Foundry Networks Switch and Router Installation And Configuration Manual page 250

Foundry Switch and Router Installation and Configuration Guide
Packets that are not located in the address table are forwarded to all other Layer 2 Switch ports unless VLANs are
operating on the Layer 2 Switch. If the Layer 2 Switch is operating with VLANs, then the packet is forwarded only
to other ports within its VLAN.
If the source address of the packet received at the Layer 2 Switch is not resident in its address table, or if the
source port of the packet has changed, both the source address and its source port will be programmed into the
address table.
Layer 3 Architecture
When a packet arrives at either a Foundry backbone Layer 2 Switch or Layer 3 Switch, an address lookup is
initiated.
IP Version 4 Packets
If the IP address is located, then the Stackable performs the following Layer 3 IP operations on the IP packet:
• Decrements the TTL value.
• Checks to see if TTL value is greater than zero. If so, the packet will be forwarded.
• Performs destination MAC address substitution of the next hop router or end station. The source MAC
address will be replaced by the MAC address of the interface.
• Updates the header checksum.
Once Layer 3 operations are completed, the packet's priority is determined, and it is placed in the appropriate
buffer for forwarding to the target output port.
IPX Packets
When an IPX packet is received, the frame type is determined. When the incoming and outgoing frame types are
Ethernet 802.2 or 802.3, the Layer 2 Switch or Layer 3 Switch will perform hardware forwarding:
• If the destination network number is an internal or remote network number.
• If the destination network number is an internal or remote network number, the system will provide network
assistance.
• If the frame type is Ethernet II or Ethernet SNAP, the packet is sent directly to the CPU. The CPU will
increment the transport control count and forward it to its target output port.
Other Protocol Packets
Protocols not supported by Foundry Layer 3 Switches will be switched at Layer 2.
9 - 14 December 2000