We discussed routers in general in an
earlier section. We also focused on the
TCP/IP protocol suite and how it operates on a LAN. We also
talked about some particular implementations in
network protocols. Now we
will focus more on how TCP/IP actually operates on the Internet or global
WAN in fairly broad terms. I will also have a couple extra web pages
over and above what Littlejohn Shinder covers.
We talked previously about the advantages of subnetting, particularly due to reducing broadcast traffic. IP forwarding is a phrase that describes how messages addressed to devices on separate subnets reach their destination. The following steps broadly outline what happens.
Again, a router must have an interface on more than one network. These networks can be LANs or WANs. A WAN interface can be a modem, an ISDN adapter or maybe even another WAN device.
Routers make use of routing tables to keep track of the IP addresses of other networks, including other routers and servers. Static routing requires the sys admin to manually enter the IP addresses for desired network addresses into the routing table. This can be done with the route command that comes with TCP/IP. This command line utility allows the sys admin to both add and delete routing table entries. You can also print out the routing table using a variant of this command.
Static routing gives a lot more control over the routes that can be used. But it also requires a huge amount of continually ongoing effort to maintain the table. In larger networks, this can easily become unmanageable.
Dynamic routing uses protocols to build and change routing tables automatically. These protocols allow routers to interact and exchange their routing table information. This offers several advantages including the following.
Dynamic routing almost always proves to be better for all but the smallest networks.
Routing Protocols. Almost all routing protocols can be lumped into one of two categories.
When using dynamic routing protocols, problems can arise such as the following.
Cisco's most common protocols are the following.
RIP - Routing Information Protocol is a distance vector protocol with specifications given in RFC 1058. RIP routers announce their routing table information to other routers at periodic intervals. RIP version 1 uses broadcast messages. RIP version 2 adds the capability for multicast messages.
One of the potential difficulties in these approaches is described in the following sequence of steps.
This sort of looping can arise unless certain countermeasures are put into effect so that router tables can converge to correct information. To do this Cisco adds in three methods to process routing information from other sources.
RIP works well for small or medium sized networks, but something more sophisticated has turned out to be necessary for larger networks. OSPF - Open Shortest Path First standards can be found in RFC 1247 and RFC 1583. The algorithm used to determine routes is not prone to the looping problems of RIP and is more efficient even though larger databases of information arise.
OSPF works by dividing the network into areas. This provides a hierarchical structure to OSPF routing tables. Each OSPF router belongs to an area and maintains a database only on the gateways in its area. A special router called an ABR - Area Border Router connects every area to a special backbone area. Whenever a packet needs to travel from one area to another it goes through this backbone area. This sort of hierarchy reduces the size of routing tables and reduces the time spent recalculating routes when changes are made.