ISDN BRI Configuration

BRI circuits are treated by the router in much the same way that an analog connection is. They're both temporary, low-speed links. A BRI makes an excellent solution for a remote site with sporadic traffic to a central site. The differences are handled in Open Systems Interconnect (OSI) Layers 1 and 2. That said, you must configure several items for proper ISDN operation.

ISDN Switch Type

There are several ISDN switch types, depending on who the manufacturer is, where the switch is located, and what type of ISDN is being used. When ordering ISDN, you must gain this information from your provider. If the router does not know what switch type is being used, no ISDN calls can take place.

You can configure the switch type in two places. You can configure it in global configuration mode, where it gives each BRI interface a default setting, or you can set it in interface configuration mode. Any interface-specific switch type setting overrides the switch type configured globally for the router.

You tell the router what switch type it is using with the following command:

 Router(config)#isdn switch-type type 

The following command shows an example of setting an AT&T 5-ESS switch as the one the router will be using:

 Router(config)#isdn switch-type basic-5ess 

Configuring Interesting Traffic

One of the properties of dialup connections is the ability to control the traffic that starts the connection. If you pay for a circuit for every minute it's up, do you want Router Information Protocol (RIP) updates crossing? Setting a value for interesting traffic doesn't affect what can cross, only what starts the connection and when it ends, after no interesting traffic crosses in a certain amount of time. To configure what type of traffic can cross, you need to establish a dialer list. The dialer list can specify a whole protocol, such as IP, or it can refer to an access list number. The access list is then used to define a more granular list of traffic that can activate the link. Configure a dialer list statement in the following format:

 RouterA(config)# dialer-list # protocol protocol [permit | list #] 

The first example of a dialer list allows all IP traffic to activate the circuit so that traffic can cross:

 RouterA(config)#dialer-list 1 protocol ip permit 

The next example shows a dialer list statement that references an access list. The access list permits only Telnet traffic to activate the link:

 RouterA(config)#dialer-list 1 protocol ip list 101 RouterA(config)#access-list 101 permit tcp any any eq 23 

Setting only Telnet traffic to bring up the link would be unusual, but using only Internet Control Message Protocol (ICMP) ping traffic wouldn't be. Dialer lists typically do not permit only certain types of traffic to start the circuit but prevent certain types from starting it. Access list 101 in the previous example doesn't only allow Telnet to cross the circuit, but rather permits only Telnet to bring up the circuit. Remember that once a circuit is up, all traffic can cross. The access list isn't used to filter the interface but instead filters the startup process.

Interface Configuration

You can configure the BRI interface in many ways to support anything from a simple ISDN configuration to a complex configuration. Although Chapter 8, "Dial-on-Demand Routing," goes into more detail about the options that are available, we discuss the basics here.

Although it's possible for a BRI interface to bridge traffic, it's more common for the traffic to be routed, and that's what the examples in this text assume. One of the first things that you should do is place a network address on the interface.

Configuring a Dialer Map

Once you place the address on the interface, you need to tell the router what phone number it can call and what network address is attached to the remote interface. To do so, you need to configure a dialer map statement.

The dialer map links a protocol (IP, Internetwork Packet Exchange [IPX], and so on), an address, the name of a remote device (optional, used for Challenge Handshake Authentication Protocol [CHAP] authentication), and a phone number to call. You configure a dialer map as follows:

 RouterA(config)# dialer-map protocol address {name name} phone_number 

An example follows:

 RouterA(config)# dialer-map ip 10.1.2.2 name RouterB 4085551212 

When the router receives a packet and checks the routing table, it sees that it needs to send the traffic out a particular interface. Before that can happen, it needs to see what traffic is allowed to bring up the link. We already talked about dialer lists but not how they're linked to an interface.

A dialer group tells the interface to use the properties defined in a particular dialer list. This process allows a policy to be established router-wide with simple pointers configured on each interface to the policy. It allows multiple interfaces to use the same policy or more than one. The format of the command is

 Router(config-if)#dialer-group 1 

The only thing you have to worry about configuring is the number being used. The dialer-group number is the same number used in the dialer list that defines the policy you want to use on the interface. Dialer group 1 links to dialer list 1.

