OSPF Commands

[kevin.hu]

Hi,

We have a mixed of old 2610 routers and new 3800 routers. I am not sure if these OSPF exponential backoff timers have global or local significance because I cannot configure exponential backoff on those older routers.

Please advice. Thanks a lot!

ispf (local significant?)

limit retransmissions non-dc 12 (what does this do?)

timers throttle spf 500 1000 10000 (local or global significant?)

timers throttle lsa all 100 10000 45000 (local or global significant?)

timers pacing lsa-group 120

(local or global significant?)

[Giuseppe Larosa · Accepted solution]

Hello,

give a look at the following link

http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_s3g.html#wp1086707

All these commands apply within an OSPF process and allow a tuning of OSPF behaviuor node by node (local not global)

This was not possible on older IOS versions that had fixed values for most of these timers or were not capable of some opmitizations (lsa grouping means instead of sending a single OSPF packet with a new version of an LSA wait for 120 seconds collect all LSAs that have to be refreshed and send out a single OSPF update with all of them).

All this shouldn't compromise interoperability between new routers and old 2610s.

You can design your network, if possible, by putting the 2610s in OSPF stub areas, where the amount of link state info that has to be exchanged / refreshed is reduced to a minimum and also recalculations are less frequent.

I put some details:

timers throttle spf spf-start spf-hold spf-max-wait

they decide after how many msec a spf calculation occurs, minimum time between two calculations, maximum wait time between two calculations.

non-dc

Nondemand circuit retransmissions.

limit retransmissions non-dc 12 = max 12 times an update is retransmitted on a standard link

timers throttle lsa all 100 10000 45000

this is 12.4

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp3.html#wp1017690

To set rate-limiting values for Open Shortest Path First (OSPF) link-state advertisement (LSA) generation, use the timers throttle lsa all command in router configuration mode. To restore the default values, use the no form of this command.

usage guidelines:

The "same LSA" is defined as an LSA instance that contains the same LSA ID number, LSA type, and advertising router ID. We suggest you keep the milliseconds value of the timers lsa arrival command less than or equal to the hold-interval value of the timers throttle lsa all command.

all this helps OSPF to scale better with an higher number of IP prefixes because it allows to package LSAs and to do some traffic shaping of them.

I suggest you to use default values for timers and use stub areas for 2610s as much as possible.

hope to help

Giuseppe

[kevin.hu]

Thanks Giuseppe.

Another question. Is there a guideline on how to choose different time values for different exponential backoff timers?

[Giuseppe Larosa]

Hello Kevin,

how to choose depends from your design goal:

- fast convergence

- scalability

if you can give a look at networkers presentations about OSPF design and IP fast convergence they are interesting subjects, otherwise look for these on CCO

The default values were thought to avoid to hang up the router cpus that were less powerful 15 years ago.

These timers decide how much your router is reactive.

Ideally after a single LSA change you would like to recalculate as soon as possible.

However, if another LSA change happens within a few msecs what to do ?

recalculate as soon as possible or is better to wait some time in order to give the network time to propagate other changes/LSAs that likely can arrive on some interface, so that a single SPF execution can accomodate more then one LSA change ?

These should be the meaning of the timers.

Best Regards

Giuseppe