User guide

Layer 2 switch functional description Switching
2-3
iMG/RG Software Reference Manual (Switching)
2.2.2.3 Address management
The primary function of the layer 2 switch is to receive good packets from the ports, process them and forward
them to the appropriate ports for transmission. This frame processing involves the Ingress Policy, Queue Con-
troller, Output Queues and Egress Policy.
The normal packet flow involves learning how to switch packets only to the correct ports. The switch learns
which port and end station is connected to by remembering each packet's Source Address along with the port
number on which the packet arrived - and the vlan that it is on.
When a packet is directed to a new, unlearned MAC address, the packet is flooded out of all the ports (as long
as they belong on the same VLAN) except for the one on which it arrived. Once a MAC address/port number is
learned, all future packets directed to that end station's MAC addresses are directed to the learned port num-
ber only. This ensures that the packet is sent to the correct end station. This table can be displayed via the CLI
The address database is stored in the embedded switch memory and has a default aging time of about 300 sec-
onds (5 minutes). If no packets are received from that MAC Address during that aging interval, then the address
is purged from the database. If a MAC Address is received from a different port during this time, then the MAC
address is learned on that new port and all traffic is then routed to that new port.
The number of MAC addresses that can be learned differs between devices. (Kendin, BCM, Marvell, Marvell Gig)
2.2.2.4 Rate limiting support
The integrated layer 2 switch supports hardware rate limiting on receive and transmit independently on a per
port basis. The rate limiting applies to all the frame types: unicast, broadcast and multicast.
Some devices do provide the ability to rate limit the Multicast and Broadcast traffic. (BCM and Gig Marvell)
If the number of bytes exceeds the programmed limit, the switch will stop receiving or transmitting packets on
the port. In the transmit direction, extra packets are placed in one or more FIFO queues and sent as soon as
possible given the configured limit. Note that when multiple queues are configured, the highest priority queue is
emptied first.
In the receive direction, on some devices, there is an option provided for flow control to prevent packet loss. In
this case, if the configured limit is reached, and Flow Control is enabled, then a PAUSE frame will be sent to the
peer device. This will stop transmission of packets until the Gateway is ready to receive packets again.
2.2.2.5 Loop Detection
Loop detection is a feature available at layer 2 used to disable automatically one or more switch ports when a
loop is verified on one or more of these ports.
Ethernet loops are likely to happen when a Ethernet-to-Coax balun is used in installations where there are
appliances connected to coax cable that need to the6 ethernet ports. In this case, if the coax cable is not prop-
erly terminated, a signal reflection is generated on the coax cable segment and then reported to the ethernet
segment too causing high network degradation.