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Understanding the network.pdf - Back to Home

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access-list 100 permit tcp any any gt 1000<br />

dialer-list 1 pro<strong>to</strong>col ip list 100<br />

ISDN BRI Interface <strong>Back</strong>up Configuration<br />

IOS uses HSRP <strong>to</strong> provide LAN gateway redundancy, but in most cases, it is <strong>the</strong> WAN<br />

link that presents <strong>the</strong> greatest possibility of failure. In scenarios where multiple<br />

external paths exist, this vulnerability <strong>to</strong> failure is minimized somewhat. But,<br />

because most WAN-like failures are due <strong>to</strong> PSTN switch and <strong>network</strong> failures, unless<br />

<strong>the</strong>se circuits have been engineered for complete path redundancy, when one link<br />

fails, <strong>the</strong> o<strong>the</strong>r very likely will go down as well. Complete path redundancy is quite<br />

costly and difficult, unless your site is serviceable from multiple LEC/CLEC COs. To<br />

defend against such failures, <strong>the</strong> IOS provides <strong>the</strong> capability <strong>to</strong> assign a <br />

asynchronous serial or ISDN interface <strong>to</strong> be activated in <strong>the</strong> event of a failure or if<br />

additional bandwidth is needed.<br />

To configure <strong>the</strong> router <strong>to</strong> use link backup, <strong>the</strong> primary interface is configured first.<br />

You specify which interface will be used as <strong>the</strong> backup and under what conditions<br />

<strong>the</strong> backup link will be enabled and disabled. Three interface configuration<br />

subcommands are used:<br />

• specifies <strong>the</strong> backup interface.<br />

• defines <strong>the</strong><br />

primary load requirements that need <strong>to</strong> be met <strong>to</strong> bring up and bring down<br />

<strong>the</strong> link. This command can also be used <strong>to</strong> configure <strong>the</strong> backup interface <strong>to</strong><br />

come online during high bandwidth demand periods. For example, <strong>the</strong><br />

configuration entry configures <strong>the</strong> interface <strong>to</strong> come<br />

online when <strong>the</strong> primary interface has a utilization load of 90 percent and<br />

disable when <strong>the</strong> load returns <strong>to</strong> 40 percent.<br />

• specifies<br />

<strong>the</strong> time between <strong>the</strong> primary link failure and <strong>the</strong> backup link activation, and<br />

how long <strong>the</strong> backup link will remain active once <strong>the</strong> primary link has been<br />

res<strong>to</strong>red.<br />

Now let's examine a "backup only" configuration:<br />

leibniz(config-if)#ip address 172.16.44.1 255.255.255.248

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