarding method is used.
When the CoS for 802.1Q VLAN tag is enabled, the IP1726 LF will recognize 3 bits of precedence
carried by the VLAN tag and map it to the specified priority. The data precedence 0 to 3 will be treated as
low priority packet, and the packet with other values will be stored in high priority queue. Setting register
25H and 26H to “1” will enable the CoS function for the corresponding port. The LSB of both registers
controls the lowest port ID for that group. For example, the bit 0 of register 25H corresponds to port 1.
The IP1726 LF provides the IP layer CoS function by recognizing the DSCP(Differentiated Service Code
Point) Octet and mapping it to the specified priority. For IPv4 packet, DSCP is embedded in the TOS(type
of Service) Octet. For a IPv6 data packet, the Traffic Class Octet can be used to differentiate the Class of
Service. When this function is enabled, the IP1726 LF will automatically recognize the IP version and
capture the either the TOS field(IPv4) or Traffic Class(IPv6). The following tables show the location of
DSCP in a packet.
Destination MAC
Address(6 bytes)
Source MAC
Address(6 bytes)
0800H
IP version =
0100b(4 bits)
IHL(4 bits)
TOS(8 bits).
IP1726 LF maps
TOS[5:0] to 2 levels of
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