1 2 3 Previous Next 36 Replies Latest reply: Jun 23, 2013 6:30 PM by Brandon RSS

    EIGRP Split Horizon

    jamesh

      Hello all,

       

      I have a HS EIGRP FR topo up, and In all the documentation I have read it says that on a physical FR interface, that split horizon is disabled, and doing the sho ip int command verifies this. However, I am unable to get the routes from one spoke to the other. Even the lab books I have mention that split horizon being disabled by default would allow the spoke routes to be propagated to the other spoke.

       

      Am I missing something ? (and no, my hub interface is not a sub-interface)

       

      Thanks,

       

      James

       

      R1 config (hub)

       

      Serial0/0 is up, line protocol is up
        Internet address is 172.16.124.1/29
        Broadcast address is 255.255.255.255
        Address determined by setup command
        MTU is 1500 bytes
        Helper address is not set
        Directed broadcast forwarding is disabled
        Multicast reserved groups joined: 224.0.0.10
        Outgoing access list is not set
        Inbound  access list is not set
        Proxy ARP is enabled
        Local Proxy ARP is disabled
        Security level is default
        Split horizon is disabled  -----------------> the default, as I understand it.
        ICMP redirects are always sent
        ICMP unreachables are always sent
        ICMP mask replies are never sent
        IP fast switching is enabled
        IP fast switching on the same interface is enabled
        IP Flow switching is disabled
        IP CEF switching is enabled
        IP CEF Fast switching turbo vector
        IP multicast fast switching is enabled

       

       

      interface Serial0/0
      ip address 172.16.124.1 255.255.255.248
      encapsulation frame-relay
      clock rate 2000000
      frame-relay map ip 172.16.124.2 102 broadcast
      frame-relay map ip 172.16.124.3 103 broadcast
      no frame-relay inverse-arp

       

      Gateway of last resort is not set

           172.16.0.0/29 is subnetted, 1 subnets
      C       172.16.124.0 is directly connected, Serial0/0
           10.0.0.0/24 is subnetted, 2 subnets
      D       10.10.10.0 [90/2297856] via 172.16.124.2, 00:12:13, Serial0/0
      D       10.10.20.0 [90/2297856] via 172.16.124.3, 00:08:56, Serial0/0

       

      R2 routing table (spoke)

       

      Gateway of last resort is not set

           172.16.0.0/29 is subnetted, 1 subnets
      C       172.16.124.0 is directly connected, Serial0/0.1
           10.0.0.0/24 is subnetted, 1 subnets
      C       10.10.10.0 is directly connected, Loopback10

       

      R3 routing table (spoke)

       

      Gateway of last resort is not set

           172.16.0.0/29 is subnetted, 1 subnets
      C       172.16.124.0 is directly connected, Serial0/0
           10.0.0.0/24 is subnetted, 1 subnets
      C       10.10.20.0 is directly connected, Loopback20

        • 1. Re: EIGRP Split Horizon
          Keith Barker - CCIE RS/Security, CISSP

          The "IP Split-Horizon" command is related to RIP only, not EIGRP.

           

          The interface command to disable the EIGRP split horizon is:

           

          R1(config-if)#no ip split-horizon eigrp x  (where x is the AS number)

           

          The show ip interface will still show only the RIP attitude for split-horizon, regardless of how the EIGRP is set.

           

          Best wishes,

           

          Keith

          • 2. Re: EIGRP Split Horizon
            jamesh

            Keith,

             

            So if I understand correctly, the no ip split-horizon eigrp [AS] must be used whether the interface is strictly physical or whether it's a sub-interface ?

             

            Thanks,

             

            James

            • 3. Re: EIGRP Split Horizon
              Martin
              Displaying the EIGRP Split Horizon state of an interface is an unusually difficult piece of
              information to find without simply displaying the configuration. By default, IOS enables
              EIGRP Split Horizon. To find the setting for an interface, look for the presence or absence
              of the no ip split-horizon eigrp command on the configuration. Also, the debug ip eigrp
              command output displays messages stating when prefixes are not advertised out an interface
              due to split horizon.

              Source: W.Odom, Route, 2010 ciscopress.

