You can use access lists to be more specific about what is interesting traffic. In the preceding
examples, we set the dialer list to allow any IP traffic to bring up the line and keep it up.
That’s great if you are testing, but it can defeat the purpose of why you use a DDR line in
the first place. You can use extended access lists to set the restriction, for example, to only
e-mail or Telnet.
Here is an example of how you define the dialer list to use an extended access list:
804A(config)#dialer-list 1 protocol ip list 110
804A(config)#access-list 110 permit tcp any any eq smtp
804A(config)#access-list 110 permit tcp any any eq telnet
804A(config)#int bri0
804A(config-if)#dialer-group 1
In the previous example, you configure the dialer-list command to look at an IP extended
access list. This doesn’t have to be IP; it can be used with any protocol. Create your dialer list and
then apply it to the BRI interface with the dialer-group command.
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Using Optional Commands
You should configure two other commands on your BRI interface: dialer load-threshold
and dialer idle-timeout. The dialer load-threshold command is used in conjunction
with the ppp multilink command for multilink PPP (MPPP).
The dialer load-threshold command tells the BRI interface when to bring up the second
B channel. The value specified is from 1–255, where 255 tells the BRI to bring up the second B
channel only when the first channel is 100 percent loaded. The second option for that command
Dial-on-Demand Routing (DDR) 847
is in, out, or either. This calculates the actual load on the interface either on outbound traffic,
inbound traffic, or either inbound or outbound traffic. The default is outbound.
The dialer idle-timeout command specifies the number of seconds to wait for interesting
traffic before a call is disconnected. The default is 120 seconds.
RouterA(config-if)#dialer load-threshold 127 either
RouterA(config-if)#dialer idle-timeout 180
The dialer load-threshold 127 command tells the BRI interface to bring up the second B
channel if either the inbound or outbound traffic load is 50 percent. The dialer idle-timeout
180 command changes the default disconnect time from 120 to 180 seconds.
MPPP allows load-balancing between two or more B channels on a BRI or PRI interface. It is
non-vendor-specific and provides packet fragmentation and reassembly, along with sequencing
and load-calculating. Cisco’s MPPP is based on RFC 1990, which is referred to as PPP Multilink
Protocol (MP). The configuration would then look like this:
RouterA(config)#int BRI0
RouterA(config-if)#dialer load-threshold 127 either
RouterA(config-if)#dialer idle-timeout 180
RouterA(config-if)#ppp multilink
Not a tough configuration, but you should use it nonetheless. The ppp multilink command will
fragment packets and send them over both lines, which provides a load-balancing effect on the data
being sent over the link. You can verify that the Multilink Protocol is working by using the show ppp
multilink command.
and dialer idle-timeout. The dialer load-threshold command is used in conjunction
with the ppp multilink command for multilink PPP (MPPP).
The dialer load-threshold command tells the BRI interface when to bring up the second
B channel. The value specified is from 1–255, where 255 tells the BRI to bring up the second B
channel only when the first channel is 100 percent loaded. The second option for that command
Dial-on-Demand Routing (DDR) 847
is in, out, or either. This calculates the actual load on the interface either on outbound traffic,
inbound traffic, or either inbound or outbound traffic. The default is outbound.
The dialer idle-timeout command specifies the number of seconds to wait for interesting
traffic before a call is disconnected. The default is 120 seconds.
RouterA(config-if)#dialer load-threshold 127 either
RouterA(config-if)#dialer idle-timeout 180
The dialer load-threshold 127 command tells the BRI interface to bring up the second B
channel if either the inbound or outbound traffic load is 50 percent. The dialer idle-timeout
180 command changes the default disconnect time from 120 to 180 seconds.
MPPP allows load-balancing between two or more B channels on a BRI or PRI interface. It is
non-vendor-specific and provides packet fragmentation and reassembly, along with sequencing
and load-calculating. Cisco’s MPPP is based on RFC 1990, which is referred to as PPP Multilink
Protocol (MP). The configuration would then look like this:
RouterA(config)#int BRI0
RouterA(config-if)#dialer load-threshold 127 either
RouterA(config-if)#dialer idle-timeout 180
RouterA(config-if)#ppp multilink
Not a tough configuration, but you should use it nonetheless. The ppp multilink command will
fragment packets and send them over both lines, which provides a load-balancing effect on the data
being sent over the link. You can verify that the Multilink Protocol is working by using the show ppp
multilink command.
Configuring the Dialer Information
There are five steps in the configuration of dialer information:
1. Choose the interface.
2. Set the IP address.
Dial-on-Demand Routing (DDR) 845
3. Configure the encapsulation type.
4. Link interesting traffic to the interface.
5. Configure the number or numbers to dial.
Here is an example of how to configure the five steps:
804A#config t
804A(config)#interface bri0
804A(config-if)#ip address 172.16.60.1 255.255.255.0
804A(config-if)#no shutdown
804A(config-if)#encapsulation ppp
804A(config-if)#dialer-group 1
804A(config-if)#dialer string 8358662
Instead of the dialer string command, you can use a dialer map command, which
provides more security:
804A(config-if)#dialer map ip 172.16.60.2 name 804B 8358662
The dialer map command is used to configure the IP address of the next hop router, the
name of the remote router for authentication, and the number to dial to get there. The name is
usually the host name of the remote router, but it must be the name used by the remote router
to identify itself.
Take a look at the following configuration of an 804 router:
804B#show run
Building configuration...
