This submit will element the issue situation of OSPF Caught in EXSTART state as a consequence of MTU mismatch between OSPF neighbors. (For higher understanding please undergo the associated video.)
OSPF Caught in EXSTART: Instance Situation
The above diagram depicts 2 Routers R1 and R2 related to every by way of Ethernet interface fa0/0 over 192.168.12.0/24 subnet. Interface fa0/0 of R1 is assigned IP 192.168.12.1 whereas R2’s Fa0/0 is assigned 192.168.12.2.Routers are operating OSPF (Space 0).The routers kind full adjacency with R2 being the DR (Designated Router).
As depicted within the above diagram, for understanding the impact of MTU on OSPF neighbor ship, we modify the MTU measurement on Interface Fa0/0 of R2 from 1500 to 1499 bytes. The command on interface stage is “ip mtu 1499”. Now, we allow “debug ip ospf adj” command on every of R1 and R2 to have an in depth view of OSPF actions whereas OSPF adjacency between R1 and R2 is in course of.
R1 debug output exhibits that it’s receiving smaller MTU from R2 whereas R2 debug output exhibits it’s receiving bigger MTU from R1.
On this case, R2 will drop the OSPF packet obtained from R1 and can keep caught in “EXSTART” state because it can not settle for OSPF packet with measurement bigger MTU than its interface MTU measurement. Nonetheless, R1 accepts the OSPF packet and strikes to alternate state.
As proven in above diagram, so as to resolve the problem of MTU mismatch and due to this fact caught in “EXSTART” state, we will
(1) Both change MTU measurement on Interface of R1 to 1499 Bytes
(2) Inform R1 to disregard MTU verify throughout EXSTART stage.
Right here we are going to use choice 2.
For performing this alteration, we go to interface fa0/0 of R1 and add “ip ospf mtu-ignore” command.
As soon as finished, R2 will bypass the mtu verify course of in EXSTART stage and additional transfer to full adjacency. The “present ip ospf“output exhibits each the Routers having full OSPF state.
Watch this video for higher understanding:
Proceed Studying:
OSPF Multi-Space Adjacency : Instance Situation
Perceive & Configure OSPF SHAM LINKS
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If you wish to be taught extra about OSPF, then verify our e-book on OSPF Interview Questions and Solutions in straightforward to grasp PDF Format defined with related Diagrams (the place required) for higher ease of understanding.
