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Monday, January 2, 2012

How to use the clear ip bgp *soft command

The command "clear ip bgp *soft command" performs soft reset of bgp neighbors relationship in both direction i.e in and out. Since this command also performs Soft reset for " in" direction so does it not require command " neighbor soft-reconfiguration inbound" as well ? Is this correct?

clear ip bgp * soft
neighbor soft-reconfiguration inbound.

Let say we have a router that support refresh capability and so does its neighbor R2. Also assume" neighbor soft-reconfiguration inbound" is configured. Now router has two options to achieve the same goal. The question is which method router will prefer, will it send refresh message? will it just use the database of unfiltered packets?

You have to distinguish between the Soft Reconfiguration and the Dynamic Inbound Soft Reset.

The Soft Reconfiguration is the older method that had to be configured manually on a per-neighbor basis using the soft-reconfiguration inbound whose point was to store a separate unfiltered database of all advertisements sent by the particular neighbor. Using the clear ip bgp * soft [ in ] command simply caused this unfiltered database to be re-filtered anew depending on the actual inbound routing policies. This approach, obviously, was very memory-intensive.

The Dynamic Inbound Soft Reset is the Cisco name for the Route Refresh capability defined by the RFC 2918. This capability is negotiated dynamically with the neighbor and there are no configuration commands related to it. This capability will be negotiated automatically on a per-neighbor basis during the BGP neighborship establishement. After using the clear ip bgp * soft [ in ], the router will automatically send the ROUTE-REFRESH message to all neighbors supporting this feature, requesting them to resend their routing advertisements for the particular address family.

In other words, the clear ip bgp * soft command has the same effect, whether the Soft Reconfiguration or the Route Refresh is used. Obviously, the means to achieve the same effect are very different.

We see a lot of BGP configurations even here on CSC to have the neighbor soft-reconfiguration inbound command configured for various neighbors. I would like to highlight the fact that this is absolutely useless, ineffective and missing the point. It seems that people are confusing the Soft Reconfig with the Route Refresh capability. Every solid BGP implementation nowadays supports the Route Refresh without needing to configure anything and without consuming those inordinate amounts of RAM. The RFC 2918 was published in September 2000, so it's nearly 12 years old and its support is nearly ubiquitous. I cannot stress this enough - configuring the neighbor soft-reconfiguration inbound is neither necessary nor called for.

If both neighbors support the Route Refresh and at the same time, they are configured with neighbor soft-reconfiguration inbound then the Soft Reconfiguration is used instead of the Route Refresh. In other words, the Route Refresh capability dynamically negotiated during the peering establishment will be ignored, the ROUTE-REFRESH messaging will not be used, and instead, both routers will store both the unfiltered and the filtered databases.

Please click here to find more documentation on "BGP Soft Reset Enhancement".

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