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Install default route with PPP

In my home office, I'm using DSL access to the Internet with ISDN backup to another ISP, as shown on the next figure:

Obviously, I would like the ISDN backup to kick in whenever the primary connection goes down; two static default routes and reliable static routing on the primary default seem like a perfect solution.

However, as I'm using PPP encapsulation on the primary connection, there's another option: PPP can insert a dynamic default route whenever IPCP negotiations succeed (and remove it when the line protocol goes down). To configure this feature (introduced in IOS releases 12.3(11)T and 12.4), use the ppp ipcp route default interface configuration command on the primary dialer interface.

GW#show ip route | begin Gateway
Gateway of last resort is 10.0.0.33 to network 0.0.0.0
 
     10.0.0.0/32 is subnetted, 2 subnets
C 10.0.0.34 is directly connected, Serial1/0
C 10.0.0.33 is directly connected, Serial1/0
     192.168.0.0/28 is subnetted, 1 subnets
C 192.168.0.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 10.0.0.33

Contrary to the DHCP-installed default route, the PPP-installed default route has administrative distance 1 (and is thus impossible to override)

7 comments:

  1. You have right but it doesn't works if I have ethernet as WAN interface. I mean clean ethernet, without pppoe.
    Reliable static routing helps in this case.

    ReplyDelete
  2. Hm.. interesting, thx Ivan, I'll test it out with my PPPoE connection

    ReplyDelete
  3. Also ran into an issue at home where CEF would not work if a static default route to the dialer interface was used. I changed

    ip route 0.0.0.0 0.0.0.0 Dialer0

    to

    interface Dialer0
    ppp ipcp route default

    and cpu usage dropped a lot

    ReplyDelete
  4. Hello friends, Ivan, you are very intelligent.

    I have a question, Is it good if I use the static default route ip route 0.0.0.0 0.0.0.0 Dialer0. Istead of ppp ipcp route default?

    Regards!

    ReplyDelete
    Replies
    1. As always, it depends on what you're trying to do. If you want the default route to always point to Dialer0, then your solution works perfectly.

      Delete
  5. Mike Veneziano04 May, 2017 23:40

    Hi Ivan,

    I am huge fan of yours, just to verify if I have 2 DSL links and I want to use one as primary, I could to ppp ipcmp route default on my primary and do a floating weighted static route on the secondary? or let us say I have an ethernet(bgp) + dsl service and I want to make the DSL as my primary, I could just use this as well and if my dsl goes down my ethernet bgp would just take over?

    ReplyDelete
    Replies
    1. "I could to ppp ipcmp route default on my primary and do a floating weighted static route on the secondary" << This should work. Do keep in mind the blog post is 10 years old and test first ;)

      "... if my dsl goes down my ethernet bgp would just take over?" << assuming you're getting just the default route over BGP, otherwise the router would use the more-specific entries you get over BGP.

      Delete

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