12 Replies Latest reply: Mar 17, 2011 6:58 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    EIGRP Stub and Passive Interfaces

    Trostka

      Hello,

       

      Configuring a router as a stub or configuring all of this router's interfaces which are not connected to other EIGRP speakers as passive interfaces have dentical outcomes?

       

      I believe both comands could be used on remote / branch routers who have only one EIGRP neighbour and do not need to advertise any other EIGRP route to the rest of the network, other than their main connected network (interface connecting the remote router to the core/ distribution / central site)?

       

      With the passive interface, all those "internal" interfaces will stop sending hello out, trying to establish neighbor's relationships (as in fact they are not connected to any EIGRP speaker) with the view of exchanging routing inormation.

       

      With the remote router configured as a stub, it's still sending Hello and participating in the EIGRP Neighborship but it's not advertising out any EIGRP route and will not receive any query from its EIGRP neighbors...Surely a stub EIGRP router connected to other routers who do not need to have routing information past the stub router and do need their routing information to be shared past the stub will also require lots of summary routes?

       

      Now, does anybody know of a modern production environment where either some EIGRP routers are configured as stubs or where some of their "inside" interfaces are passive? Any recomendation about EIGRP scalabilities issues?

       

      Thanks!

        • 1. Re: EIGRP Stub and Passive Interfaces
          Paul Stewart  -  CCIE Security

          EIGRP Stub and passive interfaces are completely different topics.  EIGRP stub is sort of like being a BGP speaker and not wanting to become a transit AS.  Cisco says it limits the query scope by telling the neighbors it is a stub.  This is true, but it also keeps the stub router from advertising certain routes.  By default connected and summary routes are advertised (assuming summarization).  So if there are redundant connections from "Branch" to "HQ", we can use stub to make sure that Branch never becomes a transit for an HQ that somehow became split.  It also limits HQ from asking for "querying" for routes when a recalculation must occur.

           

          Passive interface simply disables adjacencies on an interface that is included in the EIGRP process.  Passive-interface and stub can work on the same router without issue. 

          • 2. Re: EIGRP Stub and Passive Interfaces
            Trostka

            Ok, so assuming that a branch router is configured as a a default stub (hence just passing the connected routes that also have a corresponding network statement, and the static routes, if any) and has multiple leased lines connecting it to the branch office, because it does not see the full EIGRP routes that HQ holds, a failure at another site would not cause it to become a transit / hence wrongly advertising to HQ a route to the failed net...that behaviour would be due to a default route on Branch being readvertised into EIGRP at HQ?

             

            I'm not sure I understand corectly what you meant....

            • 3. Re: EIGRP Stub and Passive Interfaces
              Brian

              Draw a little diagram to help display the network topology you are trying to reference.  This will go along way in helping us answer your question.

               

              In short,  a router configured as an EIGRP Stub, advertises only connected and summary routes by default.  In addition, nonstub routers connected to the stub routers will not send query messages to the stub routers when a route goes "active".

               

              HTH

               

              • 4. Re: EIGRP Stub and Passive Interfaces
                Trostka

                Will do tonight. Thanks Brian

                • 5. Re: EIGRP Stub and Passive Interfaces
                  Trostka

                  Hi Paul,

                   

                  Thanks - with a lot of delay ...I definitely had some trouble grasping the concepts you mentioned.

                   

                  So now I believe - as you said- that EIGRP Stub (setting which affects the entire router, as anyone should know) and passive interface (which, as the command says, only affect an interface / directly connected networks and neighbour) are more opposite than similar.

                   

                  IN: Simply, an interface configured as passive will neither send out Hello or listen to routes updates from neighbours as "Passive interface simply disables adjacencies on an interface that is included in the EIGRP process. " But a passive interface will still advertise out connected, summary, redistributed or configured networks.

                   

                  OUT: whereas a Stub router will not advertise its own routing table. But a Stub router will still be a EIGRP neighbour, even if a very silent and secretive one!

