Cisco IOS – BGP Soft Reconfiguration

Introduction

When the BGP routing policy is amended the BGP neighbor session must be reset in order for the changes to take effect. Of course in a production environment performing a hard reset i.e clear ip bgp … ,  isn’t something that is recommended.

Within this article, we will look at the various options available to us, within Cisco IOS to ensure our BGP changes are correctly committed.

Inbound Soft Reconfiguration

With inbound soft configuration, the router stores a record of all advertisements received from the neighbors prior to any policy being applied. When a soft reconfiguration is performed the router reads through the advertisements and applies the current policies, and implements any changes that it detects due to current policy changes. [1] Below shows the commands to enable and run an inbound soft reconfiguration.

router bgp 500
  ...
  neighbor 100.1.1.1 soft-reconfiguration inbound

clear ip bgp <neighbor> soft out

However, the key disadvantage with this method is the memory overhead required, as the router has to store all of the previous advertisements.

Outbound Soft Reconfiguration

An outbound soft reconfiguration works by the router reading through its route table as if it were preparing an advertisement. It applies the current policies and advertises to neighbors any changes that it finds due to changes in current policy. [2] A soft reconfiguration can be run using the command:

clear ip bgp <neighbor> soft out

Route Refresh

To overcome the memory issues with the inbound soft reconfigurations, Cisco introduced (in version 12.1) route refresh.

Simply put, rather than store all of the neighbor advertisements in memory the route refresh feature allows you to request the neighbor to resend the routes. To invoke a route refresh the command clear ip bgp <neighbor> soft out is used. Note: This command will ONLY perform a route refresh instead of an inbound soft reconfiguration when neighbor 100.1.1.1 soft-reconfiguration inbound is NOT configured.

Route Refresh vs Soft Reconfiguration

Reference

[1] “Difference Between Inbound Vs Outbound … – Cisco Support Community.” https://supportforums.cisco.com/t5/wan-routing-and-switching/difference-between-inbound-vs-outbound-soft-reconfig/td-p/2495727 . Accessed 10 Nov. 2017.
[2] “Difference Between Inbound Vs Outbound … – Cisco Support Community.” https://supportforums.cisco.com/t5/wan-routing-and-switching/difference-between-inbound-vs-outbound-soft-reconfig/td-p/2495727 . Accessed 10 Nov. 2017.

Rick Donato
Latest posts by Rick Donato (see all)

Want to become a networking expert?

Here is our hand-picked selection of the best courses you can find online:
Cisco CCNA 200-301 Certification Gold Bootcamp
Complete Cyber Security Course – Network Security
Internet Security Deep Dive course
Python Pro Bootcamp
and our recommended certification practice exams:
AlphaPrep Practice Tests - Free Trial