convergence in bgp

[sarahr202]

Hi everybody.

Please consider the following scenario:               

R1---ibgp------R2----ebgp-------R3------network 10.0.0./8

Suppose 10.0.0.0/8 goes down, R3 check its advertisement interval for ebgp peer R2. It finds It is still running, R3 waits for the timer to expire ( default is 30 sec for ebgp peer). R3 then sends update withdrawing 10.0.0.0/8 network to R2.

Now some documents say default advertisement interval time for ibgp peer is 5 sec and some say zero second.

What will R2 do next? Will it send the update right away to R1 or will R2 wait for the 5 sec timer to expire and then sends it to R1?

Thanks and have a great weekend.

[Reza Sharifi · Accepted solution]

Hi Sarah,

The default values are 30 seconds for eBGP and 5 seconds for iBGP/eiBGP  sessions (intra-AS). This timer really starts playing its role only for  “Down-Up” or “Up-Down” convergence, as any rapid flapping changes are  delayed for the amount of advertisement-interval seconds. This becomes  especially important for inter-AS route propagation, where the default  advertisement-interval there is 30 seconds.

Here is a really good doc to read on BGP convergance:

http://blog.ine.com/2010/11/22/understanding-bgp-convergence/

HTH

Reza

[sarahr202]

Hi Reza

R1 starts sending updates to R1 and RR1, RR2. This will take some time,  depending on the BGP TCP transport settings and BGP table size. However,  before R1 will ever start sending any updates to any peer/update group,  it checks if  Advertisement Interval timer is running for this  peer. BGP speaker starts this timer on per-peer basis every time its  done sending the full batch of updates to the peer. If the subsequent  batch is prepared to be sent and the timer is still running, the update  will be delayed until the timer expires.

When does bgp router send a full batch of updates?  At the time when when neighbor relationship first comes up?

Thanks

[Reza Sharifi]

Hi Sarah,

That is correct.  When the peer establishes a neighbored relationship with another router, it will send the full batch.

Thanks

Reza

[jcramalho]

Hi Sarah and Reza:

Now, 8 years after the last reply to this question I may update the default iBGP advertisement interval as below:

IOS-XR: 0 seconds.

https://www.cisco.com/c/en/us/td/docs/routers/crs/software/crs_r4-/routing/command/reference/rr40crs1book_chapter1.html

IOS-XE: 0 seconds

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/command/iproute_bgp-xe-3se-3850-cr-book/iproute_bgp-xe-3se-3850-cr-book_chapter_010.html#wp3459951926

Please notice the same document also states: 

12.4T, 12.2SB, 12.2SE, 12.2SG, 12.2SR, 12.2SX, Cisco IOS XE 2.1

This command was modified. The default value for eBGP sessions in a VRF and for iBGP sessions changed from .5 seconds to 0 seconds.

So there was a change in the command, previously to the above mentioned IOS versions, the advertisement time was in fact 5 seconds.

HTH.

Very kind regards,

Joao