RIPv2 Conditional Default Routing

I cant understand why I do "not" need to filter out default route on R4.

 

I tried this lab and, as mentioned in the configuration section, default route did not get installed in R4, but why?

-Path of G1.45: split horizon is enable on both R4 and R5, so default route advertisement from R4 will not reflect back by R5.

-Path of Tunnel0:split horizon is "disabled" on R5, so default route will reflect back to R4.

Therefore, by reflecting default route by R5 via tunnel0, I assumed R4 will install default route.

 

My assumption why R4 did not installed reflected default route is because reflected RIP advertisement packet "next hop" field

has its own(R4) ip address, and therefore R4 ignored it.  Is my assumption correct?

(I tried to find document that states my assumption but I could not find it)

 

Another issue I found while debugging this issue is that "debug ip rip" will not display some of the prefix received by neighbor.

It seems like it will not show the prefix router itself ignored. (EX: Above default route, I confirmed its in the packet by wireshark, but

couldn't find it in debug).

Then, are there any way to check all the advertised prefix received by neighbor?

Comments

  • I would think that doing the debug ip rip on the neighbor should show those updates being sent.

    Did you look at the explanation on the previous section where it shows the default route being fed back to R6? I would expect this would happen here by going from R4 to R5 over the G1.45 and then from R5 to R4 over Tu0. I haven't looked at the initial config to see if there is something that prevents that.

    The reflected route from split-horizon probably isn't accepted because the next hop is your own interface. The way I read the RFC is that if the next hop is there then it should match the interface where the update came in on to be used.

  • Thank you for reply.

    >I would think that doing the debug ip rip on the neighbor should show those updates being sent.

    Yes, I know I can see the exact prefixes by getting debug information on other (in this case, R5) side, but

    I wanted to know if there is any way to figure out in R4 side.

     

    >Did you look at the explanation on the previous section where it shows the default route being fed back to R6? I would expect this would >happen here by going from R4 to R5 over the G1.45 and then from R5 to R4 over Tu0. I haven't looked at the initial config to see if there is >something that prevents that.

    Yes, I thought same thing will happen in this section too.(Well the reason why I thought that is tunnel0 advertisement reflects back the default information, which was wrong).  However, it didn't installed in R4 routing table.

    >The reflected route from split-horizon probably isn't accepted because the next hop is your own interface. The way I read the RFC is that if >the next hop is there then it should match the interface where the update came in on to be used.

    This is very interesting information!  Can you specify where in RFC states that?

     

     

     

  • Thanks alot.  It was very informative.

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