In the Daily Drill Down “Using Windows 2000 Server as a router on your network,” I explained how to deploy a Windows 2000 router to support IP routing, including how to configure and use static routes. Static routes enable a router to route IP packets without relying on a protocol, which routers use to share route table data. In this Daily Drill Down, I’ll take a closer look at Routing Information Protocol (RIP), one of the routing protocols included with Windows 2000 Server. In an upcoming Daily Drill Down, I’ll focus on the other routing protocol, Open Shortest Path First (OSPF).
What’s a router again?
Here’s a quick review: A router resides as a node on two or more subnets, each of which is connected through one of the router’s network interfaces. By residing on each subnet, the router is accessible to each node on a given subnet and is able to route IP packets between the two subnets.
As the network grows beyond two subnets, routing becomes a little more complex. Not only might a router be tasked with routing packets from subnet A to subnet B, but it might also need to route traffic to other local subnets or the Internet. A router concerns itself only with routing traffic to the next router up the line (the next hop), which simplifies the problem somewhat. The router doesn’t have to know the absolute route for every packet, but it needs to know through which of its interfaces it needs to route the packet to move the packet one hop closer to its destination.
Assuming a simple network with three local subnets and an Internet connection, for example, the router needs to know which interface to use to transmit Internet traffic coming from the internal subnets. The router at the other end of that connection passes the traffic on to its next hop.
Configuring and using RIP
As explained in “Using Windows 2000 Server as a router on your network,” routers use one of two methods to determine how to route traffic: static routing or dynamic routing. In that Daily Drill Down, we took a look at static routes. With static routing, you manually create the router’s routing table by adding static routes that tell the router how to route traffic destined for specific subnets or hosts. The default route tells the router how to route packets that don’t fit the criteria of any of the other routes in the routing table.
Dynamic routing becomes a necessity as the number of subnets and routers grows, particularly in a dynamic environment where new routers come on line frequently. Instead of using static routes in this situation, a dynamic means for routers to share routing information is needed.
RIP is the most common routing protocol routers used to share routing data. RIP is relatively easy to configure but is limited to a maximum of 15 hops, making it suitable mainly for small to midsized networks. RIP considers to be unreachable any destination more than 15 hops away.
A router that uses RIP builds its routing table dynamically when it first boots. Initially, the routing table contains only the routes for the networks that are physically connected to the router. RIP then periodically broadcasts announcements containing its routing table entries, and adjacent routers update their own routing tables based on these RIP announcements. RIP also supports triggered updates, which occur when a router detects a network change, such as another router going down. The router that detects the condition generates a triggered update broadcast, which other routers use to modify their routing tables. When the adjacent router comes back on line, another triggered update occurs. Windows 2000 supports both RIP version 1 and RIP version 2.
The first step in configuring RIP is to add the protocol, which you do through the Routing and Remote Access Service (RRAS) console. Expand the server you want to manage in the RRAS console, expand the IP Routing branch, and then right-click General and choose New Routing Protocol. Select RIP Version 2 For Internet Protocol and click OK. You’ll see a new node named RIP under the IP Routing branch. The next step is to specify the interface(s) on which RIP will operate. In the RRAS console, right-click RIP and choose New Interface. Select the appropriate interface from the list and click OK.
Configuring RIP interface properties
Next, you need to configure RIP’s properties. If you’ve just specified an interface for RIP, Windows 2000 automatically pops up the property sheet for the interface. Otherwise, select the RIP branch and then right-click the interface and choose Properties. The General page lets you configure several properties, as shown in Figure A.
|Here’s the General property page for a RIP interface.|
The Operation Mode property specifies the way in which RIP updates routes. The Auto-Static Update Mode option configures RIP to send out route announcements only when adjacent routers request an update. Routes learned through Auto-Static Mode are treated as static routes and are not removed from the routing table even if the router is rebooted, although you can manually remove the routes. Auto-Static Update Mode is the default mode used for demand-dial interfaces.
The second option for operation mode is Periodic Update Mode. When you enable this option, RIP automatically generates RIP announcements at a predefined interval (configured through the Periodic Announcement Interval on the Advanced property page). Any routes added using this mode are handled as RIP routes and are flushed when the router is rebooted. They must be added again through RIP advertisements. Periodic Update Mode is the default mode for LAN interfaces.
The Outgoing Packet Protocol property specifies the protocol that RIP uses for outgoing RIP announcements. If all adjacent routers support RIP v2, select RIP Version 2 Multicast. In a mixed environment where RIP v1 and RIP v2 routers are present, select RIP Version 2 Broadcast. You can’t use the multicast option in this scenario because RIP v1 doesn’t support multicast announcements. If none of the adjacent routers supports RIP v2, select RIP Version 1 Broadcast. The final option, Silent RIP, prevents the router from generating RIP announcements and causes it to operate in Listen-Only Mode. In this mode, the router listens for RIP announcements from other routers and updates its routing table based on those RIP announcements, but it doesn’t broadcast its own announcements.
The Incoming Packet Protocol property specifies the protocol the router uses for incoming packets. Select an option based on the capabilities of the adjacent routers. Or select Ignore Incoming Packets if you want the router to ignore RIP announcements from adjacent routers. This option places the router in Announce-Only Mode.
