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9733 Views 24 Replies Latest reply: Aug 14, 2010 4:37 AM by Deepak RSS 1 2 Previous Next

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Two default routes...

Aug 12, 2010 8:14 AM

Deepak 312 posts since
Jun 25, 2008

Hi!

 

Lets say we've following topology

 


(Loopback1) R1 (s1)<---->(s1) R2 (s0)<---->(s0) R3 (Loopback2)

 

All router interfaces are configured with respective ip addresses so that neighbors can ping each other. NO other configuration made other than TWO static default routes on R2..1st one is pointing towards R1 and 2nd one is pointing towards R3, with both next hop ip and routers own interface options used that are present while creating default routes.

 

If I ping Loopback1 or Loopback2 it fails...Please explain the behaviour of routers here...why its happaning the way it is happaning.

 

 

Regards,

Deepak

  • Conwyn 9,683 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Aug 12, 2010 9:39 AM (in response to Deepak)
    Re: Two default routes...

    Hi Deepak

     

    Are you using ping 1.2.3.4 source interface-name?

     

    Regards Conwyn

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  • Wendell Odom 350 posts since
    Jun 19, 2008
    Currently Being Moderated
    2. Aug 12, 2010 10:02 AM (in response to Deepak)
    Re: Two default routes...

    Deepak,

     

    I agree with Conwyn about the source interface, since R1 or R3 will need to route with which to send back the Echo Reply.

     

    Another thing to check: if you have <no ip classless> configured on R2. If so, change it to <ip classless> and try it again. That may do the trick.

     

    Regards,

    Wendell

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  • Claudius 117 posts since
    Jan 7, 2009
    Currently Being Moderated
    3. Aug 12, 2010 10:15 AM (in response to Deepak)
    Re: Two default routes...

    Hi Deepak,

     

    If you ping from Lo1 (R1) to lo2 (R2)

     

    It's needed that all routers on the way could handle the destination and/or source of ping packet. As I understand there's no routing configured but 2 static routes on R2. If I am correct, neither R1 nor R3 are aware of remote loopback networks, so...

     

    How Can it ber delivered? How Can it be Delivered Back?

     

    Unreachable Destination.

     

    Regards,

     

    Carlos

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  • Sonu Khandelwal 21 posts since
    May 25, 2009
    Currently Being Moderated
    5. Aug 12, 2010 11:05 AM (in response to Deepak)
    Re: Two default routes...

    Hi Deepak,

    Lets say You want to ping 3.3.3.3 from R2. R2 wont have any way to know if packet should be send to R1 or R3 becuase it has both the routes in its table. Lets say if it sent to R3 and its doing per prefix loadbalancing then your ping will succeed.

    If R2 is doing per packet load balancing then alternate ping packets will succeed.

     

    Experts can correct me, If I am wrong here.

     

    Thanks,

    Sonu

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  • Chad Spears CCNP CCDA CCNAS 751 posts since
    Jul 27, 2009
    Currently Being Moderated
    6. Aug 12, 2010 11:15 AM (in response to Sonu Khandelwal)
    Re: Two default routes...

    Deepak-

     

         I agree with Sonu; it appears that you should have the following on R2:

     

    ip route 1.1.1.1 (*Subnet Mask*) FastEthernet 0/0

    ip route 3.3.3.3 (*Subnet Mask*) FastEthernet 0/1

     

     

    This should work....As always Experts let me know if I am wrong.

     

    Thanks,

    CS

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  • Chad Spears CCNP CCDA CCNAS 751 posts since
    Jul 27, 2009
    Currently Being Moderated
    Re: Two default routes...

    nevermind I just read the title of the post. .....Oops!

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  • Currently Being Moderated
    8. Aug 12, 2010 11:33 AM (in response to Deepak)
    Re: Two default routes...

    I don't think you can have two static default routes, how will R2 know which one to use. Also, in order to be able to ping the loopback interfaces, you will have to provide a route to those interfaces.