              • 4. Re: EIGRP Split Horizon
                Martin
                The second problem occurs due to Split Horizon logic on Router WAN1, as noted with
                Step 2 in the figure. As shown with Step 2 in the figure, B1 could advertise its routes to
                WAN1, and WAN1 could advertise those routes to B2–and vice versa. However, with
                default settings, WAN1 will not advertise those routes due to its default setting of Split
                Horizon (a default interface subcommand setting of ip split-horizon eigrp asn.) As a
                result, WAN1 receives the Update from B1 on its S0/0/0.9 subinterface, but Split Horizon
                prevents WAN1 from advertising that topology data to B2 in Updates sent out interface
                S0/0/0.9, and vice versa.

                Source: Odom, W. ROUTE, 2010, ciscopress, p.66

                • 5. Re: EIGRP Split Horizon
                  jamesh

                  I appreciate the posts. One of the problems with reading (at least with me it seems), is that I can read this stuff over and over again ( and have read thousands of pages), but still not see it clearly( the tougher concepts anyway). Not sure what that's about.

                   

                  Thing is, when I post on here, and interact with people,( by asking questions that i'm sure are fundamental to most), I am able to clear up the muddy waters so to speak.

                   

                  Im sure that somwhere in the over three thousand pages of Cisco books and articles that I have thus far read,  it has probably mentioned that the statement under 'sho ip int' command that says 'split horizon disabled' only applies to RIP, but untl I am able to get a photographic memory I just won't be able to remember details like this until I screw it up enough to remember, or have this back and forth with you guys on here.

                   

                  Thanks guys,

                   

                  James

                  • 6. Re: EIGRP Split Horizon
                    Martin

                    Split Horizon applies to both RIP and EIGRP!

                    I think Split Horizon is enabled on physical and multi-point interfaces, and it is disabled on p-t-p.

                    We had discussed this topic recently; search cisco.com for EIGRP FAQ.

                    • 7. Re: EIGRP Split Horizon
                      jamesh

                      I'm not concerned with how much it has been discussed on here before, as I said, sometimes the only way for me to clear things up is to have this interaction with you guys.

                       

                      That's ok with you isn't it Martin ?

                      • 8. Re: EIGRP Split Horizon
                        Martin

                        yes of course, there is some confusion on this topic.

                        Let's see what others have to say....

                        • 9. Re: EIGRP Split Horizon
                          Keith Barker - CCIE RS/Security, CISSP

                          jamesh wrote:

                           

                          Keith,

                           

                          So if I understand correctly, the no ip split-horizon eigrp [AS] must be used whether the interface is strictly physical or whether it's a sub-interface ?

                           

                          Thanks,

                           

                          James

                          Yes. 

                           

                          Keith

                          • 10. Re: EIGRP Split Horizon
                            Brian

                            Aloha james,

                             

                            EIGRP split-horizon is DISABLED on the physical interface when you issue the "encapsulation frame-relay" command.

                             

                            Remove these three lines they are not needed:

                             

                            frame-relay map ip 172.16.124.2 102 broadcast
                            frame-relay map ip 172.16.124.3 103 broadcast
                            no frame-relay inverse-arp

                             

                            Also, it look like you do have a subinterface configured on one of the spoke routers:

                             

                            C       172.16.124.0 is directly connected, Serial0/0.1

                             

                            This is the minimum router configuration with EIGRP over FR when usingh the main physical interface:

                             

                            interface Serial0/0
                            ip address 172.16.124.x 255.255.255.248
                            encapsulation frame-relay

                             

                            This is all you need on each router, where x is the router # RT1 = .1, RT2 = .2 and RT3 = .3.

                             

                            Check out my Quick Facts document on EIGRP over Frame Relay.  You can find it under the "document" section of the CCNP Study Group.

                             

                            HTH

                             

                            • 11. Re: EIGRP Split Horizon
                              Brian

                              Aloha Martin,

                               

                              In this example the hub interface is a subinterface and not the main physical interface.

                               

                              the "no ip split-horizon eigrp x" is only needed when a multipoint subinterface is used.

                               

                              HTH

                               

                              • 12. Re: EIGRP Split Horizon
                                Keith Barker - CCIE RS/Security, CISSP

                                Hello Brian-

                                 

                                The EIGRP split-horizon, by default, is still enabled on a physical interface.

