Steelhead's OSPF Routing

Elect Designated Routers
When the network topology changes, neighboring routers start to exchange routing information until their link-state databases become the same. If n routers are in a segment, n *(n-1)/2 pairs of routers will exchange routing information. For example, Figure 1 shows 4 routers in the subnet 120.10.8.0 in the area 0.0.0.0; therefore, 6 pairs of routers will exchange information. To minimize the amount of routing information exchanged, when the routers are powered on, OSPF elects a designated router (DR) on every segment except point-to-point segments. All other routers in the same segment establish an adjacency with the designated router, exchange routing information, and synchronize the link-state database with the DR. If n routers are in a segment, only n-1 adjacencies or n-1 pairs of routers participate in routing information exchange. So, 4 routers require only 3 adjacencies. In terms of efficiency, using a DR reduces the amount of routing information exchanged from order (n*n) to order (n). Routing information exchange between the DR and other routers uses multicasting in a broadcast network but unicasting in a nonbroadcast network.

Each interface in a router has a priority. The priority can range from 0 (the lowest priority) to 255 (the highest priority); the default priority is 1. The network uses router priorities included in hello packets to elect a DR; the router with the highest priority is the DR. If two or more routers have the same priority, the router with the highest router ID will win the election. A router with priority 0 doesn't participate in the election. Therefore, if you want a router to be a DR, you can simply assign the highest priority to it.

For redundancy, OSPF uses the same criteria to elect a backup designated router (BDR). When the DR fails, the BDR becomes the new DR, and OSPF elects a new BDR.

Configure Microsoft OSPF
Familiarity with how OSPF works makes configuring a Microsoft OSPF router straightforward. Microsoft provides an intuitive administrative tool for router configuration. You can also install this tool on an NT Workstation to manage routers remotely.

The following steps explain how to configure a Microsoft OSPF router. You can use the flowchart shown in Figure 2, as a quick guide.

1. Prepare an NT server. You need an Intel- or Alpha-based server with NT Server 4.0 (with Service Pack 3 or later).
2. Install NICS. Make sure that your NICs are in the NT 4.0 Hardware Compatibility List (HCL--see http://www.microsoft.com/hwtest). Install NICs from the Network applet of Control Panel in the usual way. You install WAN cards in the same way that you install a LAN card. Assign an IP address to each interface. OSPF supports VLSM, so you can use different subnet masks in your network.
3. Install RRAS. You can download RRAS from http://www.microsoft.com/ntserver/info/routing&ras.htm. Microsoft offers three options (RAS, routing, and demand-dial); you must install at least the routing option to get the OSPF function.
4. Enable OSPF. Add OSPF to IP routing, and add each NIC to OSPF.
5. Configure OSPF for the router. Screen 1 shows the initial interface you use to define a router's properties and its areas.
6. Further configure OSPF for the router. After you have defined the basic properties of the router, including a router ID and the areas that the router belongs to, you must configure OSPF further for the router and its interfaces. A router can be an IR, an ABR, or an ASBR. If the router is an IR and is in a stub area, you need to enable Stub area from the Areas tab. If the router is an ABR, you need to configure a route summary (the network range) for each area that the ABR belongs to. If the router is an ABR in a stub area, you need to enable Stub area for the stub area from the Areas tab. You also need to enable Import summary advertisements if the area is an ordinary stub area, and disable Import summary advertisement if the area is a totally stubby area.
   If the ABR is on a virtual link, you need to set up the ABR's virtual interface by linking it to the other end via a transit area. If the router is an ASBR, you need to choose which routing protocols the ASBR will talk to. You can use RIP or static routing in the ASBR to communicate with an external network. You can even define which external routes the ASBR will accept.
7. Configure OSPF for each interface. You need to configure OSPF for the properties of each interface in the router. The properties include the area that the interface belongs to, the priority for DR election, the cost based on the bandwidth, the password for authentication, and the network type that the interface is attached to. If the network type is NBMA, you must define NBMA routing to reach neighboring routers. You can change the defaults of hello, dead, and poll intervals; but if you do, make sure these intervals are the same in all neighboring routers.

Good Opportunities
Microsoft aims to offer its customers a midrange router with packet-forwarding rates greater than 40,000 packets per second in regular use. Whether Microsoft's new RRAS can survive in or win today's highly competitive router market is questionable because Microsoft is still working on supporting quality of service, IP multicast routing protocols, and BGPs. RRAS, however, is tightly integrated into the NT Server operating system. Using RRAS in an NT network can reduce the cost of ownership because all NT services (file, print, applications, and routing) are in one box. RRAS offers a good cost-saving opportunity for companies that want to roll out NT to remote or branch offices. Using RRAS­ and the routing protocol for TCP/IP networks, OSPF­NT administrators can keep on top of routing and internetworking technologies.