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2248 Views 8 Replies Latest reply: Apr 13, 2011 8:03 AM by Brian RSS

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Default Routing Question

Apr 11, 2011 8:55 PM

Brian 22 posts since
Feb 7, 2010

I have a question regarding default routing, default routing is best used in a network with only one exit path out of the network. In the diagram below the author states that default routing can be used on R1 and R2. I am confused with this because R1 has two exit paths out of the network. I thought R2 and R3 would be the two router where default routing would best be placed. Can someone clear this up for me. Thanks!

 

network.jpg

  • Martin 13,077 posts since
    Jan 16, 2009
    Currently Being Moderated
    1. Apr 11, 2011 9:46 PM (in response to Brian)
    Re: Default Routing Question

    yes, if he meant R1 as Remote 1 and R2 as Remote 2.

     

    "default routing is best used in a network with only one exit path out of the network" is often used with Stub routers. R1 and R2 are stub routers; only one link, one way out of those is possible.

     

    R1 has two links, probably for redundancy; you can use default route for link 1 and backup default route for link 2.

     

    R2 is ideal for default routing; R3 has 2 routers connected to it, so not so much. By definition, R3 is not a Stub router.

     

    Default routing is not so much back and  white area; it is more grey.

     

    Likely, (and with some caution) I can add default routes to all routers on your diagram; Corp router is likely to have Internet connection, that is his default routing.

    871 wireless should have default route pointing to R3; R3 points to Corp; same goes to R1 and R2.

     

    Don;t worry too much; it will become clear as you read more about it; just remember about Stub routers: have one way out and that is default route.

    Stub routers do not need to run any dynamic protocols as all routes will go out one way, right.  They will go towards Hub router, your Corp rotuer.

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    2. Apr 12, 2011 9:50 PM (in response to Brian)
    Re: Default Routing Question

    3 routers CLN4-no-user.png

     

    Hello Brian-

     

    Let's consider the network above with 3 routers.

     

    R1 could use a default gateway, because everything R1 is not directly connected to (2.2.2.0/24, 23.0.0.0/24, 3.3.3.0/23) are all available if R1 uses the next hop of R2 at 10.0.0.2    R1 is a leaf in this network, not the core of the tree (loosely speaking).

     

    The same is true for R3, all the remote networks that R3 is not attached to are available through R2 using the next hop of 23.0.0.2.

     

    R2, because it is not the edge of the network, can't really use a default route, as some networks are to the left, and some to the right.

     

    In your diagram, R1 and R2 and the 871w are leaf routers at the edge of the network.    That is why they can use default routes.

    Corp and R3 are not at the edge, and that is why we would need more than just a default route.

     

    On a side note, the Corp and R3 routers would need routes to the networks behind R1, R2 and the 871w.

     

    Best wishes,

     

    Keith

     

    Message was edited by: Keith Barker - CCIE RS/Security, CISSP, CCSI

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    4. Apr 12, 2011 9:52 PM (in response to Brian)
    Re: Default Routing Question

    Brian wrote:

     

    Hello Keith,

     

    In your explanation I am a bit confuse when you said "R2, because it is not the edge of the network, can really use a default route, as some networks are to the left, and some to the right." I thought since its not at the edge of the network you shouldn't use default routing on R2. Can you clairfy for me.

     

    Thanks,

    Brian

    Hello Brian.   Typo user error  on my part.   I updated and noted the original post.

     

    It should say:

     

    R2, because it is not the edge of the network, can't really use a default route, as some networks are to the left, and some to the right.

     

    The definition of why is part of the same sentence.

     

    Good catch.  Hopefully that didn't cause too much head scratching.

     

    Best wishes,

     

    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    7. Apr 13, 2011 2:28 AM (in response to Brian)
    Re: Default Routing Question

    Hello Brian-

     

    With 2 default routes, the router will load balance across both of them, which means that 1/2 of the packets will go to one default gateway, and 1/2 will go to the other.  Unfortunately, depending on the destination we are trying to reach, 1/2 of the packets are going the wrong direction.

     

    On real gear, CEF may or may not be load balancing based on additional factors, and may have the result of all pings to 1 valid but unknown destination all working, and all pings 2 a second valid but unknown destination all not working.

     

    Keith

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