In June,
Microsoft launched a new version of its Multi-Protocol Routing (MPR), Routing
and Remote Access Service (RRAS), formerly code-named Steelhead. Compared with
the earlier version of MPR, a built-in service in Windows NT Server 4.0, RRAS
has a rich set of routing and internetworking features that enable NT servers to
route data over IP and IPX LANs and WANs. Some new MPR features in RRAS are
Routing Information Protocol (RIP) 2.0 for IP, Open Shortest Path First (OSPF),
IP and IPX packet filtering, and a dial-up router. In "Steelhead Swims into
the Mainstream,", Mark Minasi describes RRAS's features.
Probably the most significant feature of RRAS is OSPF, a recommended routing
protocol for TCP/IP networks. OSPF provides more efficient network convergence
of routing information and lessand betteruse of network bandwidth
than traditional RIP. OSPF, however, is sophisticated and difficult to configure
and manage in a midsize or large network. To design, implement, and manage an
OSPF network successfully, you need a good understanding of OSPF and its
architecture, and you must know how to configure it.
Some Routing Basics
| Understanding and configuring Microsoft's
powerful link-state routing procotol for TCP/IP networks that's part of
the new Routing and Remote Access Service |
To move from one network to another, a packet needs an intermediate
connecting mechanism known as a
router (or
gateway in the IP
literature). Every router has a routing table, which specifies the next router
or network for the packet en route to a specific destination. When a router
receives a packet, it checks its routing table for the packet's destination
address to determine which attached router or network to send the packet to.
Through this procedure, routers deliver a packet from a source to a destination.
Routers use two methods to generate and maintain routing: static routing
and dynamic routing. In static routing, you manually create a routing table.
This method works for a small, stable network, but not for a large network. If
the network changes (e.g., if you add or remove a router or a link fails) you
must manually modify the routing table, which is an administrative burden. In
dynamic routing, a router maintains its routing table through a routing
protocol. A routing protocol defines the way in which a group of routers
exchanges routing information; a router chooses the best routing paths or routes
to destination networks.
A group of routers and networks under the same administration using a
common routing protocol is an autonomous system (AS); examples of ASs are
networks within a company, a university, or an Internet Service Provider (ISP).
The size of a network in an AS is not limited; an AS can be a small LAN with one
router or a large network with hundreds of routers. A routing protocol used
within an AS is an interior routing protocol (IRP), such as RIP and OSPF. A
routing protocol for ASs to exchange routing information is an exterior routing
protocol (ERP), such as exterior gateway protocol (EGP) and border gateway
protocol (BGP).
A network interface in a router is attached to a network segment or link so
that the router can communicate with its neighboring routers. A network
interface has a cost, which reflects the bandwidth, length, and priority of the
attached link, and reachable neighboring routers. A routing protocol uses the
cost to find the best routes.
Advantages of OSPF
An AS has two kinds of IRP: distance-vector and link-state. A
distance-vector routing protocol exemplifies a shortest-path algorithm; that is,
it uses the total number of hops between a source and a destination as the cost
variable in finding the best route. To continuously update their routing tables,
routers using a distance-vector routing protocol exchange information in terms
of distances from sources to destinations.
A link-state routing protocol, based on a link-state (or
shortest-path-first) algorithm, works in a different way. Instead of exchanging
distance information, routers exchange link states, or information about the
router's network interfaces. A router maintains a link-state database, which is
a map of the network. The router uses the link-state database to derive the
network topology and establish a routing table. A router using a link-state
routing protocol can compute a more accurate route than one using a
distance-vector routing protocol. This process is like reading a detailed map to
find the best route from one city to another.
RIP is the most widely used distance-vector routing protocol. In an RIP
network, each router broadcasts its routing table to neighboring routers every
30 seconds. When a router receives a neighboring router's routing table, it
updates its routing table and sends the updated table to neighboring routers.
This procedure is repeated until all routers in the network have updated their
routing tables and achieved network convergence.
RIP is simple but limited. You can easily implement an RIP network by
enabling RIP on each router. However, RIP is not good for large networks or
WANs. Broadcasting large routing tables in the network every 30 seconds consumes
network bandwidth quickly. RIP also limits a network to a maximum of 15 hops.