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Configuring the Switch
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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. After the MAC address is discovered, the packet is
reformatted and sent out to the destination. The reformat process includes
decreasing the Time-To-Live (TTL) field of the IP header, recalculating the IP header
checksum, and replacing the destination MAC address with either the MAC address
of the destination node or that of the next hop router.
When another packet destined to the same node arrives, the destination MAC can
be retrieved directly from the Layer 3 address table; the packet is then reformatted
and sent out the destination port. IP switching can be done at wire-speed when the
destination address entry is already in the Layer 3 address table.
If the switch determines that a frame must be routed, the route is calculated only
during setup. Once the route has been determined, all packets in the current flow
are simply switched or forwarded across the chosen path. This takes advantage of