OSPF hello timers

[ivan.marakovic]

HI All,

Would it be sensible to use OSPF hello timer of 1s and dead timer of 4 sec on a IP Ethernet WAN with rtt of 10-70ms and with 30 3640 routers in the area 0?

Regards

Ivan

[pkhatri]

Hi Ivan,

While these are pretty aggressive timers, there should be no problems with using such small intervals. In fact, Cisco IOS gives you the ability to use a dead interval of 1 ms now (using 'ip ospf dead-interval minimal hello-multiplier }').

With what you are suggesting, it will still take four consecutive lost Hellos before the adjacency is brought down, so you are pretty safe.

If you want to wind your timers down further, you can use BFD (Bidirectional Forwarding Detection) to get sub-second failure detection.

Hope that help - pls rate the post if it does.

Paresh.

[followurself]

Hi Paresh,

cisco MSFC and routers in the same area.we connect to the ISP who give MPLS connection. Now wht happens is over MPLS BGP runs, and our router which is connected to thier router runs OSPF.so bgp redistribute routes into ospf it goes across our internal lan. we have many sites and have the same senario.BGP redistributes ospf routes and vice versa.having some issues with convergence..BGP timers are set to 15 secs and ospf to defaults. If we adjust ospf timers here say hello to 1, it will actually help within the area.since bgp is between across sites so they are multiple areas.wht wd be the best timer settings?

[pkhatri]

Howdy,

I've successfully used a BGP holdtime of 24 seconds and a keepalive of 8 seconds with a number of customer implementations. On WAN links, I'd be hesitant to wind down the timers any further.

Hope that helps - pls rate the post if it does.

Paresh

[followurself]

Hi Paresh,

I missed one point earlier, the router which runs ospf internally is connected to another router which alos runs eigrp. so ospf here is redistributed into eigrp.

so we have say router a--running bgp and ospf ie the isp router connected to MPLS

we have router b--running ospf and communicating using ospf with the isp router..learns the route from isp router where bgp redistribution happens.

router c --connected to router b--using ospf and redistributes those routes into eigrp..

now timers here i beleive need to perfect for better convergence..ospf is on defaults so do eigrp..bgp is 15 secs..if we set ospf dead to 4 with hello 1 will it help, if so wht we shd set for both bgp and eigrp

[chad patterson]

Be careful. I ran into a timer issue today while trying to secure my OSPF implementation with MD5 auth.

https://learningnetwork.cisco.com/message/714297#714297

[Deepak Kumar]

Hi,

I think this is a very aggressive timer for OSPF. I advised to not only depend on the OSPF Hello packets but also consider as carrier detection, and Bidirectional Forwarding Detection (BFD) etc for Fast Convergence in OSPF. 

Regards,

Deepak Kumar