OSPF convergence

Answered Question
Sep 18th, 2008
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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.


Correct Answer by Giuseppe Larosa about 8 years 9 months ago

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


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Giuseppe Larosa Fri, 09/19/2008 - 02:50
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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 Fri, 09/19/2008 - 04:03
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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


Correct Answer
Giuseppe Larosa Fri, 09/19/2008 - 04:48
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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 Fri, 09/19/2008 - 03:52
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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 Fri, 09/19/2008 - 04:38
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hi there


what is this bfd detection?


is this part of ospf? how does it work?

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