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Understanding the network.pdf - Back to Home

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NOTE<br />

Administra<strong>to</strong>rs beware: Tunneling places additional load on <strong>the</strong> router's memory<br />

and CPU. Encrypted tunnels add even more processing load. Although tunneling<br />

(both encrypted and unencrypted) is supported on almost <strong>the</strong> entire Cisco router<br />

platform, this does not mean that you should implement tunneling (especially,<br />

encrypted tunnels) on Cisco router models. For most real-world tunneling<br />

applications, you will want <strong>to</strong> use at a minimum a 36×0 series router and <strong>the</strong> right<br />

software version and feature set.<br />

Unencrypted GRE Tunnels<br />

Creating multipro<strong>to</strong>col tunnels (MPTs) requires <strong>the</strong> use of GRE. A tunnel interface is<br />

created using <strong>the</strong> global configuration command . After <strong>the</strong> tunnel interface is created, it employs <strong>the</strong> same interface<br />

configuration subcommands available <strong>to</strong> o<strong>the</strong>r router interfaces. To complete <strong>the</strong><br />

tunnel configuration, however, <strong>the</strong> tunnel's mode, source, and destination must be<br />

set, plus any o<strong>the</strong>r desired Layer 3 pro<strong>to</strong>col configuration commands. The tunnel<br />

mode is configured using <strong>the</strong> subcommand . The tunnel source is set with . The tunnel destination is set with . Here is an example of a GRE tunnel moving IP and<br />

AppleTalk across an Internet link:<br />

router(config)#config t<br />

router(config)#int tunnel 100<br />

router(config-if)#ip address 147.225.96.1 255.255.255.252<br />

router(config-if)#appletalk cable-range 45-45 45.149<br />

router(config-if)#appletalk zone test-tunnel<br />

router(config-if)#tunnel source e<strong>the</strong>rnet 0<br />

router(config-if)#tunnel destination 147.225.69.1<br />

router(config-if)#tunnel mode gre ip

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