13 Replies Latest reply: Sep 9, 2011 9:06 AM by Elvin Arias RSS

    What?! - OSPF interesting findings.

    Elvin Arias

      I have some "weird" findings with OSPF. We all know that in order to form an adjacency in OSPF we have some "must match" pre-requisites like:


      Area ID

      OSPF stub flag

      Subnet mask*


      Hello and dead intervals

      Authentication (if used)


      I labbed up a topology with two routers directly connected. R1 <----> R2. The network type is the Broadcast mode.


      Here is the addressing:


      Hostname | Addressing | Mask


      R1      | /24   |

      ----------------| ------------------|--------|

      R2       | /29  |



      Please note these two routers have differents subnet masks. Here is what i found:


      R1(config)#interface fastethernet 0/0

      R1(config-if)#no shutdown

      R1(config-if)#ip address

      R1(config-if)#ip ospf network broadcast

      R1(config-if)#router ospf 1

      R1(config-router)#network area 0


      R2(config)#interface fastethernet0/0

      R2(config-if)#no shutdown

      R2(config-if)#ip add

      R2(config-if)#ip ospf network broadcast

      R2(config-if)router ospf 1

      R2(config-router)#network area 0


      After some time the adjacency doesn't come up (because the "must match" rule), but here is somethinhg interesting, lets see what happens if we change the network type to point-to-multipoint (for example):


      R1 changes:

      R1(config-if)#ip ospf network point-to-multipoint


      R2 changes:

      R2(config-if)#ip ospf network point-to-multipoint


      Adjacency outputs:


      *Mar  1 00:19:17.915: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from LOADING to FULL, Loading Done



      *Mar  1 00:19:27.403: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from LOADING to FULL, Loading Done


      Some show outputs:


      R1(config-if)#do show ip ospf neighbor

      Neighbor ID     Pri   State           Dead Time   Address         Interface        0   FULL/  -        00:01:48      FastEthernet0/0


      R2(config-if)#do show ip ospf interface brief

      Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

      Fa0/0        1        0           10    P2MP  1/1


      The adjacency came up .


      Message was edited by: Elvin Arias