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Let's consider a simple scenario:

Router R1 in AS100, which has direct connections to routers R2 (AS200), R3 (AS300), R4 (AS400), R5 (AS500). Regular eBGP neighborships are formed over these links. Let's assume they are operational for a while to avoid discussing things like MRAI, update-delay etc.

At some point in time, R1 receives prefix p from its neighbors and all incoming updates are 300 msec apart.

How many times will R1 run its best path algorithm? Is there any mechanism that throttles best path algorithm, as it is in link-state protocols?

EDIT:

After some additional research I tend to think that BGP best path algorithm run is not throttled.

I checked outputs on a BGP router carrying full Internet table with 2 Tier-1 upstreams - BGP table version increments by ~1000 per minute. This means that in a minute, there were around 1000 changes in the table (~ 1000 prefixes has new best path). Since best path runs on a per-prefix basis, I cannot imagine to have it throttled.

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    Different router manufacturers may implement this differently. – Ron Trunk Jan 15 at 13:25
  • I agree. I was wondering if there can be any technical limitation when it comes to a large scale route reflection topology. Any throttling could seriously impact convergence time within a single AS. – mkurek Jan 16 at 10:43
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The timer that controls updates according to RFC 4271 Section 9.2.1.1, is called the MinRouteAdvertisementIntervalTimer. The suggested default value is 30 seconds for EGBP and 5 seconds for IGBP.

Note that these are "suggested" default values, and vendors can allow you to adjust them (Cisco does, for example).

  • Ron, thank you for your answer. I believe that the timer you mentioned controls how often we can advertise routes, not how often we can run best path algorithm. – mkurek Jan 14 at 14:51
  • You are correct, but the RFC does not specify that parameter. There would be little point, however, to run it more often than MinRouteAdvertisementIntervalTimer. – Ron Trunk Jan 14 at 14:56
  • So, how would it behave in our sample scenario? Update from R2 arrives (first one), R1 runs best path (immediately after receiving UPDATE?), and then waits let's say 30 seconds to run another best path, which will take into account updates from R3, R4, and R5? – mkurek Jan 14 at 15:09
  • Or, R1 runs the process every 30 seconds, regardless of when updates occur. I believe this is what Cisco actually does. – Ron Trunk Jan 14 at 15:23

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