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)

5 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

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.