ppp ipcp default router over riding OSPF learned default route

Answered Question
Sep 24th, 2010
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Hello all,


I have a Cisco 2801 connected to a Sprint MPLS network. For a backup, we had a cheap Internet T1 installed and planned on setting up an IPSec tunnel back to the datacenter.


Our vendor sold us a Covad Internet T1 which setups up differently than what I am used to. They use a Frame Relay PVC and issue IP information via PPP IPCP.  I had it all setup and the circuit came up, but I soon realized the PPP IPCP DEFAULT ROUTE command ended up replacing the default route my router learned from the MPLS network. I removed PPP IPCP DEFAULT ROUTE from the Virtual-Template and bounced the serial interface, and it appears to have come back up, but I am unable to route any traffic out.


I tried adding a static route pointing to the serial subinterface, but when I issue the SHOW ROUTE command for the route I added, the router reports the subnet doesn't exist in the routing table. It doesn't appear I can add a route out a Virtual-Access interface either.


How can I make this work?


Denny

Correct Answer by Peter Paluch about 6 years 8 months ago

Denny,


I suppose that the backup T1 connection is using a stable IP address at the ISP's end. This IP address will appear in your routing table having the mask /32, i.e. a host route - this is also a part of the IPCP negotiation between you and your ISP. If this IP address is stable indeed then you can define a static default route using this particular IP address as the next hop (without referring to the outgoing interface because that is dynamic) and having the administrative distance higher than 110.


Can you try this to see if it works for you?


Best regards,

Peter

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Correct Answer
Peter Paluch Fri, 09/24/2010 - 14:20
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Denny,


I suppose that the backup T1 connection is using a stable IP address at the ISP's end. This IP address will appear in your routing table having the mask /32, i.e. a host route - this is also a part of the IPCP negotiation between you and your ISP. If this IP address is stable indeed then you can define a static default route using this particular IP address as the next hop (without referring to the outgoing interface because that is dynamic) and having the administrative distance higher than 110.


Can you try this to see if it works for you?


Best regards,

Peter

dennylester Fri, 09/24/2010 - 14:46
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Hi Peter,


That helped out. I looked at just about everything except what was in the routing table. I see what you are talking about and added it and it routes fine now.


I guess the only caveat will be if I get issued an address in a completely different subnet.


Thank you again,


Denny

Peter Paluch Sat, 09/25/2010 - 14:57
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Denny,


I am glad this helped. Yes, you are absolutely correct about the caveat - if the ISP's IP address changes over time, you will need to correct the static default route. It would be ideal if the administrative distance of the IPCP-added default route could be changed but the current IOS does not seem to support that (if there are any Cisco IOS software developers here, this is an enhancement request that should be very easy to implement!)


Best regards,

Peter

alexdelangel Mon, 04/21/2014 - 11:42
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Hello firends Denny / Peter

I´ve seen you are very Smart.

Got one question why can´t you make the static default route using the outgoing interface?

Regards! 

dennylester Mon, 04/21/2014 - 12:01
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Wow, talking about an old post being resurrected.

 

As I recall, in this case the interface was being created via a Virtual Template and you cannot point a static route to a dynamically created interface, before the Interface is created. Not sure if this has changed in IOS since I last ran into this issue.

 

Denny

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