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Ask the Expert- SD-WAN

OSPF neighbors are stuck in the two-way state

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Core Issue

When Open Shortest Path First (OSPF) is enabled on a router or when a router configured for OSPF is powered up, it tries to discover its OSPF neighbors and synchronize its database with them. Routers are said to be OSPF neighbors when they see their router ID in the received hello packet and the status of their OSPF relationship transitions to Two-way.

OSPF neighbors then exchange routing information with each other. Once their databases are updated and synchronized, the status of their OSPF relationship changes to Full. However, the status of the OSPF relationship between OSPF neighbors does not always transition to Full and might remain stuck in Two-way instead.

Resolution

This is expected behavior on routers running OSPF. OSPF routers on multi-access segments synchronize their  databases with their Designated Router (DR) or a Backup Designated Router (BDR) only. The OSPF status between two routers transitions to Full only if at least one of them is a DR or a BDR.  The state remains Two-way between a pair of routers if both are drothers. 

For more information on the relationship between OSPF routers stuck in the two-way state, refer to Why Does the show ip ospf neighbor Command Reveal Neighbors Stuck in Two-Way State?

Troubleshooting Commands:

1) show ip ospf neighbors....This command will show you states of your neighbor and dead timer.

Example:

Router#sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:38    10.1.1.1        FastEthernet0/0 

1) Show ip ospf int brief ...This command will give you OSPF property briefly on each interface.

Example:

Router#sh ip os int bri
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Fa0/0        100   0               10.1.1.2/24        10    BDR   1/1

 

3) Show ip ospf int <inteface no>...This command will give detail OSPF interface information like network-type,authentication type and key, interface mask,cost etc.

Example:

Router#sh ip os int fa0/0
FastEthernet0/0 is up, line protocol is up
  Internet Address 10.1.1.2/24, Area 0
  Process ID 100, Router ID 10.1.1.2, Network Type BROADCAST, Cost: 10
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 10.1.1.1
  Backup Designated router (ID) 10.1.1.2, Interface address 10.1.1.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:06
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1  (Designated Router)
  Suppress hello for 0 neighbor(s)

4) Sh int (interface-no>....This command will give you physical link parameter like link state,MTU(Which is imp factor in ospf while exchanging database and need to same on both site>.

Example:

Router#sh int fa0/0 | in MTU | up
FastEthernet0/0 is up, line protocol is up
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,

For MTU ,you need to check on (Routing) IP MTU that's will gonna stuck your OSPF process in ex-start state if both side device don't have same MTU , default both MTUs are same on IOS platform but you can change using "ip mtu" command under interface on the router and on Switch ,you can set system routing MTU.

Example:

Router#sh ip int fa0/0 | in MTU
  MTU is 1400 bytes

Comments

thank you very useful, Cool

Beginner

In multi access networks DR/BDR elections happens at the end of the 2-way state. Router with the highest priority will be elected as DR and second highest will be elected as BDR. Default Priority is 1. If priority is tie, Router ID is the tie breaker. RID is highest IP address configured on a Physical interface, if loop back is configure, loop back interface IP selected as RID.

 

If a router priority is configured as 0, then the router will not participate in DR and BDR elections. In this scenario routers will be stuck in 2-way state. And 2-way state is the normal state between two neighbors.

 

We can configure the Router Priority by using ip ospf priority <value> form interface configuration mode.

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