This section describes how the
configuration of the central-site access routers implements
authentication, network layer addressing, and the routing strategy.
The configuration for each central-site access router is the same
with the following exceptions:
- The IP address specified for
loopback interface 0
- The IP address specified for
Ethernet interface 0
- The name of the router as
specified by the hostname global configuration command
Username Configuration for the
Remote Sites
The configuration of each central-site access router includes
the username global configuration commands .
Each remote router can dial in to any of the three central-site
access routers, so there is a username global configuration command
for each remote router. When a remote router logs in, it specifies a
name (for example, Router2) and a password (for example, outthere)
that must match the values specified by a username command. Each
remote site uses a chat script to log in and specify its host name
(which must match a value specified by the username command) and
password.
Dial-Up Configuration for the
Remote Sites
The configuration of each central-site access router includes the
chat-script global configuration commands .
The three chat-script global configuration commands establish three
scripts named CALL1020, REM, and usrv32bis. CALL1020 and REM are
invoked by the dialer map commands to dial and log in to the remote
sites, respectively. The script reset command specifies that the
USRV32BIS script is to be run whenever an asynchronous line is reset
in order to ensure that the central-site modems are always
configured correctly.
Loopback Interface Configuration
The configuration of each central-site access router includes the
commands for configuring loopback interfaces.
The IP address for loopback interface 0 is unique for each access
router and, to satisfy the rules by which OSPF selects the router
ID, must be the highest loopback IP address on the router. The IP
address for loopback interface 1 is the same for each central-site
access router.
The goal is for all three access
routers to appear to have the same IP address during IPCP
negotiation with the remote sites. (IPCP is the part of PPP that
brings up and configures IP support.) This goal is accomplished by
creating a loopback interface, assigning to it the same IP address
on each central-site access router, and running the ip unnumbered
interface configuration command using the loopback interface
address. The problem with this strategy is that OSPF takes its
router ID from the IP address of a loopback interface, if one is
configured, which would mean that all three access routers would
have the same OSPF router ID.
The solution is to create loopback
interface 0 and assign to it a unique IP address (which results in a
unique OSPF router ID for each router). The configuration then
creates loopback interface 1 and assigns to it the same IP address
on each router. Loopback interface 1 allows the ip unnumbered
command to be applied to dialer rotary group 20 later in the
configuration.
Asynchronous Line Configuration
For each of the 16 asynchronous interfaces provided by the access
router, the configuration uses the ip
unnumbered interface
configuration command to specify that the asynchronous interface is
to use the IP address of loopback interface 1 as the source address
for any IP packets that the asynchronous interface generates. The IP
address of loopback interface 1 is also used to determine which
routing processes are sending updates over the asynchronous
interface. 
The async
dynamic address interface
configuration command enables dynamic addressing on the asynchronous
interface. This command is required to allow each remote router to
specify its IP address when it logs in. The async dynamic routing
interface configuration command allows the interface to run a
routing protocol, in this case RIP.
The async
mode interactive interface
configuration command allows a remote router to dial in and access
the EXEC command interface, which allows the remote router to start
PPP and specify its IP address.
The dialer
in-band interface
configuration command allows chat scripts to be used on the
asynchronous interface. The chat scripts allow the access router to
dial the remote sites. The dialer
rotary-group interface
configuration command assigns each asynchronous interface to dialer
rotary group 20.
Dialer Interface Configuration
The configuration of each central-site access router includes the
commands for configuring dialer rotary group 20.
The interface dialer
global configuration command defines dialer rotary group 20. Any
interface configuration commands that are applied to a dialer rotary
group apply to the physical interfaces that are its members. When
the router's configuration includes multiple destinations, any of
the interfaces in the dialer rotary group can be used to place
outgoing calls.
The ip
unnumbered interface
configuration command specifies that the IP address of loopback
interface 1 is to be used as the source address for any IP packets
that dialer rotary group 20 might generate. The dialer
idle-timeout interface
configuration command will cause a disconnection if 60 seconds elapses
without any interesting traffic.
The configuration includes a dialer
map interface configuration
command for each remote router that the central-site access router
might dial. The IP keyword specifies that the dialer map is
to be used for IP packets, the IP address is the next-hop address of
the destination that is to be called, and the name keyword
specifies the host name of the remote router that is to be called.
The modem-script keyword specifies that the CALL1020 chat
script is to be used, and the system-script keyword specifies
that the REM chat script is to be used. The last value specified by
the dialer map
command is the telephone number for the remote router. The dialer
map commands do not specify the broadcast keyword, so RIP
updates are not sent to the remote sites.
For the Dialer20 interface, the dialer-group
interface configuration command defines interesting packets to be
those packets defined by the corresponding dial-list
command. Interesting packets cause a call to be made or cause a call
to be maintained. In this case, access list 101 defines RIP as
uninteresting. (RIP uses User Datagram Protocol [UDP] port 520.) All
other packets are defined as interesting.
|