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Tunneling 411
overhead, you certainly reduce the efficiency. So why even use tunneling in the first place?
There are several reasons:
·
To allow multiple protocols to flow over a single-protocol backbone
·
To overcome discontiguous network problems
·
To allow virtual private networks (VPNs)
·
To overcome the shortcoming of some routing protocols with low maximum metric
limitations
·
To reduce the amount of overhead of routing protocols
The reduction of overhead and the capability to have an IP-only backbone are the two most
compelling reasons to use tunneling. Consider the previous Figure 6-16, which shows a
network with a pocket of Novell hosts on each end of the network, but with no Novell hosts in
the center of the network. One alternative would have been to configure IPX on all four routers.
If tunneling is used in that case, Routers B and C do not need to perform IPX routing. RIP and
SAP updates are sent once per timer over the tunnel and are not processed by Routers B and C.
The amount of overhead from these protocols is greatly reduced, particularly when non-
broadcast multiaccess (NBMA) networks such as Frame Relay are in use. So, the backbone of
the WAN network can remain IP only, and when there are only pockets of the different
passenger protocols, these protocols can be forwarded using tunnels.
Tunneling for VPNs
As cited in the previous list, one reason for using tunnels is VPNs. Consider Figure 6-17, with
the cloud representing a VPN service from a service provider.
Figure 6-17
Tunneling for VPNs
Routers A1 and A2 are owned by Company A, and Routers B1 and B2 are owned by Company
B. The two companies do not want their traffic intermingled. If the service provider simply set
up routing protocols to each company's sites and advertised all the routes into the service
provider network, a couple of undesirable situations will occur. First, route filtering would be
required to keep Company A from learning routes to Company B, and vice versa. Also, if either
VPN
Provider
B2
B1
A2
A1
ch06.fm Page 411 Monday, March 20, 2000 5:11 PM