clear ip BGP

[knaik99]

Why do we not use above command in ISP network?

Please explain

[balaji.bandi]

Because it clears all the connection and terminates. Specifies that all current BGP sessions will be reset.

Then the process start to re-establish the session.

I do not know what context you were looking, explain more ? if you like detailed reply.

[knaik99]

Can we have difference between below command---

Clear ip BGP *

clear ip bgp * soft 
clear ip bgp * soft in/out 
clear ip bgp <bgp neigbour> soft in/out 

[paul driver]

Hello

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@knaik99 wrote:

Can we have difference between below command---

Clear ip BGP *  <--- hard resets all bgp tcp neighbors connections drops peerings , incurs outage, reestablishes bpp relationships with updated adjacency and local rib tables

clear ip bgp * soft <--- soft route refresh adjacency/local rib tables relating to all bgp peers
clear ip bgp * soft in/out  <--- soft route refresh of adjacency-rib-in or out and local rib table relating to all bgp peers
clear ip bgp <bgp neigbour> soft in/out <--- soft route refresh of a specific bgp neighbor adjacency-rib-in or out and local rib table.

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[paul driver]

Hello

Clear ip bgp * - will tear-down and reset all bgp tcp connections so it must be used with caution especially on a isp rtr - however you can use a related command that will not be as intrusive as the above that will request and refresh the bgp tables via the bgp route refresh capability, the below commands can do such a job.

clear ip bgp * soft 
clear ip bgp * soft in/out 
clear ip bgp <bgp neigbour> soft in/out  

[Joseph W. Doherty]

Oh, and to add to what the other posters have already noted, remember if you're dealing with Internet sized route tables, it can take a while to rebuild the peering relationship with even one peer. That's "why" you want to avoid using that command, i.e. you normally don't want to create such a (needless) disruption.

Beside the time it takes, rebuilding the peering relationship is a control plane function, so you may hammer the CPU too. So, even if you only need to reset just one peering relationship, the "soft" option (as mentioned by Paul), is generally preferred.