Layer 3 Switching -- Introducing the Router
Pages: 1, 2
Relaying packets and hop counts
Routers maintain information about other routers in the network. A router that is on the same network segment as another router is said to be a routing neighbor. A distance metric is assigned to the neighbor router. This distance metric is called a hop.
Neighboring routers are said to be one hop away from the local router. Figure 6-3 reflects the upgraded Sprocketís LAN with routers connected to the corporate Fast Ethernet backbone. A TCP/IP packet from a host on the manufacturing segment is being sent to the IBM front end processor on a token ring segment in the Sprockets data center.
The manufacturing router examines its routing table and sees that the IBM router is one hop count away on interface Fast Ethernet 0. No other paths are available in the table. The packet is then forwarded out the manufacturing router Fast Ethernet interface and sent to the IBM router.
In a more hierarchical topology, where there are significantly more routers, the server farm router may have reported a hop count of 2. The IBM router may have reported a hop count of 3 and the WAN router a hop count of 3. In this circumstance, the packet would have been forwarded on to the server farm router, which reported a better path of only two hops to the destination network segment.
In complex routing environments, such as the Internet, the packet is relayed incrementally, hop by hop (router by router) until it reaches its destination segment. How is this accomplished? The IP packet has a header field that contains the source and destination IP address. This is a network layer process; the destination IP address in the header is examined each time it enters the interface of a router. The routing table is consulted for which interface to forward the packet out to.
Nanna has her work cut out for her. She must now design an autonomous system (AS) for the new Sprocketís WAN-LAN strategy. There are ATM-WAN routers that connect Uncle Fredís network. There are redundant routers connecting the manufacturing LAN to the data center. All these routers will be exchanging routing information using the same routing protocol.
Nanna must assign this autonomous system a unique number and she must select an interior routing protocol. Nanna selects IGRP as her interior routing protocol. An interior routing protocol manages an autonomous system, such as her corporate network. Her service provider uses an exterior routing protocol, such as BGP, to manage all their customers' autonomous systems. Nanna will have to incorporate a strategy to redistribute her IGRP autonomous system routes into the service providerís BGP routes. BGP and IGRP are different routing protocols and donít explicitly communicate with each other. This route redistribution will allow Nanna the capability of accessing her network via her cable modem in her retirement home. Obviously, security risks exist with this topology, but we are using it for demonstrative purposes.
Next installment weíll delve into the realm of routing and routed protocols. Weíll also take a closer look at how Nanna implements her autonomous network. Until then...
Michael J. Norton is a software engineer at Cisco Systems.
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