Tunneling into IP

Tunneling one protocol into another is the process by which a protocol at a specific layer of the OSI model is wrapped into another protocol of the same layer or one higher in the stack. An example of this would be IPX, which is a Layer 3 protocol being wrapped inside IP, another Layer 3 protocol. Other examples include AppleTalk inside IP, and NetBIOS or source-route bridging encapsulated in IP—this is an example of a Layer 2 protocol being wrapped inside a Layer 3 protocol.

Figure 2-9 illustrates the steps of the IPX protocol being encapsulated and transported through an IP tunnel:

Step 1 Data from the application layer is passed down the OSI model to the network layer.

Step 2 At the network layer, the data is encapsulated in an IPX packet.

Step 3 At the network layer, the IPX packet is encapsulated into an IP packet.

Step 4 The IP packet is then inserted into the frame format at the data link layer for the media in which the frame will be sent out.

Why would anyone want to configure anything quite so torturous? Such a configuration certainly will not reduce or control the traffic propagated onto the network. Conversely, it will increase the traffic load because the original data will now have the additional header.

On the end routers, which are responsible for adding this extra header, there are obviously increased CPU requirements.

Figure 2-9 Tunneling IPX Within IP Workstation A

Application

Presentation

Session

Transport

Physical

Data

Session

Transport

Network

IPX

Data

->-

IP

IPX

Data

Data link

Frame Hdr.

IP

IPX

Workstation B

Application

Presentation

Session

Transport

Network

Data link

Physical

The reasons for this configuration are not justified in the name of network optimization. However, tunneling does make sense to ease the management of the entire network.

The administrator of the core no longer has to understand or worry about the vagaries of the disparate protocols. The client/server traffic should be kept locally to the user LAN networks, with traffic to remote networks connected via IP. Now the administrator can focus on the one protocol: IP.

Although the use of IP enables the administrator to utilize all the available optimization tools for IP, it should be understood that the nature of the traffic would still be inherently that of the originating protocol. A NetBIOS application generating a broadcast will be transformed into an IPX broadcast and tunneled in the IP protocol. It is delivered to the tunnel destination and stripped of the IP header, and the IPX broadcast is dealt with as normal. Some of the reasons for tunneling IPX through IP include the following:

• The traffic can utilize the advantages of IP and its sophisticated routing algorithms.

• The two ends of the tunnel appear as a single point-to-point link, although in reality they are separated by many routers.

• The network administrator for the backbone network to which the two LANs connect needs to understand only IP.

• The addressing scheme is simplified.

• Simple routing protocols may have a limited hop count, which is extended by the tunnel, which advertises the path across the tunnel as one hop.

Some things to consider when creating an IP tunnel for IPX include the following:

• The delay, or latency, created in tunneling the IPX traffic into IP may cause some applications to time out.

• Because the tunnel is viewed as a point-to-point link, separate tunnels are required for multiple links. Many tunnels on a physical interface can cause some memory problems on the interface.

• Care must be taken in redistributing routing protocols because the tunnel is often seen as a preferred path. This is because the route is advertised as a single hop. It may involve a much less favorable path, however. The tunnel may advertise the path as two hops—one hop through the tunnel and one hop to the destination. However, the 1 hop actually represents 10 hops across mixed media. The other path, which is rejected because it advertised three hops, is really only three hops away.

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