An Open Shortest Path First (OSPF) router can get "stuck" in the EXSTART state when attempting to establish an adjacency with another router. In the EXSTART state, the router is trying to determine the proper sequence number to use for the initial exchange of Link State Advertisements (LSAs). Each router begins this process with a sequence number of 1, but if the other router is using a different sequence number, the two will never be able to establish an adjacency. This can happen if the other router has been reset, or if it is using a different version of OSPF.
If you find that your router is stuck in the EXSTART state, you can try to establish an adjacency with the other router manually. To do this, you will need to know the router ID of the other router, as well as the OSPF area that it is in. You can then use the "debug ip ospf adj" command to see if the two routers are able to establish an adjacency.
You might enjoy: Can I Doordash in a Different State?
Frequently Asked Questions
Why is my OSPF neighbor stuck in exstart/exchange state?
There can be number of reasons why the OSPF neighbors are stuck in exstart/exchange state. The most common reason is that the MTU between the two routers is different. The OSPF protocol uses 20 bytes for every data packet transmitted over an OSPF link, but some networks use smaller MTUs and the OSPF packets sent over those links get fragmented. When this happens, the router on one side of the link will send a probepacket to determine whether the other side is up and running, and if not, it'll attempt to exchange routing information with that neighbor. If there's still no response after 30 seconds, the router assumes that the network segment is down and starts attempting adjacencies with neighboring routers using adjacent-hops information from its database. Router A advertises itself as reachable via interfaces on both Routers C and D (assuming their MTUs are different) since their respective metric values match. However, when Router B tries to
What are the OSPF States for adjacency formation?
There are four OSPF States for neighbors: Init, Attempt, Down and Loading. The Init or Exstart state indicates the neighbor is initializing (starting up), the Attempt or Exchange state means it's trying to form an adjacency, the Down or full state means the neighbor is down or full and can't establish an adjacency, and finally the 2-way state indicates that two routers are exchanging routing information.
Why can't I see unicast OSPF packets in my Network?
OSPF routers in a FULL STATE only exchange Hellos, and do not transmit unicast OSPF packets.
Why is OSPF stuck in the exchange state?
OSPF is configured for maximum transmission unit (MTU) of 1572 bytes. If the packets from the remote router are too large, OSPF cannot send them through the interface and will experience an exchange failure.
What is exstart/exchange state in OSPF?
The exstart/exchange state occurs when two routers exchange database information. In this state, the routers elect a master and a slave and the initial sequence number.
Sources
- https://naz.hedbergandson.com/why-ospf-stuck-in-exstart-state
- https://community.cisco.com/t5/networking-knowledge-base/ospf-neighbors-are-stuck-in-exstart-and-exchange-state-due-to/ta-p/3131603
- https://community.juniper.net/communities/community-home/digestviewer/viewthread
- https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13684-12.html
- https://ipwithease.com/ospf-stuck-in-exstart-mtu-mismatch-post/
Featured Images: pexels.com