                                 

                                In the example below, R1 is the hub, with R2 and R3 as spokes, with EIGRP.  Loopbacks are the same as router numbers.

                                 

                                R1#show ip route eigrp
                                     2.0.0.0/24 is subnetted, 1 subnets
                                D       2.2.2.0 [90/2297856] via 123.0.0.2, 00:03:43, Serial0/0
                                     3.0.0.0/24 is subnetted, 1 subnets
                                D       3.3.3.0 [90/2297856] via 123.0.0.3, 00:00:17, Serial0/0

                                R1#show run int ser 0/0
                                Building configuration...

                                 

                                Current configuration : 220 bytes
                                !
                                interface Serial0/0
                                ip address 123.0.0.1 255.0.0.0
                                encapsulation frame-relay
                                frame-relay map ip 123.0.0.3 103 broadcast
                                frame-relay map ip 123.0.0.2 102 broadcast
                                end

                                 

                                R1#show ip route eigrp
                                     2.0.0.0/24 is subnetted, 1 subnets
                                D       2.2.2.0 [90/2297856] via 123.0.0.2, 00:05:59, Serial0/0
                                     3.0.0.0/24 is subnetted, 1 subnets
                                D       3.3.3.0 [90/2297856] via 123.0.0.3, 00:02:33, Serial0/0
                                R1#


                                ***************

                                R2#show ip route eigrp
                                     1.0.0.0/24 is subnetted, 1 subnets
                                D       1.1.1.0 [90/2297856] via 123.0.0.1, 00:06:50, Serial0/0
                                R2#show run int ser 0/0
                                Building configuration...

                                 

                                Current configuration : 178 bytes
                                !
                                interface Serial0/0
                                ip address 123.0.0.2 255.0.0.0
                                encapsulation frame-relay
                                frame-relay map ip 123.0.0.1 201 broadcast
                                end

                                *************************

                                R3#show ip route eigrp
                                     1.0.0.0/24 is subnetted, 1 subnets
                                D       1.1.1.0 [90/2297856] via 123.0.0.1, 00:04:01, Serial0/0
                                R3#show run int ser 0/0
                                Building configuration...

                                 

                                Current configuration : 178 bytes
                                !
                                interface Serial0/0
                                ip address 123.0.0.3 255.0.0.0
                                encapsulation frame-relay
                                frame-relay map ip 123.0.0.1 301 broadcast
                                end

                                 

                                R3#

                                 

                                On the spokes, note the missing loopback of the remote spoke.


                                After adding this command on the hub:


                                R1(config)#int ser 0/0
                                R1(config-if)#no ip split-horizon eigrp 1


                                The spokes then recieved the routes from the other spokes.


                                R3#show ip route eigrp
                                     1.0.0.0/24 is subnetted, 1 subnets
                                D       1.1.1.0 [90/2297856] via 123.0.0.1, 00:08:20, Serial0/0
                                     2.0.0.0/24 is subnetted, 1 subnets
                                D       2.2.2.0 [90/2809856] via 123.0.0.1, 00:00:13, Serial0/0

                                 

                                 

                                RIP split horizon, on the other hand, is a different story than the example above.

                                 

                                Best wishes,

                                 

                                Keith

                                • 13. Re: EIGRP Split Horizon
                                  jamesh

                                  Heya Brian,

                                   

                                  Yea, the one sub-interface on the spoke was just for me testing what the default split horizon was under the 'sho ip int' command, but I knew it wouldn't matter what I did with that particular router, because it would'nt be dealing with split horizon.

                                   

                                  My confusion was with the fact that I had originally understood the line 'split horizon disabled' under the 'sho ip int' command to apply to EIGRP, but it does not. Just one of those confusing points. 

                                   

                                  Also, I wanted to ask about you saying that I do not need the map statements, because without them, I noticed I was unable to form a neighborship ?

                                   

                                  Are you saying to just leave it up to inverse-arp ? ( I removed inverse-arp to test the neighborship with and without the broadcast keyword in the map statement).

                                   

                                  Thanks,

                                   

                                  James

                                  • 14. Re: EIGRP Split Horizon
                                    jamesh

                                    Understood Keith, thanks.

                                     

                                    James

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