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Contributor

BGP refresh capabilty

Hi everybody,

I hope you guys are doing fine.

Please consider the following example:

R1 12.12.2.12.1-------BGP------12.12.12.2 R2

Both R1 and R2 supports BGP refresh capability, R2 is announcing 20.20.20.2/32, 200.200.200.2/32 to R1.

R1#show ip bgp neighbors 12.12.12.2 | section refresh
Route refresh: advertised and received(old & new)

This is my understanding when we use " clear ip bgp 12.1.2.12.2 in"  or show ip bgp neighbor x.x.x.x , R1 will send refresh request to R2 which then sends the fresh copy of NLRI.

but  i get the following error:

R1#show ip bgp neighbors 12.12.12.2 received-routes
% Inbound soft reconfiguration not enabled on 12.12.12.2

Why do we need soft reconfiguration?  

Below is my understanding:

1) In old BGP implementation, every time we modify inbound bgp policy, we need to to perform hard reset on BGP session, drawback; re convergence and interruption for ingress/egress traffic; enters " BGP soft reconfiguration"

2) " BGP soft reconfiguration" feature allows BGP router to retain the copy of NLRI in ADJ-RIB-IN , usually once a BGP router applies inbound policy on received routes in ADJ-RIB-IN, it discards all the routes in ADJ-RIB-IN. with this feature a BGP router now retains the routes in ADJ-RIB-IN, drawback: waste of memory; enters " BGP refresh feature"

3) BGP refresh feature allows BGP router to request fresh copies of NLRI without requiring router to store them in ADJ-RIB-IN, thus overcoming the memory waste issue.

Thanks and have a great weekend!!

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

The thing I am trying to understand why we need  bgp soft reconfiguration when we are using route refresh.

We don't need it. I see soft reconfiguration still being configured primarily for two reasons:

  1. People don't understand the difference between soft-reconfig inbound and route refresh, thinking it's the same thing, or they don't know about route refresh at all.
  2. For management and operation purposes, people want to see all routes that have been sent by a BGP neighbor to this router, regardless of whether or not they have been filtered out.

Have a great weekend, too - and please feel welcome to ask further!

Best regards,
Peter

Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

But then how can we see received unedited routes  routes from a peer when route refresh feature is being used and bgp inbound reconfiguration is not configured?

We cannot. We can only see routes that have been received and have passed through the inbound policies.

It looks like even though we  might be using BGP refresh feature  but in order to to see the unedited received routes we still need BGP reconfiguration .

Very correct - that is the reason "2" I have mentioned in one of my previous posts why people still use soft reconfiguration.

Best regards,
Peter

10 REPLIES 10
Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

I hope are doing fine, too!

R1#show ip bgp neighbors 12.12.12.2 received-routes
% Inbound soft reconfiguration not enabled on 12.12.12.2

Yes - the received-routes is a command specifically showing you the unfiltered RIB for the neighbor if the neighbor has soft-reconfiguration inbound configured. If you just want to see routes received from the neighbor that have made it into your filtered RIB, use

show ip bgp neighbors 12.12.12.2 routes

This command should work nicely with neighbors that are using the route refresh feature.

Why do we need soft reconfiguration?  

Below is my understanding:

Your understanding as you have explained it is perfect.

Best regards,
Peter

Contributor

Hi Peter,

Hi Peter,

Thanks for the response, I am still confused:

BGP refresh feature allows to save memory because a BGP router can simply send refresh request and get fresh copy of NLRI thus we can save memory by not using bgp soft reconfiguration.and use this new feature.

The thing I am trying to understand why we need  bgp soft reconfiguration when we are using route refresh.

Much appreciated and have a great weekend!!

Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

The thing I am trying to understand why we need  bgp soft reconfiguration when we are using route refresh.

We don't need it. I see soft reconfiguration still being configured primarily for two reasons:

  1. People don't understand the difference between soft-reconfig inbound and route refresh, thinking it's the same thing, or they don't know about route refresh at all.
  2. For management and operation purposes, people want to see all routes that have been sent by a BGP neighbor to this router, regardless of whether or not they have been filtered out.

Have a great weekend, too - and please feel welcome to ask further!

Best regards,
Peter

Contributor

Thanks Peter and Kumar,

Thanks Peter and Kumar,

I got it, but then I am wondering why my router is complaining:

R1#show ip bgp neighbors 12.12.12.2 received-routes
% Inbound soft reconfiguration not enabled on 12.12.12.2

Both R1 and R2 supports route refresh feature:

R1#show ip bgp neighbors 12.12.12.2 | section refresh
Route refresh: advertised and received(old & new)

IOS versions:

R1#show version
Cisco IOS Software, 3600 Software (C3660-JS-M), Version 12.4(25),

Thanks

Hall of Fame Cisco Employee

Sarah,

Sarah,

I got it, but then I am wondering why my router is complaining:

It is complaining because you are using a show command that only shows you the routes stored in the soft reconfig database, and you have not enabled the soft reconfig for the neighbor 12.12.12.12.

Best regards,
Peter

Contributor

Thans Peter.

Thans Peter.

But then how can we see received unedited routes  routes from a peer when route refresh feature is being used and bgp inbound reconfiguration is not configured?

It looks like even though we  might be using BGP refresh feature  but in order to to see the unedited received routes we still need BGP reconfiguration .

Thanks

Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

But then how can we see received unedited routes  routes from a peer when route refresh feature is being used and bgp inbound reconfiguration is not configured?

We cannot. We can only see routes that have been received and have passed through the inbound policies.

It looks like even though we  might be using BGP refresh feature  but in order to to see the unedited received routes we still need BGP reconfiguration .

Very correct - that is the reason "2" I have mentioned in one of my previous posts why people still use soft reconfiguration.

Best regards,
Peter

Contributor

Thanks , now I got it,

Thanks , now I got it, appreciate your help:)

Hall of Fame Cisco Employee

Hi Sarah,

Hi Sarah,

You are welcome as always! :)

Best regards,
Peter

Highlighted
Cisco Employee

Hi Sarah,

Hi Sarah,

Like Peter said, we dont need soft-reconfiguration inbound configured for route-refresh. Infact they both are different way of achieveing the end result. 

When ever there is a change in local inbound policy (permitting or denying prefixes), we need either route-refresh or soft-reconfiguration inbound to have the BGP table updated based on incoming (new) policy. When route-refresh is used, local BGP speaker will send a refresh request to neighbor. Neighbor inturn will send all prefixes.

In case of soft-reconfiguration, all prefixes will be stored locally in ADJ-RIB-IN. So soft clear will not send route-refresh to neighbor. instead it will run the best path selection on ADJ-RIB-IN.

HTH,

Nagendra

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