                  • 6. Re: EIGRP Stub and Passive Interfaces
                    Paul Stewart  -  CCIE Security

                    Trostka wrote:

                     

                    Hi Paul,

                     

                    Thanks - with a lot of delay ...I definitely had some trouble grasping the concepts you mentioned.

                     

                    So now I believe - as you said- that EIGRP Stub (setting which affects the entire router, as anyone should know) and passive interface (which, as the command says, only affect an interface / directly connected networks and neighbour) are more opposite than similar.

                     

                    IN: Simply, an interface configured as passive will neither send out Hello or listen to routes updates from neighbours as "Passive interface simply disables adjacencies on an interface that is included in the EIGRP process. " But a passive interface will still advertise out connected, summary, redistributed or configured networks.

                     

                    OUT: whereas a Stub router will not advertise its own routing table. But a Stub router will still be a EIGRP neighbour, even if a very silent and secretive one!

                     

                    Actually a passive interface will not advertise anything.  No EIGRP will be sent out the interface in passive mode and received EIGRP on a passive interface will be discarded.  This interface will be included in the EIGRP process, so the network represented by the interface will be made known to the EIGRP topology and will be advertised out other, non-passive, interfaces. 

                     

                    Stub router will advertise part of its own routing table.  By default, it advertises its connected routes (those derived from IP addresses on UP/UP interfaces) and summary routes (valid auto and manual summary routes).  It does not readvertise out its EIGRP learned routes.  So in BGP AS terminology, this is not a "transit" router. 

                    • 7. Re: EIGRP Stub and Passive Interfaces
                      Scott Morris - CCDE/4xCCIE/2xJNCIE

                      Passive interface has to do with participation in the protocol.  If passive, as you noted, you will not get to form a peer at all.

                       

                      Stub on the other hand does two things:  (and it assumes you DO have a peer, which makes it mutually exclusive with passive-interface)

                       

                      1.  Limits what routes are sent out (based on the details you configure it with, such as "default" only)

                      2.  Limits what peers are queried in case of route failure

                       

                      Even if you put ALL the options into a stub where you'd end up sending most, if not all of your learned routes to the peer, you still would not query the other end if a route disappeared.

                       

                      Passive interface dictates WHO you will talk to, or not...  Stub dictates HOW you will talk to them.

                       

                      HTH,

                       

                      Scott

                      • 8. Re: EIGRP Stub and Passive Interfaces
                        Trostka

                        Thanks Scott and Paul, thank you very much for taking the time to answer my question.

                        • 9. Re: EIGRP Stub and Passive Interfaces
                          Scott Morris - CCDE/4xCCIE/2xJNCIE

                          If you didn't ask the questions, you wouldn't get answers!  (and we'd be bored...   never a good thing!)

                           

                           

                           

                           

                          Glad to help though.

                           

                           

                          Scott

                          • 10. Re: EIGRP Stub and Passive Interfaces
                            Trostka

                            Just a last question: so there is a "stub" flag in the EIGRP Hello, like the OSPF hello?

                             

                            I can not find anywhere a description of what TLV are in the EIGRP Hello.

                             

                            thanks again! 

                            • 11. Re: EIGRP Stub and Passive Interfaces
                              Scott Morris - CCDE/4xCCIE/2xJNCIE

                              It does modify the Hello packets.

                               

                              If you take a look at "show ip eigrp neighbor detail" you'll be able to see whether stub routing is enabled or not.

                               

                              The header you could always look at in Wireshark, but being that it's a proprietary protocol, I'm not sure how much detail we would be allowed to know about. 

                               

                              Scott

                              • 12. Re: EIGRP Stub and Passive Interfaces
                                Scott Morris - CCDE/4xCCIE/2xJNCIE

                                Yeah, Cisco isn't really forthcoming with the details about what flags are for what feature in the TLVs.

                                 

                                Scott