Telindus 1031, User And Reference Manual

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Telindus 1031 Router Chapter 7
User manual Configuring the router
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7.1 Introducing routing
What is routing?
Routing is the act of moving information across an internetwork from a source to a destination.
Routing versus bridging
Routing is often contrasted with bridging. At first sight, bridging might seem to do the same as routing.
The primary difference between the two is that bridging occurs at layer 2 (the link layer) of the OSI ref-
erence model, whereas routing occurs at Layer 3 (the network layer). In other words, bridging occurs at
a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where
the software component is more important. And because routing occurs at a higher level, it can perform
more complex analysis to determine the optimal path for the packet.
Basic routing activities
Routing involves two basic activities:
• determining optimal routing paths,
• transporting information groups (typically called packets).
Determining the optimal routing path
In order to determine a routing path, routers initialise and maintain routing tables. These routing tables
contain a variety of information. For example:
• Destination/next hop associations tell a router that a particular destination can be reached optimally
by sending the packet to a particular router representing the "next hop" on the way to the final desti-
nation. When a router receives an incoming packet, it checks the destination address and attempts
to associate this address with a next hop.
• Desirability of a path. Routers use metrics to evaluate what path will be the best for a packet to travel.
Routers communicate with one another and maintain their routing tables through the transmission of a
variety of messages. The routing update message is one such message that generally consists of all or
a portion of a routing table. By analysing routing updates from all other routers, a router can build a
detailed picture of network topology.
Transporting packets
In most cases, a host determines that it must send a packet to another host. Having acquired a router's
address by some means, the source host sends a packet addressed specifically to a router's physical
(i.e. Media Access Control or MAC) address, this time with the protocol (i.e. network) address of the des-
tination host.
As it examines the packet's destination protocol address, the router determines that it either knows or
does not know how to forward the packet to the next hop. If the router does not know how to forward the
packet, it typically drops the packet. If the router knows how to forward the packet, however, it changes
the destination physical address to that of the next hop and transmits the packet.
The next hop may be the ultimate destination host. If not, the next hop is usually another router, which
executes the same switching decision process. As the packet moves through the internetwork, its phys-
ical address changes, but its protocol address remains constant.