Ericsson ECN330, User Manual

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Configuring the ECN330-switch
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1553-KDU 137 365 Uen D 2006-06-16
• 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 ECN330-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 the ECN330-switch, the packet
can be routed directly to the destination node. However, if the packet belongs to
a subnet not included on the ECN330-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 through 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 ECN330-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 through the ICMP process.
When the ECN330-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 ECN330-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 ECN330-switch determines that a frame must be routed, the route is
calculated only during setup. Once the route has been determined, all packets