Demand-Dial Routing
Demand-Dial Routing |
|
The recommendation of static routing for on-demand demand-dial connections is based on the fact that the routing protocols provided with the Windows 2000 Routing and Remote Access Service (Routing Information Protocol [RIP] for IP, Open Shortest Path First [OSPF], RIP for IPX, Service Advertising Protocol [SAP] for IPX) have a periodic advertising behavior that can cause the connection to be made for each advertisement or to keep the connection up permanently if the advertising interval is less than the idle time-out. Due to the time, distance, and cost-sensitive nature of typical dial-up WAN links, running routing protocols over on-demand connections is not recommended.
Static routes for demand-dial routing are either manually configured or automatically configured by using autostatic updates, discussed later in this section.
Manual configuration of static routes is the adding of static routes that are available across the demand-dial interface using the Routing and Remote Access snap-in. For TCP/IP traffic, static IP routes must be added.
To add a static IP route that uses a demand-dial interface
Note
The Gateway field is not available when a demand-dial interface is selected. A demand-dial interface is a point-to-point connection and the forwarding IP address of the Gateway field is not needed to forward the IP traffic.
For IPX traffic, static IPX routes and static SAP services must be added.
Make sure to use the default IP route (0.0.0.0/0) carefully. While the default route can be used to simplify configuration of static routing over on-demand connections, you must consider its implications. The default IP route effectively summarizes all IP destinations and becomes the route used to forward IP packets when another more specific route is not found.
The use of the default route is an assumption that all other destinations is in the direction of the default route. When you are using a demand-dial interface to connect to the Internet, this is a valid assumption. However, when you are using a demand-dial interface to connect a branch office to a corporate office in a private intranet, the use of a default route needs to be carefully considered.
If a default IP route is configured to use a demand-dial interface, then the demand-dial connection can be initiated for IP traffic that is unreachable. For example, if an organization is using the private IP network 10.0.0.0/8 for its address space and a branch office uses 10.1.1.0/24 for the hosts of the branch office, then the static routing of the branch office router can be configured in the following ways:
If someone in the branch office attempts to send traffic to the destination IP address of 192.168.0.1, the router at the branch office does not have a route in its routing table for the packet. The packet is discarded and an ICMP Destination Unreachable-Host Unreachable message is sent to the sending host.
If someone in the branch office attempts to send traffic to the destination IP address of 192.168.0.1, the router at the branch office initiates the connection and forward the packet across the demand-dial connection to the corporate router. Neither the corporate router nor another router on the corporate intranet has a route in its routing table for the packet. The packet is discarded and an ICMP Destination Unreachable-Host Unreachable message is sent to the sending host.
Using a default route for branch office connectivity can produce undesirable results for unreachable traffic.
While manually entering a small number of static routes might be a feasible solution, when the number of routes is large or routes change, manual configuration is no longer a viable administrative option. To automatically add routes and services to the routing tables of a Windows 2000 router, the Routing and Remote Access service supports autostatic updates across demand-dial interfaces.
An autostatic update requests all known routes or services from the router on the other side of the connection and adds them to the routing tables of the requesting router. An autostatic update is a one-time, one-way exchange of routing information. After the routes are sent, the two routers do not periodically advertise even though the connection remains in a connected state.
Note
The "auto" in autostatic is the automatic adding of routes as static routes in a routing table. Autostatic updates do not occur automatically when the demand-dial connection is made.
To use autostatic updates for IP routes, the demand-dial interface must be added to the RIP routing protocol. The default operation mode for demand-dial interfaces for RIP is Autostatic update mode. The default outgoing packet protocol is RIP version 2 multicast. The default settings are correct when initiating a connection with another Windows 2000 router.
To use autostatic updates for IPX routes and SAP services, the demand-dial interface must be added to the RIP for IPX and SAP for IPX routing protocols. The default update mode for demand-dial interfaces for RIP for IPX is No update. You must change the update mode to Autostatic. The default update mode for demand-dial interfaces for SAP for IPX is No update. You must change the update mode to Autostatic.
To change the update mode
Note
When an autostatic update is requested, the existing routes that were obtained through a previous autostatic update are deleted before the request for routes is sent. If there is no response to the request, then the router cannot replace the routes it has deleted. This might lead to a loss of connectivity to remote networks.
Autostatic updates can be made manually or on a scheduled basis.
To manually update static IP routes across a demand-dial interface, perform the following procedure.
To manually perform an IP autostatic update
If the demand-dial interface is in a disconnected state, the connection is automatically made. After the link is in a connected state, the autostatic update begins. The autostatic update only transfers routing information from the answering router to the calling router. To transfer routing information from the calling router to the answering router, perform the preceding procedure on the answering router.
The transfer of IP routing information occurs through RIP for IP. The router on which the update was initiated sends a General RIP Request. The router on the other end of the connection responds with a RIP Response containing all of the appropriate routes in its IP routing table. The RIP routes received by the requesting router are automatically added as static routes to the requesting router's IP routing table. For more information about RIP messages, see "Unicast IP Routing" in this book.
To manually update static IPX routes and SAP services across a demand-dial interface, perform the following procedure.
To manually perform an IPX and SAP autostatic update
Just as in the case of an IP autostatic update, if the demand-dial interface is in a disconnected state, the connection is automatically made. After the link is in a connected state, the auto static update begins. The autostatic update only transfers routing and service information from the answering router to the calling router. To transfer routing and service information from the calling router to the answering router, perform the preceding procedure on the answering router.
The transfer of IPX routing information occurs through RIP for IPX. The router on which the update was initiated sends a RIP General Request. The router on the other end of the connection responds with a RIP Response containing all of the appropriate routes in its IPX routing table. The RIP for IPX routes received by the requesting router are added as static routes to the requesting router's IPX routing table. For more information about RIP messages, see "IPX Routing" in this book.
The transfer of SAP service information occurs through SAP for IPX. The router on which the update was initiated sends a SAP General Request. The router on the other end of the connection responds with a SAP Response containing all of the appropriate services in its SAP service table. The SAP services received by the requesting router are added as static services to the requesting router's SAP service table. For more information about SAP messages, see "IPX Routing" in this book.
Autostatic updates can be scheduled to occur periodically through a combination of a batch file or netsh script and Windows 2000 Scheduled Tasks. To perform an autostatic update using RIP for IP and the netsh script, the following netsh commands are run:
netsh interface set interface name=<Demand-Dial interface name> connect=CONNECTED
netsh routing ip rip update <Demand-Dial interface name>
netsh interface set interface name=<Demand-Dial interface name> connect=DISCONNECTED
For example, to update IP routes using a demand-dial interface called CorpHub, the netsh commands are:
netsh interface set interface name=CorpHub connect=CONNECTED
netsh routing ip rip update CorpHub
netsh interface set interface name=CorpHub connect=DISCONNECTED
The netsh commands can be run from a Windows 2000 batch file or they can be placed in a netsh script file. For example, the following script file Corphub.scp is created to run the preceding commands:
interface set interface name=CorpHub connect=CONNECTED
routing ip rip update CorpHub
interface set interface name=CorpHub connect=DISCONNECTED
To run the preceding script file, run the following at the command line:
netsh -f corphub.scp
After the Windows 2000 batch file or netsh script file is created, you can configure it to run periodically through Windows 2000 Scheduled Tasks.