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Figure 10-10 Dialer Profiles: Pooling Multiple BRIs

Pool 1

As Figure 10-10 illustrates, dialer profiles allow great flexibility. You can put the same physical interface into multiple pools, enabling better usage of the interfaces and connected lines. You can use separate logic as defined on different dialer interfaces. In fact, you can use IP addresses in different IP subnets on different dialer interfaces, giving you much greater control over your network design and IP addressing choices. So, whether you just want to allow a router to use multiple interfaces to dial the same set of sites, or if you want to do something more complex, dialer profiles help give you the necessary functionality.

Multilink PPP

Multilink PPP allows multiple links between a router and some other device over which traffic is balanced. The need for this function is straightforward; some of the motivations for using it can be subtle. Figure 10-11 illustrates the most obvious need for multilink PPP.

Figure 10-11 Multilink PPP for a Dial-In Device

PC

For faster service, the PC dials two B channels instead of one. To use both channels at the same time, and to get the most out of the extra available bandwidth, MLP can be used. MLP breaks each packet into fragments, sends some fragments across each of the two links, and reassembles them at the other end of the link. With two dialed circuits, it breaks each packet into two equal-sized packets. The net result is that the links are used approximately the same percentage.

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Multilink PPP is also useful between routers. For example, in Figure 10-12, videoconferencing between Atlanta and Nashville uses six B channels between two routers.

Figure 10-12 Multilink B Channels Between Routers

In this example, if multilink PPP is used, the links have almost identical usage. However, the 384 KB needed for the videoconference is available.

Now consider the alternative to MLP—six parallel links without multilink PPP. Six routes to subnet 10.2.2.0/24 would exist in Router A’s routing table, assuming that Router A uses the ip maximumpaths 6 command. Presumably, some traffic would use each of the six routes in the routing table.

The problem with not using MLP in this case relates to the default internal switching method in a Cisco router, called fast switching. Fast switching balances across equal-cost routes by sending all packets to the same IP address over the same link. The result is that router Atlanta sends some packets over one link and some over the other, but the balancing is unpredictable. More importantly, all packets to the videoconference system’s single IP address in Nashville use the same link, effectively limiting the videoconference to 64 kbps. MLP solves this problem.

With MLP, the router treats the multiple links as a single link, with one route in the routing table, and lets MLP balance the traffic. Example 10-11 shows a sample multilink PPP configuration. The Atlanta and Nashville routers use two B channels of the same BRI.

Example 10-11 Multilink PPP Configuration for Atlanta

username Nashville password Robert interface bri 0

ip addr 10.3.3.1 255.255.255.0 encapsulation ppp

dialer idle-timeout 300

dialer load-threshold 25 either

dialer map 10.3.3.2 name Nashville 16155551234 dialer-group 1

pppauthentication chap ppp multilink

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MLP requires just a few additional commands compared to typical DDR and ISDN configuration. The two key commands are ppp multilink and dialer load-threshold. The ppp multilink command enables multilink PPP. The router can dynamically sense when two links are actually parallel links to the same other device based on some hidden PPP flows, so the command does not have to explicitly identify which links are working together with MLP.

The dialer load-threshold command tells the router to dial another B channel if the utilization average on the currently used links is more than 25 percent for either inbound or outbound utilization. Although it isn’t necessarily required for MLP, this command is useful so that you can simply add more B channel calls when the load requires it; MLP then ensures that the additional bandwidth is used.

Table 10-16 summarizes the additional commands used with MLP in dial environments.

Table 10-16 Summary of the New Configuration Needed for MLP Versus Legacy DDR

ISDN and DDR Configuration Summary

ISDN and DDR configuration can be overwhelming. They include a lot of different commands, and most people simply do not get to practice ISDN in a lab. You should be ready to interpret configurations and recognize the commands for each feature.

Legacy DDR configuration includes the most commands. You need static routes to send packets out the dial interfaces. You need dialer groups, sometimes referring to ACLs, to define what traffic causes a dial to occur. In other words, you define the “interesting” traffic. You then define the phone numbers to dial, either with the dialer string command when you want to dial a single site, or with the dialer map command when you want to dial multiple different sites. Finally, you might change the default settings for both idle timers.

For BRIs, you only need to worry about configuring the ISDN switch type and, in some cases, the ISDN SPIDs. For PRIs, you configure the switch type, and you never configure SPIDs. You also need to configure the encoding, framing, and list of channels inside the PRI that will be used. These are the commands that are configured under the controller configuration mode.

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With dialer profiles, you essentially pool multiple physical interfaces and create a dialer interface. The DDR configuration causes packets to be routed out the dialer interface. If a dial needs to be made, the dialer interface picks one of the physical ISDN interfaces that has an available B channel to dial the line.

Finally, MLP allows you to efficiently use multiple parallel B channels connected between two devices. To configure it, you use the ppp multilink command.