OSPF convergence

[mauricio.rodriguez]

Hello,

I need an information about OSPF convergence time.

On the network we have installed VoIP with IP-Phones. We need to customize OSPF convergence response time in order to have a minimum "blank" on the conversation.

I precise that the routers OSPF are configured on L3 switches and are connected with Gigabit Ethernet link and are all on the LAN network.

What are the values that are recommended that I have to customize in order to have a best convergence.

Thank you by advance.

Best regards.

[Giuseppe Larosa]

Hello Mauricio,

modern IOS images support sub-second hello timers so you can speed up convergence.

Another possibility if supported is to use BFD (Bidirectional Forwarding detection)

see

OSPF Support for Fast Hello Packets

By specifying the minimal and hello-multiplier keywords with a multiplier argument, you are enabling OSPF fast hello packets. The minimal keyword sets the dead interval to 1 second, and the hello-multiplier value sets the number of hello packets sent during that 1 second, thus providing subsecond or "fast" hello packets.

When fast hello packets are configured on the interface, the hello interval advertised in the hello packets that are sent out this interface is set to 0. The hello interval in the hello packets received over this interface is ignored.

The dead interval must be consistent on a segment, whether it is set to 1 second (for fast hello packets) or set to any other value. The hello multiplier need not be the same for the entire segment as long as at least one hello packet is sent within the dead interval.

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1014268

then there the SPF timers that can be tuned otherwise the convergence time is 1+5 = 6 seconds.

in router ospf config mode

timers throttle spf

see

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

Hope to help

Giuseppe

[mauricio.rodriguez]

hi

thank you for your answer

I don't know if cat 3750 with IP SERVICES ios support this image, so If I cannot use Fast Hello I heard that it's possible to configure manually hello to 1s and Dead interval to 4 * hello = 4s

Do you confirm this ?

What are the commands line ?

Thank you by advance

[Giuseppe Larosa · Accepted solution]

Hello Mauricio,

the commands are given in interface config mode

int vlan 50

ip ospf hello-interval 1

ip ospf dead-interval 4

you need to set the same on all devices otherwise the OSPF adj cannot be formed

Hope to help

Giuseppe

[Joseph W. Doherty]

As Giuseppe's notes, there are some tuning knobs that can improve OSPF convergence. Another is "Incremental OSPF", see http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfispf.html

However, to maximize OSPF convergence time, you should also consider the overall network design. Below are three links that will provide information to further understand what all should/can be done.

http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/HA_campus_DG/hacampusdg.html#wp1107567

http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/HA_recovery_DG/campusRecovery.html#wp998494

http://www.cisco.com/en/US/docs/nsite/campus/ha_campus_routed_access_cvd_ag.pdf

[carl_townshend]

hi there

what is this bfd detection?

is this part of ospf? how does it work?

[Joseph W. Doherty]

See http://www.cisco.com/en/US/technologies/tk648/tk365/tk480/technologies_white_paper0900aecd80244005.html

PS:

NB: Easily found on Cisco web site, searched using terms "OSPF BFD".