OSPF Graceful Restart


(Rene Molenaar) #1

This topic is to discuss the following lesson:

https://networklessons.com/cisco/ccie-routing-switching-written/ospf-graceful-restart/


(Mohammad Hasanuz Zaman) #2

Hi Rene,
Thanks for your nice stuff .
So , how can we configure the Graceful restart and First we have to check the device is supported or not ?? Thx

br//zaman


(Lazaros Agapides) #3

Hello Zaman

You can find out about how to configure OSPF graceful restart at this Cisco Documentation. Note that OSPF graceful restart comes in two flavours: one which was implemented by Cisco, and one which is the IETF equivilent. Both can be implemented on Cisco devices. Because cisco Calls it Non Stop Forwarding, the command in both cases is nsf. nsf cisco is used to implement the Cisco version while nsf ietf is use for the IETF version.

Both commands are available in IOS version 12.2(33)SXH and later.

I hope this has been helpful!

Laz


(saif s) #4

Dear Agapides,
kindly,

  1. i have confuse about sentecnce “Once OSPF has restarted, it will re-establish neighbor adjacencies with the helper routers” it contradict ( sorry if wrong with using word I am not native), with concept of gracefull restart that neighbor adjacency is not down during ospf process restart…
    2.usually i clear ospf process when i make change in LSDB using filtering or summerization or any thing else change LSDB ,so you mentioned if thers is changed in LSDB before & after gracefull restart , it terminate gracefull restart ( mean neighbor reset ) & interupt in network happen ,so how can compromise between two concept .need your nice clearification

(saif s) #5

Dear Agapides,
from cisco documents,
During the NSF restart process, if neighbors that are not NSF-aware are detected on a network interface,NSF restart is aborted on the interface; however, NSF restart will continue on other interfaces.
is it mean if router 2 detect router 3 is non helpler capable " support or configure ",it will reset adj with r3 during restart ospf process ,keep traffic not interrup with R1 need your clearfication


(saif s) #6

Dear sir
kindlty, related with nsf ietf helper strict-lsa-checking, if defalut behavior when there have been changes in LSA type 1-5 and 7. helper mode will be exit …why i need this command…


(Lazaros Agapides) #7

Hello saif

I will attempt to answer all your questions in this one post.

Graceful restart informs OSPF neighbors that the router will reset its OSPF process. The neighbor routers maintain their neighbor adjacencies, but the router performing the graceful restart actually disengages its adjacencies. So from the point of view of the neighbours, the adjacency stays up, but from the point of view of the router performing the graceful restart, the OSPF process is stopped, restarted, and adjacencies are reestablished.

An OSPF router that is gracefully restarting will exit the graceful restart mode only under certain circumstances. These are clearly described. A change in the LSDB during the restart will cause it to exit. Any LSDB change after, will not affect the graceful restart since it will have been completed.

Yes, this is exactly correct.

This command is necessary to be enabled on helper routers when you want the helper to terminate the graceful restart of its neighbor if the helper receives an LSA that will have to be relayed to the restarting router. This will eliminate the need for the helper router to send the LSA to the restarting router and having the restarting router detect this and cancel the graceful restart. It saves the router an extra step.

I hope this has been helpful!

Laz


(saif s) #8

dear sir

High appreciated yout support , your answer reflect high level unterstansding of features
Thanks thanks :heart_eyes:
god safe you


(Lazaros Agapides) #9

Hello Saif

I appreciate your high praise! Just doing our best here at Networklessons. :sunglasses:

God be with you too!

Laz