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Chapter 17Wan Configuration

Configurando HDLCserial interfaces on Cisco routers that use HDLC typically need no specific Layer 1 or 2configuration commands. The cabling needs to be completed as described in Chapters 4

and 16, but there are no required configuration commands related to Layer 1. IOS defaults

to use HDLC as the data link protocol, so there are no required commands that relate to

Layer 2. As on Ethernet interfaces, the only required command to get IP working on theinterface is the ip address command and possibly the no shutdown command

Step 1 Configure the interface IP address using the ip address interface subcommand.Step 2 The following tasks are required only when the specifically listed conditions aretrue:

a. If an encapsulationprotocol interface subcommand that lists a protocol besides HDLC

already exists on the interface, use the encapsulation hdlc interface subcommand to enable

HDLC.b. If the interface line status is administratively down, enable the interface using thenoshutdown interface subcommand.c. If the serial link is a back-to-back serial link in a lab (or a simulator), configure the

clocking rate using the clock rate speedinterface subcommand, but only on the one router

with the DCE cable (per the show controllers serial numbercommand).Step 3 The following steps are always optional, and have no impact on whether the linkworks and passes IP traffic:

a. Configure the links speed using the bandwidth speed-in-kbps interface subcommand.b. For documentation purposes, configure a description of the purpose of the interface

using the description textinterface subcommand.


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the output from the show controllers command for S0/1/1 confirms that R1 indeed has a

DCE cable installed. The show interfaces S0/1/1 command lists the various configuration

settings near the top, including the default encapsulation value (HDLC) and defaultbandwidth setting on a serial interface (1544, meaning 1544 kbps or 1.544 Mbps).

PPPConfiguring the basics of PPP is just as simple as for HDLC, except that whereas HDLC is

the default serial data-link protocol and requires no additional configuration, you must

configure the encapsulation ppp command for PPP. Other than that, the list of possibleand optional configuration steps is exactly the same as for HDLC. So, to migrate from a

working HDLC link to a working PPP link, the only command needed is anencapsulationppp command on each of the two routers serial interfaces.

You can configure the DHCP client, DHCP server, and PAT functions with SDM using the

following five major steps:Step 1 Establish IP connectivity. Plan and configure (from the CLI) IP addresses on thelocal LAN so that a PC on the LAN can ping the routers LAN interface.

Step 2 Install and access SDM. Install SDM on the router and access the router SDM

interface using a PC that can ping the routers IP address (as implemented at Step 1).

Step 3 Configure DHCP and PAT. Use SDM to configure both DHCP client services andthe PAT service on the router.

Step 4 Plan for DHCP services. Plan the IP addresses to be assigned by the router to the

hosts on the local LAN, along with the DNS IP addresses, domain name, and defaultgateway settings that the router will advertise.Step 5 Configure the DHCP server. Use SDM to configure the DHCP server features on

the router.

Step 1: Establish IP Connectivity

For this step, you should choose the following details:Step a Choose any private IP network number.

Step b Choose a mask that allows for enough hosts (typically the default mask is fine).

Step c Choose a router IP address from that network.

Step 2: Install and Access SDM

To be able to install the SDM software on the router (if it is not already installed on therouter), and to allow the engineers host to access the

router using a web browser, theengineer needs to use a host with IP connectivity to reach the router. Typically, the

engineer would use a host on the local LAN, configure the routers local LAN interface

with the IP address planned at Step 1, and configure the host with another IP address in that

same network. Note that SDM does not use Telnet or SSH, and the PC must be connected

via an IP networkthe console can only be used to access the CLI.

From the SDM Home page shown in Figure 17-3:1. ClickConfigure near the top of the window.

2. ClickInterfaces and Connections at the top of the Tasks pane on the le ft side of the

window.


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On the Create Connection tab, do the following:1. Choose the Ethernet (PPPoE or Unencapsulated Routing) radio button.2. Click the Create New Connection button near the bottom of the tab.These actions open the SDM Ethernet Wizard, shown in Figure 17-5. The page in Figure

17-5 has no options to choose, so just clickNext to keep going.


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The first two items in the list relate to planning on the local LAN, and the last two items are

just values learned from the ISP that need to be passed on to the hosts on the local LAN.1. Recall the private IP network and mask used on the local LAN and then choose a subsetof that network that can be assigned to hosts using DHCP.

2. Make a note of the routers IP address in that network; this address will be the local

hosts default gateway.3. Find the DNS server IP addresses learned by the router using DHCP client services,using the show dhcp server EXEC command; the routers will then be able to inform the

DHCP clients on the local LAN about the DNS server IP address(es).

4. Find the domain name, again with the show dhcp server EXEC command.

Cisco uses the termDHCP pool for the IP addresses that can be assigned using DHCP.

shows how to find those values using the show dhcp server command. This command lists

information on a router acting as a DHCP client, information learned from each DHCPserver from which the router has learned an IP address.

The show ip dhcp binding command output lists information about the IP addresses

assigned to hosts on the local LAN by the DHCP server function in the access router.

The show ip nat translations command output provides a few insights that confirm the

normal operation of NAT and PAT. The output shown in Example 17-4 lists one heading

line plus two actual NAT translation table entries. The two highlighted parts of the headingline refer to the inside global address and the inside local address. The inside local addressshould always be the IP address of a host on the local LAN, in this case 192.168.1.101. The

router translates that IP address to the one globally routable public address known to the

routerthe 64.100.1.1 IP address learned via DHCP from the ISP.

The last command in the example, clear ip nat translation *, can be useful when the

problem symptom is that some users connections that need to use NAT work fine, and

other users connection that need to use NAT do not work at all. NAT table entries mightneed to be removed before a host can start sending data again, although this is probably arare occurrence today. However, this command clears out all the entries in the table, and

then the router creates new entries as the ensuing packets arrive. Note that thisclearcommand could impact some applications.

Note

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