Use the Added Cost For Routes property to modify the cost for the route. You would increase this number to increase the cost of the route and direct traffic through other, less costly routes when possible. Keep in mind that RIP is limited to a maximum of 15 hops, and routes with an effective cost of more than 15 are considered unreachable.
The Tag For Announced Routes property lets you assign a tag number to be included with all RIP announcements. Inclusion of a tag number is applicable only to RIP v2. The tag is used to mark specific routes for administrative purposes and is generally not required.
The Activate Authentication option lets you enable a password for incoming and outgoing RIP announcements. With this option enabled, all other routers connected to the selected interface must also be configured for the same password. The plain-text password specified by the Password field is added to all outgoing RIP announcements. Incoming announcements are scanned for the password, as well. This feature serves as a means for routers to recognize one another but doesn’t actually provide security. It simply provides a way for adjacent routers to exclude RIP announcements from routers that are not configured with the specified password. In effect, it gives you a means of grouping routers into a logical set.
Use the Security page of the interface’s properties to specify which routes are accepted or rejected based on the routes’ destination addresses. For outgoing routes, specify which routes are broadcast based on their destination addresses.
The Neighbors page lets you configure how your Windows 2000 router interacts with adjacent routers. Use the option Use Broadcast Or Multicast Only to restrict RIP announcements to the protocol specified for outgoing packets on the General page. Select the option Use Neighbors In Addition To Broadcast Or Multicast if you want to specify routers to which your router sends unicast RIP announcements as well as RIP announcements using the protocol specified for outgoing packets on the General page. To disable broadcast announcements, select the option Use Neighbors Instead Of Broadcast Or Multicast and define a list of routers to which your router sends unicast RIP announcements. This latter option is useful in networks with routers that don’t support RIP broadcasts but do accept unicast announcements.
The Advanced property page for a RIP interface, shown inFigure B, offers several options. I’ll look at each of these options.
|The Advanced property page for a RIP interface offers several options.|
- Periodic Announcement Interval: This value specifies the frequency of RIP announcements from the local router. This value is used in conjunction with Periodic Update Mode, which you set through the General property page for the RIP interface. You can specify a value in seconds between 15 seconds and 24 hours.
- Time Before Routes Expire: This setting specifies the time-to-live (TTL) for routes that are learned from other routers through RIP. Routes that do not update before they exceed the specified TTL are marked as invalid. As with the announcement interval, this setting is applicable only with Periodic Update Mode.
- Time Before Route Is Removed: Use this setting to specify the amount of time a route will remain in the routing table before it expires and is removed. Valid values are between 15 seconds and 72 hours. This setting is applicable only with Periodic Update Mode.
- Enable Split-Horizon Processing: This option, when enabled, prevents routes learned on a given network from being announced on that same network. Deselecting the option allows those routes to be announced.
- Enable Poison-Reverse Processing: Use this option to assign a metric of 16 to those routes learned on a given network that are announced on the same network. Assigning a metric higher than 15 marks the routes as unreachable.
- Enable Triggered Updates: Use this option to allow the router to generate triggered updates, as discussed earlier.
- Send Clean-Up Updates When Stopping: Selecting this option causes the local router to broadcast RIP announcements for all routes with a metric of 15 to indicate to adjacent routers that the routes are unreachable. When the router comes back up, it generates additional announcements that reannounce the routes with their default metrics, making them available again.
- Process Host Routes In Received Announcements: Use this option to include host routes received in incoming RIP announcements. By default, host routes are ignored.
- Include Host Routes In Sent Announcements: Use this option to include host routes in outgoing RIP announcements. By default, host routes are not included.
- Process Default Routes In Received Announcements: Use this option to include default routes learned through incoming RIP announcements. By default, the default routes are ignored. Enabling this option could result in the router being disabled if the default routes learned through RIP are not applicable to the local router. So, use this option with discretion and only if the default routes apply to all routers on the interface.
- Include Default Routes In Sent Announcements: Use this option to include default routes in outgoing RIP announcements. See the previous item for an explanation of why this can cause problems.
- Disable Subnet Summarization: Use this route to prevent subnet routes from being summarized by class-based network ID for outgoing RIP announcements generated to networks that are not part of the same class-based network. Subnet summarization can improve routing performance by, in effect, sorting the routes. Subnet summarization requires that all adjacent routers support either RIP v2 Broadcast or RIP v2 Multicast. The option is disabled by default.
That takes care of configuration for the selected interface. Go through the process again for any other interfaces on which you want to use RIP. If you’re using all of the default settings, you need to add RIP to the desired interface only.
There are a few general properties you can configure for RIP, as well. In the RRAS console, right-click RIP under the IP Routing branch and choose Properties. You can configure the maximum delay for triggered updates as well as logging options, and you can specify how the local router handles RIP announcements. You can configure the router to accept all announcements, accept all announcements from a list you specify, or reject all announcements from a list you specify.
When your Windows 2000 server acts as a router on your network, it communicates with other routers in one of two ways—by using RIP or by using OSPF. In this Daily Drill Down, I’ve shown you how to configure your Windows 2000 server to support RIP.
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