10G Core Routing Switch User Manual
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Figure
About hops
As the networks may have different sizes, the segment lengths connected between two different pairs of routers are
also different. The number of route segments multiplies a weighted coefficient can serve as a weighted measurement
for the actual length of the signal transmission path.
If a router in a network is regarded as a node and a route segment in the Internet is regarded as a link, message
routing in the Internet works in a similar way as the message routing in a conventional network. Message routed
through the shortest route may not always be the optimal way route. For example, routing through 3 LAN route
segments may be much faster than that through 2 WAN route segments.
7.1.2.
Route Selection through the Routing Table
The key for a router to forward packets is the routing table. Each router saves a routing table in its memory, and each
entry of this table specifies the physical port of the router through which the packet is sent to a subnet or a host.
Therefore, it can reach the next router in via a particular path or reach a destination host via directly connected
network.
A routing table has the following key entries:
Destination address: It is used to identify the destination IP address or thedestination network of IP packet, which
is 32 bits in length.
Network mask: It is made up of several consecutive "1"s, which can be expressed either in the dotted decimal
format or by the number of the consecutive "1" s in the mask. Combining with the destination address, it is used
to identify the network address of the destination host or router. If the destination address is ANDed with the
network mask, you will get the address of the network segment where the destination host or router is located.
For example, if the destination address is 129.102.8.10, the address of the network where the host or the router
with the mask 255.255.0.0 is located will be 129.102.0.0.
Output interface: It indicates an interface through which an IP packet should be forwarded.
Next hop address: Indicates the next router that an IP packet will pass through.
Priority added to the IP routing table for a route: There may be different next hops to the same destination. These
routes may be discovered by different routing protocols, or they can just be the static routes configured manually.
The one with the highest priority (the smallest numerical value) will be selected as the current optimal route.
According to different destinations, the routes can be divided into the following:
Subnet route: The destination is a subnet.
