Routing path management – LevelOne GTL-2691 User Manual

Page 623

HAPTER

| General IP Routing

IP Routing and Switching

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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 the next hop 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.

OTE

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 the high throughput and low latency

of switching by enabling the traffic to bypass the routing engine once the

path calculation has been performed.

OUTING

ATH

ANAGEMENT

Routing Path Management involves the determination and updating of all

the routing information required for packet forwarding, including:

◆

Handling routing protocols

◆

Updating the routing table

◆

Updating the Layer 3 switching database