Understanding Routing
Routing Overview
Routing can be defined as the process and its associated technologies that convey data over the internetwork from one local area network (LAN) to another. There are numerous technologies available that can enable connectivity between remote networks. The Internet can also provide LAN-to-LAN connectivity. Routing takes place in IP networks, based on IP routing tables and its entries. The information in the IP routing tables is used by IP hosts to transfer data over the internetwork. Routers are devices operating at the network layer of the OSI model that use the IP routing tables to forward traffic which it receives from a host or from a router.
Before actually configuring LAN routing, one of the first factors to decide on is the connection type that will be used for the remote site connection.
A number of technologies exist that can be used for remote network connections, including:
- Frame Relay: This is a WAN technology that uses other hardware components to establish remote site connections. A frame relay connection uses a standard leased line which connects the network site to the frame relay provider's nearest point of presence (POP). The frame relay provider then delivers the connection to the frame relay cloud. In order to use the frame relay provider for a LAN-to-LAN connection, you have to install a leased line at each site which connects the network to the nearest point of presence (POP) of the frame relay provider. The frame relay provider is then responsible for connecting the lines to the same frame relay cloud so that a connection can be established between the two networks.
- Frame relay provides flexibility.
- Each of your sites can be connected to a local point of presence (POP) which in turn leads to reduced cost of the leased lines.
- You can connect to multiple sites using a single frame relay connection.
- You pay for only the bandwidth that is used.
- Contracted bandwidth can be exceeded when heavy traffic conditions are present.
- Leased lines: Dedicated leased lines are also typically used to connect remote networks. While dedicated leased lines are commonly used for WAN links to enable remote network connectivity, purchasing and maintaining leased lines are expensive. In addition to this, you have to pay for allocated bandwidth all the time. This is due to leased lines being classed as persistent connections. This means that the connections are permanent connections, and remain open all the time.
- Dial-on demand connections: While the WAN connections provided by Integrated Service Digital Network (ISDN) and standard asynchronous modems are typically slower than dedicated leased lines, they can be disconnected at any time, and can also be used to enable connectivity to different locations. One of the main characteristics of dial-on demand connections is that you pay for the actual bandwidth that you are using.
- Virtual private networks (VPNs): Remote access VPNs provides a common environment where many different sources such as intermediaries, clients and off-site employees can access information via web browsers or email. Many companies supply their own VPN connections via the Internet. Through their ISPs, remote users running VPN client software are assured private access in a publicly shared environment. By using analog, ISDN, DSL, cable technology, dial and mobile IP; VPNs are implemented over extensive shared infrastructures. Remote access VPNs offer a number of advantages, including the elimination of WAN circuit and modem costs, cable modems enable fast connectivity and are relatively cost efficient, new users can be added with hardly any costs, and information is easily and speedily accessible to off-site users through Internet connectivity.
The benefits of using the frame relay WAN technology are:
The Routing and Remote Access Service (RRAS) provides multiprotocol routing services for Microsoft Windows 2000 Server and Windows Server 2003 computers. RRAS includes a wide variety of features that support unicast and multicast IP routing, IPX routing, AppleTalk routing, and remote access.
The components of unicast IP routing are described below:
- Static IP routing: You can use the Routing And Remote Access management console to configure and manage static routes. Static routes are manually created, and need to be modified whenever a change occurs to the network configuration.
- RIP versions 1, RIP version 2: RIP is a distance-vector routing protocol which is normally used for dynamic routing in small to medium sized internetworks.
- OSPF: This is a link-state routing protocol that is used for dynamic routing in medium to large sized internetworks
- Network address translation (NAT): NAT translates private IP addresses to Internet IP addresses that can be routed on the Internet.
- IP packet filtering: This is a security feature that enables you to define the traffic types that are allowed to pass over an interface. When you configure IP packet filters, you specify what traffic is allowed/denied, based on the following:
- Source address
- Destination address
- TCP port number
- UDP port number
- IP protocol numbers
- ICMP types and codes
- DHCP Relay Agent: This is a relay agent that forwards DHCP messages between the DHCP servers and DHCP clients that are located on different network segments.
- ICMP router discovery: ICMP router discovery makes is possible to advertise and reply to router solicitations.
