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National Massage Certification Exam

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ISDN is a huge topic on both your Cisco CCNA and BCRAN CCNP exams. While many ISDN topics seem straightforward, it's the details that make the difference in the exam room and working with ISDN in production networks. Configuring and troubleshooting multilink PPP is just one of the skills you'll need to pass both of these demanding exams.

With BRI, we've got two B-channels to carry data, and both of them have a 64-kbps capacity. You might think it would be a good idea to have both channels in operation before one reaches capacity, and it is a great idea Problem is, it's not a default behavior of ISDN. The second b-channel will not begin to carry traffic until the first one reaches capacity.

With Multilink PPP (MLP), a bandwidth capacity can be set that will allow the second b-channel to bear data before the first channel reaches capacity. The configuration for MLP is simple, but often misconfigured. We'll use our good friend IOS Help to verify the measurement this command uses.

Enabling MLP is a three-step process:

Enable PPP on the link

Enable MLP with the command ppp multilink

Define the threshold at which the second b-channel should start carrying data with the dialer load-threshold command.

Let's say you wanted the second b-channel to start carrying data when the first channel reaches 75% of capacity. It would make sense that the command to do so would be dialer load-threshold 75... but it's not.

R1(config)#int bri0

R1(config-if)#ppp multilink

R1(config-if)#dialer load-threshold ?

Load threshold to place another call

The dialer load-threshold value is based on 255, not 100. To have this command bring the line up at a certain percentage, multiply that percentage in decimal format by 255. Below, I multiplied 255 by .75 (75%) to arrive at 191.

R1(config-if)#dialer load-threshold 191 ?

either Threshold decision based on max of inbound and outbound traffic

inbound Threshold decision based on inbound traffic only

outbound Threshold decision based on outbound traffic only

R1(config-if)#dialer load-threshold 191 either

As illustrated by IOS Help in the above configuration, dialer load-threshold has additional options as well. You can configure the interface to consider only incoming, outgoing, or all traffic when calculating when to bring the next channel up.

Configuring Multilink PPP is just one of the skills you'll need to earn your CCNA and pass the CCNP BCRAN exam. Don't underestimate ISDN on Cisco's certification exams!

National Massage Certification Exam

Earning your Cisco CCNA and CCNP is a tough proposition, and part of that is the fact that you quickly learn that there's usually more than one way to do things with Cisco routers ? and while that's generally a good thing, you better know the ins and outs of all options when it comes to test day and working on production networks. Working with Frame Relay subinterfaces and split horizon is just one such situation.

One reason for the use of subinterfaces is to circumvent the rule of split horizon. You recall from your CCNA studies that split horizon dictates that a route cannot be advertised out the same interface upon which it was learned in the first place. In the following example, R1 is the hub and R2 and R3 are the spokes. All three routers are using their physical interfaces for frame relay connectivity, and they are also running RIPv2 172.12.123.0 /24. Each router is also advertising a loopback interface, using the router number for each octet.

R1(config)#int s0

R1(config-if)#ip address 172.12.123.1 255.255.255.0

R1(config-if)#no frame inverse

R1(config-if)#frame map ip 172.12.123.2 122 broadcast

R1(config-if)#frame map ip 172.12.123.3 123 broadcast

R1(config-if)#no shut

R2(config)#int s0

R2(config-if)#encap frame

R2(config-if)#no frame inver

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R2(config-if)#frame map ip 172.12.123.3 221 broadcast

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R3(config)#int s0

R3(config-if)#encap frame

R3(config-if)#no frame inver

R3(config-if)#frame map ip 172.12.123.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

R3(config-if)#ip address 172.12.123.3 255.255.255.0

R1#show ip route rip

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.123.2, 00:00:20, Serial0

3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.3, 00:00:22, Serial0

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:06, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:04, Serial0

The hub router R1 has a route to both loopbacks, but neither spoke has a route to the other spoke's loopback. That's because split horizon prevents R1 from advertising a network via Serial0 if the route was learned on Serial0 to begin with.

We've got two options here, one of which is to disable spilt horizon on the interface. While doing so will have the desired effect in our little network, disabling split horizon is not a good idea and should be avoided whenever possible. We're not going to do it in this lab, but here is the syntax to do so:

R1(config)#interface serial0

R1(config-if)#no ip split-horizon

A better solution is to configure subinterfaces on R1. The IP addressing will have to be revisited, but that's no problem here. R1 and R2 will use 172.12.123.0 /24 to communicate, while R1 and R3 will use 172.12.13.0 /24. R3's serial0 interface will need to be renumbered, so let's look at all three router configurations:

R1(config)#interface serial0

R1(config-if)#encap frame

R1(config-if)#no frame inverse-arp

R1(config-if)#no ip address

R1(config-if)#interface serial0.12 multipoint

R1(config-subif)#ip address 172.12.123.1 255.255.255.0

R1(config-subif)#frame map ip 172.12.123.2 122 broadcast

R1(config-subif)#interface serial0.31 point-to-point

R1(config-subif)#ip address 172.12.13.1 255.255.255.0

R1(config-subif)#frame interface-dlci 123

R2(config)#int s0

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R2(config-if)#encap frame

R2(config-if)#frame map ip 172.12.13.3 221 broadcast

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R3(config)#int s0

R3(config-if)#ip address 172.12.13.3 255.255.255.0

R3(config-if)#encap frame

R3(config-if)#frame map ip 172.12.13.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

A frame map statement always names the REMOTE IP address and the LOCAL DLCI. Don't forget the broadcast option!

Show frame map shows us that all the static mappings on R1 are up and running. Note the "static" output, which indicates these mappings are a result of using the frame map command. Pings are not shown, but all three routers can ping each other at this point.

R1#show frame map

Serial0 (up): ip 172.12.123.2 dlci 122(0x7A,0x1CA0), static,

broadcast, CISCO, status defined, active

Serial0 (up): ip 172.12.13.3 dlci 123(0x7B,0x1CB0), static,

broadcast, CISCO, status defined, active

After the 172.12.13.0 /24 network is added to R1 and R3's RIP configuration, R2 and R3 now have each other's loopback network in their RIP routing tables.

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:20, Serial0

3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.1, 00:00:22, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.13.1, 00:00:20, Serial0

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.13.1, 00:00:22, Serial0

While turning split horizon off is one way to achieve total IP connectivity, doing so can have other unintended results. The use of subinterfaces is a more effective way of allowing the spokes to see the hub's loopback network.

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Chris Bryant has sinced written about articles on various topics from CISCO CCNA, Personal Desktop and Cisco CCNP. Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the. Chris Bryant's top article generates over 27100 views. Bookmark Chris Bryant to your Favourites.

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