wan backup paul flynn. 2 dial backup mission-critical applications require additional wan links in...
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![Page 1: WAN Backup Paul Flynn. 2 Dial Backup Mission-critical applications require additional WAN links in order to prevent downtime. Multiple T1s may be provisioned](https://reader035.vdocument.in/reader035/viewer/2022062714/56649d395503460f94a12721/html5/thumbnails/1.jpg)
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.