The components of IP multicast routing are described below:
- Multicast forwarding: Multicast forwarding is an element of the TCP/IP protocol suite. You can use the Routing And Remote Access management console or the Netsh command-line tool to examine the content of the multicast forwarding table.
- IGMP version 1 and IGMP version 2: IGMP is a protocol of the TCP/IP protocol suite that is used to manage and control multicast group membership.
- Multicast boundaries: You can configure multicast boundaries, based on the following:
- Time-To-Live (TTL) specified in the IP header.
- IP multicast group address.
- Maximum multicast traffic allowed in kilobytes per second.
- Specific forwarding and routing: A Windows 2000 Server or Windows Server 2003 router can support specific multicast forwarding and routing if IGMP Router mode and IGMP Proxy mode are enabled for interfaces.
The components of IPX routing are described below:
- IPX packet filtering: This is a security feature that enables you to define the traffic types that are allowed to pass in and out of an interface. When you configure IPX packet filters, you specify what traffic is allowed/denied, based on the following:
- Source and destination IPX network
- Packet type
- Node
- Socket numbers
- RIP for IPX: This is a distance-vector routing protocol that is used on IPX internetworks. You can configure both static IPX routes and RIP route filters through the Routing and Remote Access Service (RRAS).
- SAP for IPX: This is a distance-vector advertising protocol that advertises services and each service's location on IPX internetworks. Through the Routing and Remote Access Service (RRAS), you can configure:
- Static SAP services
- SAP service filters
- NetBIOS over IPX: Through the Routing and Remote Access Service (RRAS), you can configure:
- RRAS to forward NetBIOS over IPX broadcasts
- Static NetBIOS names
Understanding Static Routing
With static routing, routing protocols are not used to communicate routing information between IP routers. Administrators have to manually create and modify the routing table entries. Each time a change occurs in the network configuration, the entries in the routing table have to be modified to reflect these changes. Static routing works well in a small network where it is easier to configure a small number of static routes than it is to configure dynamic routing.
A few advantages of using static routing are:
- Static routing is easy to deploy and configure.
- Because static routing does not involve routers communicating between each other, it works well for low bandwidth WAN links.
- Static routes can offer support for unnumbered connections.
- Static routes are not as resource intensive as the dynamic routing protocols.
A few disadvantages of using static routing are:
- Static routing only works for small networks where expansion is not likely.
- Maintaining static routes becomes costly as the network expands.
- Static routing provides no fault tolerance. If a route is incorrectly configured, the route remains unavailable until the issue is manually resolved.
How to add a static route using the Routing And Remote Access console
- Click Start, Administrative Tools, and then Routing And Remote Access to open the Routing And Remote Access management console.
- In the console tree, right-click Static Routes and then select New Static Route from the shortcut menu.
- When the Static Route dialog box opens, provide the appropriate information for the following settings:
- Interface
- Destination
- Network Mask
- Gateway
- Metric parameters
- Click OK.
How to add a static route using a command prompt
A few important factors to consider before adding a static route from the command prompt are summarized below:
- If you add a static route from the command prompt using the Route command, the static route will not appear in the list of static routes displayed in the Routing And Remote Access management console.
- If you use the Route command, you have to define an interface by a hexadecimal number. You cannot use the interface address.
- For a static route added from the command prompt to be persistent, you have to use the -p switch.
To add a static route from a command prompt,
- Open a command prompt
- Enter the following command:
- route add destination mask netmask gateway metric costmetric if interface
Understanding Dynamic Routing
With dynamic routing, the need to manually create and maintain static routes is eliminated. Dynamic routing use routing protocols so that IP routers can communicate with each other. The routing protocols also enable routers to share the information they have in their routing tables. A router that is configured to use dynamic routing forwards its routing table's content to the other routers configured for dynamic routing at regular time periods or intervals. When a router does not send its routing table at the specified time interval, the other routers simply remove the router from their routing tables. This process prevents traffic from being forwarded to the failed router. Once the failed router is online again, the router starts sending dynamic routing messages which enables the other routers to determine that traffic can be forwarded to the router again. This in turn causes the other routers to update their routing tables to include the specific router once more.
