|
The remote routers and the central-site
access routers have no need to connect to the Internet, so they use
RFC 1597 addresses (NAT). The Class B address 172.16.0.0 is used for
the entire access portion of the network, and Class C equivalent
addresses are assigned to the remote routers. Each subnet gets one
Class C equivalent (172.16.x.0 with a mask of 255.255.255.0), which
makes addressing easy to manage. Network 172.16.1.0 is reserved for
numbering the dialer cloud later if needed. (The dialer cloud is
defined as the subnet to which all of the asynchronous interfaces
are attached.)
Initially, the dialer cloud is
unnumbered. If, in the future, the dialer cloud were to be numbered,
the following questions must be considered:
- Can the dialer cloud use the same
subnet mask as the remote sites? If not, variable length subnet
mask (VLSM) support will be required. (RIP version 1 does not
support VLSM.)
- Would the use of multiple
subnetted Class C addresses cause discontiguous subnets at the
remote sites? If so, discontiguous subnet support will be
required. (RIP version 1does not support discontiguous subnets.)
In this network, these issues are not
a problem. A mask of 255.255.255.0 can be used everywhere, so there
are no VLSM concerns. All subnets are from the same major Class B
network, so there are no discontiguous subnet concerns. The table in
the graphic summarizes the addressing for the access portion of the
network.
To facilitate an accurate routing
table, all next-hop IP addressing must be accurate at all times. To
accomplish this, the remote sites need to know the IP address that
they will dial in to, and the central site needs to know the IP
address of the remote site that has dialed in.
All central-site access routers use
the same IP address on all of their asynchronous interfaces. This is
accomplished by configuring the Dialer20 interface for IP unnumbered
off of a loopback interface. The IP address of the loopback
interface is the same on all of the central-site access routers.
This way, the remote routers can be configured with the IP address
of the router to which it connects, regardless of which router the
remote router dials in to.
The remote router needs to announce
its IP address to the central-site router when the remote router
connects. This is accomplished by having the remote router start PPP
on the central site using the EXEC command ppp 172.16.x.1. To
support this, each central-site access router is configured with the
async dynamic address
interface configuration command.
Note: The autoselect feature
allows the router to start an appropriate process, such as PPP,
automatically when it receives a starting character from the router
that has logged in. To use autoselect, a mechanism for supporting
dynamic IP address assignment would be required, such as per-user
address support in TACACS+.
|