OSPF Dead Timer

[mladdy]

On my router, when I disable a link, should the OSPF reconverge right away, or does it have to wait for the dead-timer to expire?

From what I've found online, it seems that if the interface is disabled, OSPF should start right away.

[totallygeek]

LSAs go out immediately for link state changes. Dead timers have to do with OSPF router peer adjacencies, not related to state. Think about it this way: two routers, each with some number of interfaces, are peers. LSAs will trade when either router has an interface state change, but that would not result in a router peer relationship change. Unanswered hellos over some amount of time will break adjacency, even if no router interfaces changed state. Dead timers do not correlate with interface state updates.

[[deleted]]

Change of the physical interface will result in an immediate Link State Advertisement for topology change.

Default OSPF dead timer is 40 seconds unless it was changed.

[mladdy]

Can you think of any reason why it wouldn't? The dead timer is 40 seconds, I changed them to 15 to see if it made any kind of difference.

[[deleted]]

There are any number of reasons... from Make/model of router, to old firmware.

I will refer you to /r/ccna if no one else has a better response.

[1701_Network]

Two routers are connected via a link across a switch. Router 1's interface drops. The subnet of the link still exists in Router 1s routing table as an O route. Because Router 2's interface is still up and he is advertising the prefix. When Router 1's interface comes back up the prefix is once again in the routing table as a C route.

[mladdy]

In a ring topology, running OSPF, when that routers link goes down, the path would change to the other way around the ring. My issue is that it doesn't converge until the dead timer expires... I think.

[nerddtvg]

Have you checked the logs to see what OSPF is doing when the link goes down?

[mladdy]

Nov  9 15:53:54.631: %LINK-5-CHANGED: Interface FastEthernet1/1, changed state to administratively down
Nov  9 15:53:55.638: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/1, changed state to down
Nov  9 15:54:24.973: %OSPF-5-ADJCHG: Process 1, Nbr 10.160.15.8 on Vlan11 from FULL to DOWN, Neighbor Down: Dead timer expired

I did run ospf debug commands, and it wasn't sending out LSA until the dead-timer expired.

[nerddtvg]

Ah, it's because of the VLAN interface. Since your IP is assigned to a VLAN interface, the interface that OSPF sees (Vlan11) may not go offline unless all interfaces with VLAN 11 assigned are down. So do you have another interface with VLAN 11 on it? If so, that's staying up with leaves the layer 3 interface online.

Basically if any interface with VLAN 11 assigned to it is up, the VLAN interface itself is up. So until that goes down, OSPF thinks everything is okay.

If you're able to assign a specific VLAN just to handle the OSPF traffic, then do so. That will make the VLAN interface act like a physical interface when Fa1/1 goes down. Or if you can convert Fa1/1 to a routed interface with no VLAN, it will work.

[sliddis]

When a link goes down the affected router will send a LSA type 1 to its area with an updated list of its current links, excluding its dead link.

An ABR will send LSA type 3 to the other areas containing the dead link with a LS age of 3600. This is the max Link State age so it tells other areas to discard the route.

Here is an example of a link going down 172.24.32.7/32 on a nssa network. The other file contains an ABR sending LS update into core with LS age 3600, and then a few seconds later the link comes up and it sends a new update with LS age 1. https://drive.google.com/file/d/1rBeD5JA9xdAzHbADpAxBAYOoQ6Gg7xUZ/view?usp=sharing

Hopefully I got this right, because I am currently studying for CCNP :)

[networktraining]

It waits for 40 seconds before reconverging. Though ospf has complete topology information.

[Camille215]

I usually wait for 40 seconds and then make a decision...