One of the main factors to consider when deciding on implementing dynamic routing is the actual routing protocol that you will use. The Routing and Remote Access service (RRAS) includes integrated support for the following dynamic routing protocols:
- Routing Information Protocol (RIP) version 2
- Open Shortest Path First (OSPF)
How to configure RRAS to operate as a LAN router
- Click Start, Administrative Tools, and then click Routing And Remote Access to open the Routing And Remote Access management console.
- In the console tree, select the server that you want to configure.
- From the Action menu, select Configure And Enable Routing And Remote Access.
- The Routing And Remote Access Server Setup Wizard starts.
- Click Next on the initial page of the Routing And Remote Access Server Setup Wizard.
- On the Configuration page, select the Custom Configuration option.
- Select the Any Combination Of The Features Available In Routing And Remote Access option and then click Next.
- On the Custom Configuration page, select the LAN Routing checkbox and then click Next.
- On the Completing The Routing And Remote Access Server Setup Wizard page, click Finish.
- Click Yes in the message box that appears, asking whether the Routing and Remote Access service should be started.
How to install the RIP routing protocol on a RRAS router
- Click Start, Administrative Tools, and then click Routing And Remote Access to open the Routing And Remote Access management console.
- In the console tree, expand the IP Routing node.
- Select the General subnode.
- From the Action menu, click the New Routing Protocol command.
- The New Routing Protocol dialog box opens.
- Select RIP Version 2 For Internet Protocol from the Routing Protocols list. Click OK.
- A RIP node is added beneath the IP Routing node in the console tree of the Routing And Remote Access console
- Select the RIP node in the console tree.
- From the Action menu, click the New Interface command.
- The New Interface For RIP Version 2 For Internet Protocol dialog box opens.
- Using the Interfaces list, select the interface which connects the computer to the LAN and then click OK.
- The RIP Properties dialog box for the interface which you have selected is displayed next.
- On the General tab, provide the following information:
- Specify whether the RIP version 1 or RIP version 2 packet format must be used for outgoing messages.
- Specify whether broadcasts or multicasts should be used.
- Specify whether incoming messages using the RIP version 1 format; or RIP version 2 format; or whether both of these formats should be processed.
- Click the Advanced tab.
- Set the value in the Periodic Announcement Interval (Seconds) setting to 300 seconds. This is the frequency at which the router transmits RIP messages.
- Set the value in the Time Before Routes Expire (Seconds) setting to 1800 seconds.
- Set the value in the Time Before Route Is Removed (Seconds) setting to 1200 seconds.
- Click OK.
How to configure RRAS packet filters
Click Start, Administrative Tools, and then click Routing And Remote Access to open the Routing And Remote Access management console.
- Right-click the server in the console tree, and then select Configure And Enable Routing And Remote Access from the shortcut menu.
- The Routing and Remote Access Server Setup Wizard starts.
- Click Next on the initial page of the Routing and Remote Access Server Setup Wizard.
- Select the Custom Configuration option. Click Next
- Click LAN routing and then click Next.
- Click Finish.
- Click Yes to enable LAN routing.
- Proceed to enable the RIP Version 2 for Internet Protocol.
- Once RIP Version 2 is enabled, right-click RIP in the console tree, and then select New Interface from the shortcut menu.
- Select the interface.
- The default setting for RIP if you are running Windows Server 2003 is:
- Outgoing packet protocol: dropdown list = RIP version 2 broadcast
- Incoming packet protocol: dropdown list = RIP version 1 and 2
- The following configuration is recommended if you are using RIP version 2; and Ethernet as the transport medium:
- Outgoing packet protocol: dropdown list = RIP version 2 multicast
- Incoming packet protocol: dropdown list = RIP version 2 only
- Click OK.
How to disable the RIP routing protocol on the RRAS router
- Click Start, Administrative Tools, and then click Routing And Remote Access to open the Routing And Remote Access management console.
- In the console tree, select the server that you want to disable RRAS for.
- From the Action menu, click the Disable Routing And Remote Access command.
- Click Yes in the message box which is displayed, warning you that the RRAS router will be disabled.
- The Routing and Remote Access service is stopped.
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