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support tagging, so you should set the PVID to the same value at both ends of
the link (if the device you are attaching to is VLAN-aware), and configure an IP
interface for this VLAN if you need to connect it to other groups. (See “VLAN
Tagging” on chapter 2 and chapter 3.) This limitation will be removed for future
firmware versions.
4.2.2.IP Switching IP Switching (or packet forwarding) encompasses tasks required to forward packets for
both Layer 2 and Layer 3, as well as traditional routing. These functions include:
• Layer 2 forwarding (switching) based on the Layer 2 destination MAC address
• Layer 3 forwarding (routing):
- Based on the Layer 3 destination address
- Replacing destination / source MAC addresses for each hop
- Incrementing the hop count
- Decrementing the time-to-live
- Verifying and recalculating the Layer 3 checksum
If the destination node is on the same subnetwork as the source network, then the
packet can be transmitted directly without the help of a router. However, if the MAC
address is not yet known to the switch, an Address Resolution Protocol (ARP) packet
with the destination IP address is broadcast to get the destination MAC address from
the destination node. The IP packet can then be sent directly with the destination MAC
address.
If the destination belongs to a different subnet on this switch, the packet can be routed
directly to the destination node. However, if the packet belongs to a subnet not included
on this switch, then the packet should be sent to a router (with the MAC address of the
router itself used as the destination MAC address, and the destination IP address of the
destination node). The router will then forward the packet to the destination node via the
correct path. The router can also use the ARP protocol to find out the MAC address of
the destination node of the next router as necessary.
Note:
In order to perform IP switching, the switch should be recognized by other
network nodes as an IP router, either by setting it as the default gateway or by
redirection from another router via the ICMP process.
When the switch receives an IP packet addressed to its own MAC address, the packet
follows the Layer 3 routing process. The destination IP address is checked against the
Layer 3 address table. If the address is not already there, the switch broadcasts an ARP
packet to all the ports on the destination VLAN to find out the destination MAC address.