SPIDs

A service provider ID (SPID) is an accounting code for a B channel. Some providers assign a separate SPID to each B channel, whereas others use the same for both. Some providers don't even use SPIDs, but when they are used, they are a required part of the configuration.

SPIDs are easy to set as long as you know the number. There are two SPIDs with a BRI interface, and they're configured in interface configuration mode. You use the following command to set the SPID:

 RouterA(config-if)# isdn [spid1 | spid2] spid 

The following shows how they are configured:

 RouterA(config-if)#isdn spid1 40855512121111 RouterA(config-if)#isdn spid2 40855512121112 

Notice that the SPID number is fairly long. It can be up to 14 characters and in the U.S. is often the phone number of the interface, including area code, with up to four additional numbers added on. Notice how the difference between SPID 1 and SPID 2 are the final four digits, 1111 and 1112. The full number indicates the customer number and individual circuit being used.

Routing

As with any other temporary connection, routing becomes an issue. The router knows what network the BRI port is connected to because it's configured on the interface. What about the networks beyond the WAN? If the connection is down, dynamic routing isn't taking place. Instead, we use static routing.

We discussed static routes before but not in context with the interface configuration. When configuring a static route, you can set it up so that the packets are sent out a given interface or sent to a particular IP address. Sometimes, you might have a static route pointing to an IP address that's a few hops away; that setup won't work with ISDN calls. You can use the local BRI interface as the destination or the IP address of the device being called. If the route is configured another way, the dialer map won't work properly.

Bandwidth Usage

ISDN has the capability to do two different things at the same time or one task twice as well. It's possible to use one channel for voice and the other for data, allowing someone to be online and receive phone calls. Highly important before cable and DSL, now a BRI's most attractive feature is to bundle the two channels together to create a single 128Kbps path.

You can accomplish this task in one of two ways. The first is a Cisco method called Bandwidth on Demand (BOD). The router analyzes outgoing traffic and determines whether it needs to connect the second channel to help out. The router is configured with a preset load percentage to wait for. If the load is reached on the first circuit, the second circuit is enabled. Use this command to set the threshold:

 Router(config-if)# dialer load-threshold percentage 

The following commands show the load percentage being set to 60% before enabling the second channel:

 Router(config)#interface bri0/0 Router(config-if)#dialer load-threshold 60 

The threshold gets placed on the BRI interface, configuring the D channel. The problem with BOD is that it only works for traffic in one direction: out! What about traffic that needs more than 64Kbps of bandwidth coming in, such as most consumer Internet surfing? You couldn't use BOD, but later, you could use Multilink Point-to-Point Protocol (MLP).

MLP was discussed in Chapter 4, "PPP Authentication with PAP and CHAP," so it isn't covered in detail here. MLP allows the router to determine whether to bring up the second channel based on a preset load percentage for incoming traffic, outgoing traffic, or traffic in either direction. You use the following commands to activate this feature:

 Router(config-if)#dialer load-threshold percentage [incoming | outgoing | either] Router(config-if)#ppp multilink 

The following commands set the MLP load to 60% for either incoming or outgoing traffic:

 Router(config)#interface bri0/0 Router(config-if)#dialer load-threshold 60 either Router(config-if)#ppp multilink 

MLP has a limitation that is so obvious that sometimes people forget about it. Although BOD works with either high-level data link control (HDLC) or PPP, MLP requires PPP to work.

Additional BRI Options

A BRI has some additional settings that should be of interest to anyone taking the exam or trying to set up a BRI circuit. The first is setting the idle timer. The idle timer value specifies how much time should pass with no interesting traffic before disconnecting the circuit. Use the command dialer idle-timeout minutes while in interface configuration mode to set the number of minutes the dialer waits before disconnecting.

You can also set up caller ID screening as well, if you want to receive calls only from predetermined numbers. This setting can work in conjunction with PPP callback or separate from it. While in interface configuration mode, use the command isdn caller number to set what numbers your ISDN interface accepts calls from.

The late 1990s in the U.S. saw a big problem caused by this feature, however, as new area codes were being added on a rapid basis: An area code would change and the caller ID setting would be invalid. The setting must exactly match what is returned from the telco, so if you are in the 408 area code and you want to allow the number 555-1212, you can enter 4085551212. If the telco only gives 5551212, the connection fails.



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

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