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1315 Views 5 Replies Latest reply: Jul 14, 2011 10:14 PM by Paul Stewart - CCIE Security, CCSI RSS

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Which route will be taken...

Jul 14, 2011 2:04 PM

tonio1771 13 posts since
Dec 8, 2008

I have a question i want to clear up...

 

Lets say there are 3 routes in routing table and each routes being advertises by 3 different ruting protocols.

 

RIP 172.16.0.0 /24

EIGRP 172.16.0.0/16

OSPF 172.16.0.0/22

 

I know the AD on the OSPF is the most trusted out of the 3, but will the larger prefix (RIP /24) overide that and the packet will take that route. How can I have the packet take the OSPF route if the latter is true. I guess I can make the interface passive that the RIP routes is coming in from? Thank for the help

  • Martin 13,079 posts since
    Jan 16, 2009
    Currently Being Moderated
    1. Jul 14, 2011 2:11 PM (in response to tonio1771)
    Re: Which route will be taken...

    First of all; EIGRP's AD is 90; OSPF is 110; RIP 120, so EIGRP is the most trusted one;

    Since each of these routes has a different mask and lengths; they will be considered different destinations and all 3 be installed in the routing table.

     

    If two routes have the same length, then you compare (or router will) ADs of routing protocols;

  • Currently Being Moderated
    3. Jul 14, 2011 3:38 PM (in response to tonio1771)
    Re: Which route will be taken...

    Hmm, I think you missed what Martin said at the end. The ADs of routes will only be compared if the routes are to the same destination. 172.16.0.0/16 and 172.16.0.0/24 are two different routes, and the route with the longest prefix for the particular destination will be chosen.

     

    HTH

    DelVonte

  • Martin 13,079 posts since
    Jan 16, 2009
    Currently Being Moderated
    4. Jul 14, 2011 8:46 PM (in response to tonio1771)
    Re: Which route will be taken...

    sorry I were not clear enough;

     

    your 3 routes are different; have different masks; therefore all 3 will be in the table;

     

    but if some of those were same, like :

     

    RIP 172.16.0.0 /24

    EIGRP 172.16.0.0/16

    OSPF 172.16.0.0/24

     

    OSPF wins, RIP is gone; so you have 2 routes: one EIGRP with /16 and one OSPF /24; (RIP is removed from routing table)

  • Paul Stewart  -  CCIE Security, CCSI 6,962 posts since
    Jul 18, 2008
    Currently Being Moderated
    5. Jul 14, 2011 10:14 PM (in response to tonio1771)
    Re: Which route will be taken...

    The following three routes will be included in the route table. This is true because they have different masks. So they are "different routes"

     

    Routes from original post:

    RIP 172.16.0.0 /24

    EIGRP 172.16.0.0/16

    OSPF 172.16.0.0/22

     

    So how does this work. Let's take three exmples.

     

    Let's look at a packet going to 172.16.188.50

     

    Which route would this packet take?  Does it match the most specific route (rip)?  No.  Does it map the second most specific route (ospf)?  Let's see

     

    172.16.0.0/22 would match the following destinations--172.16.0.0-172.16.3.255 (see that subnetting stuff comes in handy).  172.16.188.50 doesn't fall into this range, so it doesn't match the ospf route.

     

    So how about the eigrp route?  That would match 172.16.0.0-172.16.255.255--agreed?  If you agree with my math, you can that 172.16.188.50 WOULD match this route.

     

    That is the order in which the match is considered.

     

    So a packet going to 172.16.0.99 would match which route?  Answer--rip (172.16.0.0/24)

     

    How about a packet to 172.16.3.0?  Answer--the OSPF route (172.16.0.0/22)

     

    Remember most specific (longest) route in the route table wins. Since those routes have different masks, they can all be included in the route table.

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