3Com Switch 7757, Configuration Manual

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31
IP R
OUTING
P
ROTOCOL
O
VERVIEW
n
When running a routing protocol, the Ethernet switch also functions as a router.
The word “router” and the router icons covered in the following text represent
routers in common sense and Ethernet switches running a routing protocol. To
improve readability, this will not be mentioned again in this manual.
Introduction to IP
Route and Routing
Table
IP Route Routers are used for route selection on the Internet. As a router receives a packet,
it selects an appropriate route (through a network) according to the destination
address of the packet and forwards the packet to the next router. The last router
on the route is responsible for delivering the packet to the destination host.
Routing Table The key for a router to forward packets is the routing table. Each router maintains
a routing table. Each entry in this table contains an IP address that represents a
host/subnet and specifies which physical port on the router should be used to
forward the packets destined for the host/subnet. And the router forwards those
packets through this port to the next router or directly to the destination host if
the host is on a network directly connected to the router.
Each entry in a routing table contains:
■
Destination address: It identifies the address of the destination host or network
of an IP packet.
■
Network mask: Along with the destination address, it identifies the address of
the network segment where the destination host or router resides. By
performing “logical AND” between destination address and network mask,
you can get the address of the network segment where the destination host or
router resides. For example, if the destination address is 129.102.8.10 and the
mask is 255.255.0.0, the address of the network segment where the
destination host or router resides is 129.102.0.0.A mask consists of some
consecutive 1s, represented either in dotted decimal notation or by the number
of the consecutive 1s in the mask.
■
Output interface: It indicates through which interface IP packets should be
forwarded to reach the destination.
■
Next hop address: It indicates the next router that IP packets will pass through
to reach the destination.
■
Preference of the route added to the IP routing table: There may be multiple
routes with different next hops to the same destination. These routes may be
discovered by different routing protocols, or be manually configured static