OSPF Fast convergence tuning

ashish-nagpal · ‎01-07-2010

Hi All,

I need your suggestions over following details.

Brief Overview of network:
- Our network is pure level-3 level ISP backbone network. It is comprissed of mostly Cisco GSRs, 6500 & 7600 series devices.
- IGP used in network is OSPF only and all the links used in backbone network are 10Gige & 1 Gige Ethernet only.
- All edge and core routers in our network are receiving full internet routes through BGP but not redistributed in OSPF.
- There are multiple leaf areas conneced to backbone area and each area is comprised of almost 10-25 devices.
- There is huge database of OSPF and the timers used in OSPF are default.

Problem Description:
- Few days back we faced link flap multiple times on one of our backbone router, which caused reconvergence of entire OSPF database. This convergernce took a long while because of huge OSPF database.
- In order to achieve faster and stable convergence we are looking to fine tune OSPF timers using OSPF SPF/LSA throttling with default parameters.

Need suggestion, if it is recommended to implement OSPF SPF and LSA throttling in core backbone network of level-3 size ISP and what are the suggested values, which can be used when entire backbone is based on 1/10Gige ethernet links.

Regards

Ashish Nagpal

Giuseppe Larosa · ‎01-07-2010

Hello Ashish,

the following chapter from OSPF configuration guide can be of help

http://www.cisco.com/en/US/docs/ios/iproute_ospf/configuration/guide/iro_short_path_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1054299

and command reference for timers spf throttle

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp3.html#wp1017802

the explanation given in usage guideline is:

Usage Guidelines

The first wait interval between SPF calculations is the amount of time in milliseconds specified by the spf-start argument. Each consecutive wait interval is two times the current hold level in milliseconds until the wait time reaches the maximum time in milliseconds as specified by the spf-max-wait argument. Subsequent wait times remain at the maximum until the values are reset or a link-state advertisement (LSA) is received between SPF calculations.

so you can use the command with a low start and hold time and an high maximum time (third parameter) as in the following example:

This example shows a router configured with the start, hold, and maximum interval values for the timers throttle spf command set at 5, 1,000, and 90,000 milliseconds, respectively.

router ospf 1

 router-id 10.10.10.2

 log-adjacency-changes

 timers throttle spf 5 1000 90000 


In case of single event the routers are still reactive but in case of flapping 
the third parameter allow to delay SPF  execution to avoid waste of cpu and memory resources


without this commmand the behaviour is not self adaptive, so I think exact values are not
important, what counts is the change to a self adaptive behaviour.

Hope to help

Giuseppe