WAN Backup

Paul Flynn

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Dial Backup• Mission-critical applications require additional

WAN links in order to prevent downtime. • Multiple T1s may be provisioned for a particular

application and even backed up by a Frame Relay PVC.

• Although the additional lines may have been leased for backup purposes, these lines can also be used for load balancing or to route certain traffic.

• Strictly speaking, such links are extra links, not backup links.

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• The primary link is typically a Frame Relay PVC, or a leased line.

• Backup interfaces can be a physical interface or a logical interface such as a dialer profile.

• Typical backup interfaces include Integrated Services Digital Network Basic rate Interface (ISDN BRI) and asynchronous interfaces.

• Serial interfaces connected to either an asynchronous modem or ISDN BRI can also be backup interfaces.

• The backup interface is also called the secondary link

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Configuring Backup

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Backup Interface Options

• If a physical link is used as a backup to a primary connection, it is in standby mode and cannot be used as a link to another site.

• Next Slide illustrates a potential drawback of placing an interface in standby mode.

• Branch Office A wants to back up its Frame Relay connection with an ISDN BRI.

• However, Branch Office A also wants to use the same BRI interface as a DDR link to Branch Office B.

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• If Branch Office A places the physical BRI link in standby mode, it is deactivated.

• The link does not activate until the primary line fails or reaches a specified threshold.

• Therefore, the BRI link cannot be used to connect to Branch Office B.

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Dialer profiles and backup

• Configure one dialer profile to act as the backup line.

• This profile is in standby mode until engaged. • Configure another dialer profile for DDR

between the Branch Office A and Branch Office B.

• Then make the physical BRI interface a member of both dialer pools.

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• When using a BRI for a dial backup, neither of the B channels can be used while the interface is in standby mode.

• When using dialer profiles, it is possible to have one B channel bound to the backup dialer profile and the other B channel bound to the DDR dialer profile at the same time.

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Configuring dial backups with dialer profiles

1. Create and configure a dialer interface.

2. Make the physical interface to be used as the backup, typically Bri 0/0, a member of the dialer pool by using the dialer pool-member number command.

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3. Configure the primary interface to use a backup:

– Enter interface configuration mode for the primary interface.

– Assign a Layer 3 address to the primary interface.

– Specify the backup interface dialer to be used with the backup interface dialer number command.

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Load backup with OSPF• OSPF uses cost as the sole routing metric. • Cost is based on bandwidth. • For example, a 64-kbps link gets a metric of

1562, while a T1 link gets a metric of 64. • Unless both the primary link and the secondary

link have the same cost, OSPF will not install both routes in the routing table.

• If one link has a lower cost than the other, all routing occurs over the link with the lower cost, even if both lines are up.

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• When the OSPF interface costs for the primary and secondary links are unequal, configure the costs to be the same by using the following command:

• Router(config-if)#ip ospf cost interface-cost

• The OSPF cost can be a value in the range from 1 to 65535.

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Load backup with IGRP and EIGRP

• With IGRP and EIGRP, unequal-cost load balancing can be configured.

• Use the variance multiplier command to configure unequal-cost load balancing by defining the difference between the best metric and the worst acceptable metric.

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Show Backup Commands

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Floating static routes as backup

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Dialer watch

• The router monitors the existence of a specified route in the routing table.

• If that route is not present, the router initiates the dialup backup link.

• Unlike the other backup methods, such as backup interface or floating static routes, dialer watch does not require interesting traffic to trigger the dial.