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[Config] Configuring an 877W for use on BT Broadband help please

Re: [Config] Configuring an 877W for use on BT Broadband help pl

One thing about that FAQ... "ip route 0 0 dialer1" is not recommended. Let PPP from the interface set the default route.

Here's my dialer setup for bellsouth (the important parts):

interface Dialer1
 description Bellsouth.net DSL
 ip address negotiated
 ip nat outside
 encapsulation ppp
 dialer pool 1
 dialer vpdn
 ppp chap hostname ...
 ppp chap password ...
 ppp ipcp header-compression ack
 !! (optional) dns servers for dhcp server
 ppp ipcp dns request
 !! make this the default route
 ppp ipcp route default
 

interface ATM0
 no ip address
 atm bandwidth dynamic
 no atm auto-configuration
 no atm ilmi-keepalive
 no atm address-registration
 no atm ilmi-enable
 atm pppatm link reset
 !! you can set this to auto
 dsl operating-mode ansi-dmt
!
interface ATM0.1 point-to-point
 pvc dsl 8/35 
  pppoe max-sessions 1
  dialer pool-member 1
  protocol ppp dialer
 

With the default route, packets will be forced to that interface, always -- dialer interfaces are always up (spoofed). If the dialer interface is down -- because the dsl interface is down (read: unbound), packets will be accepted and "lost" because there's nothing on the other side of the dialer. On the other hand, with no default route, an ICMP error is generated (dest unreachable, no route) because there's no route when the PPP session is down -- unless the router is configured to not generate them [no ip unreachables].

Plus, it's completely unnecessary. The dialer will bind the instant the PVC is available. And PPP (IPCP) will set the default route once negotiated. This is not an "ondemand" dialer interface where traffic initiates the connection; the PVC's state will maintain the process.

Yes, I am aware of the whole up/down thing with Dialer interfaces. What I am interested in is the recommendation you speak of to not use a static route.

Or is this your recommendation, which I agree with in theory, but in practice unless theres a specific reason to not use a static route (i.e. because of how your failover/backup mechanism works), I dont think its all that worth worrying about and a static will be fine. IMO anyway.

I'm not sure where I learned that. All of Cisco's documentation is cut-n-paste from dial-on-demand configs (i.e. analog modem days) -- and they're 5+ years old. The only thing I found in support of an explicit default route was 12.4(15)T specific -- and even then it's only necessary for dial-on-demand or when there are multiple clients per PVC. All the other docs include it with no explanation or reason (other than a holdover from dial-on-demand.)

I tested with my DSL router, and stand firmly behind no default. With an interface default, packets are routed blistfully into oblivion when DSL is down. Sans default, when DSL is down (thus PPP) there is no route and ICMP messages are generated as a result. That's the difference between an application waiting for a connection timeout vs. immediately failing.

reply to Asimov
I think we should qualify both the recommendations as "personal preference".

cramer is right as regards the ICMP unreachable messages being sent but that is assuming a number of things:

1.) That there is no other default route with a higher AD elsewhere on the router.
2.) That the application that is timing out will take note of ICMP unreachable messages and close the socket. Not all applications respect ICMP and hence they will wait for the timeout anyway as they continuously try and re-transmit the packet regardless. So in theory, it should work, in practice, nope. IE is an example... it will sit there trying despite the unreachable messages.

Another thing to consider is the code classification of ICMP type 3 messages, sent by the router? Are they code 0 or 1? 1 is the most common one I have seen on the Cisco platform in the situation described above.

Depending on the network model of the application, these responses may be ignored or acted upon, thus is the Cisco router's response to no default route actually helpful to the application if it wanted this information?

PPP IPCP route default is common in virtual-template implementations or as TomS_ suggested, fail over. Default route via dialer is common in single site implementations without any mechanism of fail over.

Personal preference is the only selection criterion as both methods have the same timeouts despite IPCP default route being the better method on paper, it is not so in practice as most applications do not care and will try re-transmitting regardless.

So you are both right in practice.
--
A word to the wise ain't necessary, it's the stupid ones who need the advice!

And don't forget... the router can be set not to generate unreachables -- which is very common as a rather lame "security feature" (cisco's own security scanner will recommend/demand setting no unreachables on every interface when the only one that needs it is the internet facing one.)

reply to TomS_

I think TomS_ was specifically looking for Cisco documentation, but I may be wrong.

Also, proxy-arp is mistakenly mentioned in the other post and its relationship is incorrect with respect to static routes and dialer interfaces in PPPoE which may be due to protocol confusion. Proxy arp is not relevant in the decision making process of choosing either a static route via a dialer interface or the use of the command IPCP default route.
--
A word to the wise ain't necessary, it's the stupid ones who need the advice!

reply to TomS_

Re: [Config] Configuring an 877W for use on BT Broadband help pl

reply to Asimov

interface ATM0.1
 ip nat outside
interface BVI1
 ip nat inside
 
ip access-list standard local-net
 permit 10.10.10.0 0.0.0.7
ip nat inside source list local-net interface atm0.1 overload