Router-ID in OSPF protocol

Unanswered Question
Feb 18th, 2007

When I use highest ip address of active interface as a Router-Id of the router if its active interface goes down then the router must reestablishing adjacency with other neighbor router or not.

please clarify or provide helpful link.

Thank in advance

I have this problem too.
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Danilo Dy Sun, 02/18/2007 - 22:27

If the interface associated with this IP address is ever brought down, or if the IP address is removed/replaced, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces. Yes, it will reestablish adjacency - check "show ip ospf neighbor".

By experience, I recommend you statically assign a router-id and it should be a loopback interface IP Address

Jon Marshall Sun, 02/18/2007 - 22:41


Agree with most of what you said. However i believe even if the interface that has the IP address being used as the router-id goes down OSPF will still continue to use that IP address.

The only time the router-id changes is if you manually change it with the "router-id" command or if the OSPF process itself restarts.



weerapatr Mon, 02/19/2007 - 00:24

Hi guys Thank for input.

So, the neighbor routers that associate with active interfaces must reestablish adjacency again but the router-id in hello packet has not been changed,

please clear me out

Jon Marshall Mon, 02/19/2007 - 00:28


Yes new adjancencises will need to be establised but the router-id will not change unless you manually change it with the router-id command or restart the OSPF process.



mheusinger Mon, 02/19/2007 - 06:14


interesting link - and dead wrong in this specific point, as already pointed out. Each OSPF process needs a router ID for identification. There is no requirement, that it belongs to an active interface IP. You can f.e. use an OSPF router ID of without any problem as long as it is unique.

If you need the confirmation: Jon is absolutely right.

Regards, Martin

P.S.: Just configured it as a demonstration by adding a Loopback with IP, starting OSPF and deleting the Loopback:

(Hope this clarifies it):

R1#sh ip int brie

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES TFTP administratively down down

Serial1/0 unassigned YES NVRAM administratively down down

Serial1/1 unassigned YES NVRAM administratively down down

Serial1/2 unassigned YES NVRAM administratively down down

Serial1/3 unassigned YES NVRAM administratively down down

R1#show ip ospf

Routing Process "ospf 10" with ID

Start time: 00:00:33.660, Time elapsed: 00:02:35.148

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capability

Router is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

LSA group pacing timer 240 secs

Interface flood pacing timer 33 msecs

Retransmission pacing timer 66 msecs

Number of external LSA 0. Checksum Sum 0x000000

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Number of areas transit capable is 0

External flood list length 0

Area BACKBONE(0) (Inactive)

Number of interfaces in this area is 0

Area has no authentication

SPF algorithm last executed 00:01:44.688 ago

SPF algorithm executed 3 times

Area ranges are

Number of LSA 1. Checksum Sum 0x00B9B0

Number of opaque link LSA 0. Checksum Sum 0x000000

Number of DCbitless LSA 0

Number of indication LSA 0

Number of DoNotAge LSA 0

Flood list length 0

ahmednaas Mon, 02/19/2007 - 08:11

Thanks Martin.

I guess somebody should bring this to the attention of the documentation guys at Cisco.

Jon Marshall Wed, 02/21/2007 - 06:37

Many thanks for that Martin.

I remembered reading a long time ago that the router-id did not actually change when the interface went down but I'd never actually tested it.



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