     

    I wasn't really sure how configuring two default routes would work, so I put it together in Packet Tracer:

     

    R2#ping 3.3.3.3

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
    ..!.!
    Success rate is 40 percent (2/5), round-trip min/avg/max = 3/3/3 ms

    R2#ping 3.3.3.3

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
    .!.!.
    Success rate is 40 percent (2/5), round-trip min/avg/max = 4/4/5 ms

    R2#ping 1.1.1.1

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
    .!.!.
    Success rate is 40 percent (2/5), round-trip min/avg/max = 3/4/5 ms

    R2#

     

    I was really surprised that I got any response. Heres the output after removing one default route and configuring static routes to the loopback interfaces:

     

    R2#
    R2#
    R2#config t
    Enter configuration commands, one per line.  End with CNTL/Z.
    R2(config)#no ip route 0.0.0.0 0.0.0.0 20.1.1.2
    R2(config)#ip route 1.1.1.1 255.255.255.255 10.1.1.2
    R2(config)#ip route 3.3.3.3 255.255.255.255 20.1.1.2
    R2(config)#^Z
    R2#
    %SYS-5-CONFIG_I: Configured from console by console

    R2#ping 3.3.3.3

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
    !!!!!
    Success rate is 100 percent (5/5), round-trip min/avg/max = 2/3/6 ms

    R2#ping 1.1.1.1

    Type escape sequence to abort.
    Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
    !!!!!
    Success rate is 100 percent (5/5), round-trip min/avg/max = 3/4/6 ms

    R2#

     

     

    HTH

    DelVonte

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  • Claudius 117 posts since
    Jan 7, 2009
    Currently Being Moderated
    9. Aug 12, 2010 11:54 AM (in response to Deepak)
    Re: Two default routes...

    Hi Deepak,

     

    I see, I misunderstood the question, but there's not only 2 default routes, actually they're all over the place... Interesting scenario.

     

    I belive that the point in this case is how packets will be processed, try to disable ip cef, that would work.

     

    Let me know if I am on the way .

     

    Regards,

     

    Carlos.

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  • Wendell Odom 350 posts since
    Jun 19, 2008
    Currently Being Moderated
    10. Aug 12, 2010 11:55 AM (in response to DelVonte)
    Re: Two default routes...

    Deepak and DelVonte,

    Actually, the results DelVonte sees with the defaults is what I would have expected. You can have multiple default routes in the routing table at the same time, but I agree it's not useful for real life. Deepak, I'm sure you mention somewhere that you're just testing to see what happens.

     

    Backing up a step, Deepak... any chance the cabling differs from your figure? EG, R2's Fa0/1 is actually cabled to R1's Fa0/0? That would fit the symptoms at least. One other thought: I've not used the syntax "ip route 0.0.0.0 0.0.0.0 fa0/0 10.1.1.2" often, instead just listing the next-hop IP address (ip route 0.0.0.0 0.0.0.0 10.1.1.2) in such cases. May be worth a shot. Last suggestion, and I'll stop: shut down one LAN interface on R2, and see if you can at least ping the router that's still possibly reachable, and get that working before adding back the other side.

     

    That's all I can think to try. I'll be curious to see your results.

    Thanks,

    Wendell

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  • Currently Being Moderated
    11. Aug 12, 2010 11:56 AM (in response to Claudius)
    Re: Two default routes...

    I'm not really sure if any of us know exactly what is being asked

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  • Conwyn 9,683 posts since
    Sep 10, 2008
    Currently Being Moderated
    13. Aug 12, 2010 12:53 PM (in response to Deepak)
    Re: Two default routes...

    Hi Deepak

     

    You might find this interesting especially Toor's post.

     

    See https://learningnetwork.cisco.com/message/39389#39389

     

    Regards Conwyn

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  • Currently Being Moderated
    14. Aug 12, 2010 1:43 PM (in response to Conwyn)
    Re: Two default routes...

    That is interesting and enlightening, I never thought about it from that perspective. Always assumed just one default route, now I have to play around with this. Thanks for the post Deepak, and thanks for the link Conwyn. I learn quite a bit on CLN everyday.

     

    Regards

    DelVonte

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