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    Static Routing


      i have configured static routes on all the three routers.


      L0 on R1 is succedssfully communicating with R3 L0.


      Now the link between R2 and R3 is down.

      Now if i try to ping from L0 R1 to L0 R3 the ping failed and R2 s0/0 is replying with host unreachable.


      Everytime if i try to ping it is going to R2 eventhough it does not have route to and so R2 processing is increased. Is there any way to drop packets at R1 level without using dynamic routing protocols?


        • 1. Re: Static Routing

          Hi Srikant,


          What do you mean by this??? Is there any way to drop packets at R1 level without using dynamic routing protocols?

          • 2. Re: Static Routing

            Because you have configured ststic routes, R1 doesnt know R3 is down and therefore it is still in the routing table.  If you were to use a dynamic routing protocol, once a router goes down, that router would no longer be advertising its networks and other routers would remove it from their routing tables.  Your ping would still fail, but processing onR2 shouldnt go up because the ping shouldn't make it that far.  R1 would have no route to R3 and drop the packet.

            • 3. Re: Static Routing

              the icmp packets from R1 are going to R2 to reach

              But the link between R2 and R3 is down. So How does R1 knows that R2 does not have route to So that R1 will not send them to R2 to reach

              • 4. Re: Static Routing

                I think Jared just answered your question above. Thats the whole point of dynamic routing - it is meant to adapt to change. Once the link between R2 and R3 goes down, it will be removed from R2s routing table and subsequently from R1s routing table.


                With your static routes, you are forcing R1 to go to its next hop even if the next hop does not have a path to the destination.


                Message was edited by: Aninda

                • 5. Re: Static Routing
                  Fabrizio Chessa

                  Hi Srikant,


                  I probably understand what you think.


                  Case 1 - no dynamic routing protocol:

                  • R1 deafult-route --> or static-route -->
                  • R2 default-route --> or static-route -->
                  • Question ping from Lo R1 to
                    • Is Net directly connected? --> No, it isn't
                    • How can I reach net I can reach net by default or static route
                  • In that case R1 don't know if net is reachable or not and R1 send echo-request to R2 by default and R2 replyes to R1 with net unreachable


                  Case 2 - Dynamic Routing Protocol:

                  • Note:
                    • R1 no static-route in routing table R1 learn and send only dynamic route
                    • R2 no default-route redistribution at least from/to R2-->R1
                  • Question ping from Lo R1 to
                    • Is Net directly connected? --> No, it isn't
                    • How can I reach net I don't know --> network Unreachable


                  This way is very brief resume of the principal steps that R1 do.


                  I think that there aren't way to do what you think...




                  • 6. Re: Static Routing

                    Thanks Jared.


                    I answerd same thing in my interview. But interviewer argued with me about that.

                    I explained him clearly. I asked him about the answer, he said go and refer the materials.

                    Thats why i posted here.

                    • 7. Re: Static Routing

                      Ohh now i got ur questions !!



                      What u wnt is that, if R2 doesn't have a route to R3, then u just want to drop the packet  at R1 instead of R2.



                      U can achieve this by creating the SLA to check the reachebility of R3 from R1. & can create the Track object at R1 in static route of R1 .



                      So what it does, it will use the static route only when the SLA is up.



                      & as per u, if link bw R2 & R3 is down, then SLA will also down & R1 will not use the static route of R1.




                      • 8. Re: Static Routing
                        Fabrizio Chessa

                        I think that is your interviewer that need to refresh and refer the networking materials



                        • 9. Re: Static Routing

                          If u doesn't know abt anythng then u should learn instead of arguing.

                          • 10. Re: Static Routing
                            Fabrizio Chessa

                            Good solution,


                            But I think the problem remains...


                            How SLA knows the reachability of I suppose by ping command... But the problem isn't resolved because R1 tests the reachability by ping from LoR1-->LoR3 and in case of link failure R2-->R3 it's always R2 to respond network unreachable.



                            • 11. Re: Static Routing
                              Fabrizio Chessa

                              It was ironically

                              Just kidding, I did not want to offend anyone


                              I'm sorry for the misunderstanding



                              • 12. Re: Static Routing



                                You are right?? that SLA will use ICMP packet to check the reachibility.  But I think that the question is how would R1 knows that link b/w R2 & R3 is down ??

                                • 13. Re: Static Routing
                                  Fabrizio Chessa

                                  What I understand is:

                                  • How I can avoid (by static-routing) ICMP echo-request to R2 to know the network reachability?


                                  If I misunderstood your reply is correct

                                  • 14. Re: Static Routing

                                    The scenerio, i had explained is not for the  traffic originated by R1 (bcz Yes R1 will send the ICMP msg to R2 to check the reachability). but if i had an n/w segment behind R1 then traffic originated by that segment will stop at R1 bcz static route will comes into play only when SLA is UP.


                                    & if just wnt to share one more thing that as srikanth mentioned that he wnt to tke ping from Lo of R1.



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