BGP and ISIS

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Aug 3rd, 2008
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Hi,


Is there any relation between ISIS overload bit timer and BGP graceful restart timer. Does BGP timer should be always greater than ISIS overload bit timer.


Thanks,

Prakash

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pradeepde Fri, 08/08/2008 - 07:38
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In a Service Provider's network, when a router running BGP and IS-IS reboots, BGP might take more time to converge than IS-IS.

Thus the router may drop traffic for destinations not learned via BGP. It is convenient to set the Over- load Bit until BGP has converged.


prakashj Sun, 08/17/2008 - 21:20
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Hi Pradeep,


If I am not wrong when you set overload bit, ISIS does not participate in the routing till the overload bit timer is over after a restart of the box. So how will the router reach the neighbor router's loopback address if it does not involve in the routing. How can BGP establish neighbor with the peer router's loopback without reachability.


Regards,

Prakash

Harold Ritter Mon, 08/18/2008 - 11:06
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Prakash,


Setting the overload bit only prevents the router to be used as a transit router.


Directly connected subnets are still reachable to other routers. So if you had two routers configured with overload bit, you could still establish a BGP session between their respective loopback addresses assuming they are either directly connected or the routers in between do not have their overload bit set.


Regards,

prakashj Mon, 08/18/2008 - 12:08
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Thanks Harold..


We have an MPLS network with centralised RRs. All the BGP peerings are between PEs and RRs. At present we have configured BGP graceful-restart timer (300s) higher than overload-bit(240 sec). Are there any implications to this design? We have the same configuration on all the PE routers.


Regards,

Prakash

Harold Ritter Sun, 08/24/2008 - 18:17
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Prakash,


There is no real link between the two as the overload bit is only set on system restart, whereas the 300s is time that would be allow for graceful restart.


Regards,

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