Why Is the OSPF Neighbor Stuck in a Certain State?

Latest reply: Apr 27, 2017 08:36:59 2157 1 0 0
Why Is the OSPF Neighbor Stuck in a Certain State?
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Created Apr 27, 2017 08:36:59 Helpful(0) Helpful(0)

Stuck in the Init State
The Init state indicates that a router can receive Hello packets from its neighbor but it does not see itself in the Neighbor field of the neighbor's Hello packets.

The possible causes are as follows:

The neighbor is configured with an improper ACL, which filters out the Hello packets sent by the router.
The link is faulty. For example, the link is a unidirectional link and can only receive packets rather than send packets. As a result, the Hello packets cannot reach the neighbor. Such a cause is common on an NBMA network.
On an NBMA network, mapping is required between Layer 2 and the IP address. In case of static mapping, the key parameter Broadcast is missing.
Stuck in the Attempt State
The attempt state exists only on an NBMA network. This state indicates that a router is sending Hello packets to a statically configured neighbor but has not received any response.

The possible causes are as follows:

The neighbor is configured with an improper ACL, which filters out the Hello packets sent by the router.
The Layer 2 link is faulty, which cause the Hello packets unable to reach the neighbor.
Stuck in the 2-way State
The 2-way state indicates that a router has established a two-way adjacency with a remote end.

In broadcast or NBMA mode, the 2-way state of DRothers is a normal state.

In other modes, if neighbors are stuck in the 2-way state, it is an abnormal situation and the possible cause is that the Layer 2 link is faulty.

Stuck in the Exstart/Exchange State
In the networking shown in the Figure 1, the MTU of Device A is larger than that of Device B and the router ID of Device B is larger than that of Device A.

After Device A and Device B establish two-way communication, they negotiate a master/slave relationship and determine initial DD sequence numbers for exchanging DD packets. The negotiation result is that Device B acts as the master. After Device A receives an initial DD packet from Device B, it sets the status of Device B to exchange and also sends a DD packet carrying the DD sequence number negotiated for Device B. After Device B receives the DD packet from Device A, it finds that the MTU (carried in the DD packet) of Device A mismatches its own MTU and thus ignores the received DD packet. Device B then re-sends a DD packet. On Device B, you can view that Device A is stuck in the Exstart state.

Figure 1 Stuck in the Exstart/Exchange State


Stuck in the Loading State
In the loading state, neighbors exchange LSRs and LSUs to obtain detailed LSAs. If a neighbor is stuck in this state, it indicates that the LSR or LSU exchange fails.

The possible causes are as follows: the neighbor responds with an incorrect packet after receiving an LSR: You can run the display ospf error command to find whether such an error occurs.

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