Current configuration:
!
version 12.0
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname 804B
!
ip subnet-zero
!
isdn switch-type basic-ni
!
interface Ethernet0
ip address 172.16.50.10 255.255.255.0
846 Chapter 26 Integrated Services Digital Network (ISDN)
no ip directed-broadcast
!
interface BRI0
ip address 172.16.60.2 255.255.255.0
no ip directed-broadcast
encapsulation ppp
dialer idle-timeout 300
dialer string 8358661
dialer load-threshold 2 either
dialer-group 1
isdn switch-type basic-ni
isdn spid1 0835866201 8358662
isdn spid2 0835866401 8358664
dialer hold-queue 75
ppp multilink
!
ip classless
ip route 172.16.30.0 255.255.255.0 172.16.60.1
ip route 172.16.60.1 255.255.255.255 BRI0
!
dialer-list 1 protocol ip permit
!
The BRI interface is running the PPP encapsulation and has a timeout value of 300 seconds,
which is discussed in the next section. The load-threshold command makes both
BRI channels come up immediately (if you are paying for both, you want them both up all
the time) and is used with multilink, which we will discuss later in this section. The one
thing you should really notice is the number in the dialer-group 1 command. That number
must match the number in the dialer-list command, which is used to define what is
interesting traffic. The dialer hold-queue 75 command tells the router that when it
receives an interesting packet, it should queue up to 75 packets while it is waiting for the
BRI to come up. If more than 75 packets are queued before the link comes up, the packets
beyond the 75 will be dropped.
1. Choose the interface.
2. Set the IP address.
Dial-on-Demand Routing (DDR) 845
3. Configure the encapsulation type.
4. Link interesting traffic to the interface.
5. Configure the number or numbers to dial.
Here is an example of how to configure the five steps:
804A#config t
804A(config)#interface bri0
804A(config-if)#ip address 172.16.60.1 255.255.255.0
804A(config-if)#no shutdown
804A(config-if)#encapsulation ppp
804A(config-if)#dialer-group 1
804A(config-if)#dialer string 8358662
Instead of the dialer string command, you can use a dialer map command, which
provides more security:
804A(config-if)#dialer map ip 172.16.60.2 name 804B 8358662
The dialer map command is used to configure the IP address of the next hop router, the
name of the remote router for authentication, and the number to dial to get there. The name is
usually the host name of the remote router, but it must be the name used by the remote router
to identify itself.
Take a look at the following configuration of an 804 router:
804B#show run
Building configuration...
Current configuration:
!
version 12.0
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname 804B
!
ip subnet-zero
!
isdn switch-type basic-ni
!
interface Ethernet0
ip address 172.16.50.10 255.255.255.0
846 Chapter 26 Integrated Services Digital Network (ISDN)
no ip directed-broadcast
!
interface BRI0
ip address 172.16.60.2 255.255.255.0
no ip directed-broadcast
encapsulation ppp
dialer idle-timeout 300
dialer string 8358661
dialer load-threshold 2 either
dialer-group 1
isdn switch-type basic-ni
isdn spid1 0835866201 8358662
isdn spid2 0835866401 8358664
dialer hold-queue 75
ppp multilink
!
ip classless
ip route 172.16.30.0 255.255.255.0 172.16.60.1
ip route 172.16.60.1 255.255.255.255 BRI0
!
dialer-list 1 protocol ip permit
!
The BRI interface is running the PPP encapsulation and has a timeout value of 300 seconds,
which is discussed in the next section. The load-threshold command makes both
BRI channels come up immediately (if you are paying for both, you want them both up all
the time) and is used with multilink, which we will discuss later in this section. The one
thing you should really notice is the number in the dialer-group 1 command. That number
must match the number in the dialer-list command, which is used to define what is
interesting traffic. The dialer hold-queue 75 command tells the router that when it
receives an interesting packet, it should queue up to 75 packets while it is waiting for the
BRI to come up. If more than 75 packets are queued before the link comes up, the packets
beyond the 75 will be dropped.
Specifying Interesting Traffic
After setting the route tables in each router, you need to configure the router to determine what
brings up the ISDN line. An administrator uses the dialer-list global configuration command
to define what is interesting traffic.
The command to configure all IP traffic as interesting is as follows:
804A(config)#dialer-list 1 protocol ip permit
804A(config)#interface bri0
804A(config-if)#dialer-group 1
The dialer-group command associates a dialer list to the BRI interface. Extended access
lists can be used with the dialer-list command to define exactly which traffic is interesting.
We’ll cover that in a minute.
brings up the ISDN line. An administrator uses the dialer-list global configuration command
to define what is interesting traffic.
The command to configure all IP traffic as interesting is as follows:
804A(config)#dialer-list 1 protocol ip permit
804A(config)#interface bri0
804A(config-if)#dialer-group 1
The dialer-group command associates a dialer list to the BRI interface. Extended access
lists can be used with the dialer-list command to define exactly which traffic is interesting.
We’ll cover that in a minute.
Configuring the Static Routes
To forward traffic across the ISDN link, you configure static routes in each of the routers. The suggested
routing method is static routes. Keep the following in mind when creating static routes:
All participating routers must have static routes defining all routes of known networks.
Default routing can be used if the network is a stub network.
An example of static routing with ISDN follows:
RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.60.2
RouterA(config)#ip route 172.16.60.2 255.255.255.255 bri0
What this does is tell the router how to get to network 172.16.50.0, which is through
172.16.60.2. The second line tells the router how to get to host address 172.16.60.2 and to send
traffic out the BRI0 interface.
routing method is static routes. Keep the following in mind when creating static routes:
All participating routers must have static routes defining all routes of known networks.
Default routing can be used if the network is a stub network.
An example of static routing with ISDN follows:
RouterA(config)#ip route 172.16.50.0 255.255.255.0 172.16.60.2
RouterA(config)#ip route 172.16.60.2 255.255.255.255 bri0
What this does is tell the router how to get to network 172.16.50.0, which is through
172.16.60.2. The second line tells the router how to get to host address 172.16.60.2 and to send
traffic out the BRI0 interface.
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