Using Dial Backup with Dialer Profiles

A physical interface can be either an active interface or a dial backup interface. This limitation led to customers wanting to fully utilize all of the interfaces they were purchasing. The solution was to allow a backup interface command to map to a dialer pool. Because a dialer pool is attached to a virtual interface called a dialer interface, and a physical interface can be a member of multiple dialer pools, this arrangement lets you use a physical interface in the normal course of business even though it might someday be called on to serve as a backup to a primary interface.

You need to configure a dialer interface as shown in Chapter 7, "Using ISDN." Once you finish that, go into interface configuration mode for the primary link and enter the following command:

 backup interface dialer number 

This command tells the primary link to use the dialer pool as the backup interface. When the primary link goes down, the router uses a physical interface that is a member of the specified dialer pool to serve as a backup link.

Load Sharing and Dial Backup

Using the backup load command enables you to activate one link and help out the primary link. Routing protocols, however, tend to have their own ideas about what paths traffic should take. For example, if you previously configured a router to bring up an ISDN connection to help out a T1 when the T1 had a certain load, a routing protocol such as OSPF isn't going to make use of the ISDN line without a little bit of help. OSPF determines the best path by looking at the bandwidth of the link. Because a T1 has greater bandwidth than ISDN, OSPF will prefer to send all data across the T1 and ignore the ISDN line.

You use load sharing with dial backup on an OSPF routed network by using the following command in interface configuration mode for the dial backup interface:

 ip OSPF cost cost 

Doing so tricks OSPF into thinking that a 128Kbps ISDN line is really a 10Mbps Ethernet link or whatever you need to simulate. OSPF will then load-share data across both lines.

IGRP and EIGRP aren't as difficult to work with. They both support load sharing across links of different speeds, although with a few additional commands. The problem most people have with load sharing with IGRP and EIGRP is the number of factors these protocols take into account when determining the best path.

Assume that you want to back up a T1 with an ISDN connection. A T1 has a bandwidth of 1.544Mbps, whereas an ISDN line has two channels with bandwidth of 128Kbps. It would take 12 ISDN lines to equal one T1. This gives you a variance of 12. Using the following command in routing protocol configuration mode tells the router to use the ISDN line even though it isn't nearly as fast as the T1:

 variance multiplier 

Doing so creates a problem because the router will load-balance equally across both lines. This arrangement is undesirable: either the ISDN line will become saturated or the T1 will be underutilized. To fix this issue, you have the following command, which you also configure in routing protocol configuration mode:

 traffic-share {balanced | min} 

The balanced option in this command tells the router that for every packet sent across the ISDN line, it should send 12 across the T1.

The load-sharing options for the commands discussed in this section follow:

  • cost In OSPF, the wire's bandwidth in bits per second divided into 100 million. Thus, a 10Mbps Ethernet link has a cost of 10. The cost command tricks the OSPF protocol into thinking that the link is a different speed from what it really is.

  • multiplier Determines the range of wires that may be used for load sharing. Each load-sharing link must be within a range of x to y, where x is the value of the primary link and y is x divided by the variance value.

  • balanced Tells the router to load-share according to the capabilities of each line.

  • min Tells the router to divide traffic only among routes with the best metrics.

Verifying a Dial Backup Configuration

The following command displays the status of the configured interface:

 show interface type number 

You see information on which device is serving as a backup, the delay before the backup is enabled, and the delay before the backup is disabled once the primary is restored. The section in question appears at the beginning of the interface status and looks like this:

 Backup interface Serial0/0, failure delay 0 sec, secondary disable delay 0 sec, kickin load 60%, kickout load 40% 

The kickin and kickout load values determine when the backup circuit activates and deactivates. It's common to think that you display this information via a show backup command instead of the show interface command.



CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
CCNP BCRAN Remote Access Exam Cram 2 (Exam Cram 640 - XXX)
ISBN: N/A
EAN: N/A
Year: 2003
Pages: 183

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net