Re: [TCLUG:17090] DSL with Cisco 675

[Dan Debertin <danield@bitstream.net>]

On Tue, 2 May 2000, Bob Tanner wrote:

> 
> Not NAT, but PAT and it stinks! Better to use your linux box with IP Masq then
> the cruddy PAT in the 675.

I would be surprised to hear this with any recent version of CBOS. Most
ISPs that run DSL in routing (PPP) mode use a configuration like this:
      
                                                NAT/DHCP AREA
                                       __________________________             
_____   _________________    ________  | __________________     |
|ISP|-->| USW ATM CLOUD |--->| C675 |--->| CUSTOMER PC(s) |     |
-----   -----------------    --------  | ------------------     |
>                                      -------------------------- 

What this means is that the customer router gets a public,
globablly-routable IP address, doles out 10.0.0.x address to all
computers on the local side via DHCP (you can set 10.0.0.x addresses
statically as well), and then NATs them all onto different port numbers on
that one global IP address -- this kind of NAT is called PAT. Note that
you probably aren't using this configuration if you are routing a real
network across your DSL line or something.

Most ISPs configure their residential customers in this manner IF they are
using routing(PPP) mode. If there were serious issues with the NAT/PAT
mechanism on those 675s, I doubt that its use would be so widespread. Just
to give you an example, VISI.com has around a thousand DSL users, a large
percentage of which are configured in this fashion (and yes, a good number
of them are linux-users). This is according to Dave Carter, who is welcome
to chime in and correct me if I'm wrong ;).

What are the issues that people are seeing with the PAT mechanism on the
675? The same old passive-FTP issues that plague most NAT implementations?

Dan Debertin
Senior Systems Administrator
Bitstream Underground, Inc.
danield@bitstream.net
(612)321-9290