BGP Soft Configuration

Question:

I understand WS-C3750X-48PF-L the purpose behind this idea perfectly fine, but I just have a few questions on the use the command.

If I recall ( < IOS 12.0) you had to manually include the 'neighbor x.x.x.x soft-reconfiguration in' command to enable soft reconfiguration,

for inbound BGP updates. Then if let's say you changed a prefix-list, route-map, distribute-list, you would need to do 'clear ip bgp soft neighbor-id'.

Obviously once you enable soft-reconfiguration, it will store a copy of the updates, from which it will modify.

But if you have > IOS 12.0, I know all you have to do is, 'clear ip bgp soft neighbor-id or * for all BGP sessions on the router from which it is configured'.

I was wondering if the above commands and IOS version requirements were correct?

Also, when you change a prefix-list, route-map, distribute-list, and it's for outbound BGP updates, do you have to do the soft option as well?

Answer:

You have to distinguish between Soft Reconfiguration and Route Refresh. The Soft Reconfiguration is what you describe - keeping the set of all received routes - and up to this day and the most recent IOSes, if you want to use it, you must configure the neighbor using the neighbor soft-reconfiguration in commmand.

The Route Refresh is an optional enhancement to BGP (albeit almost universally supported) that is negotiated during BGP peering establishment in the OPEN messages. The Route Refresh feature introduces a new, 5th message to BGP, the ROUTE REFRESH. Using this message, a BGP speaker can ask its neighbor to resend all routes of a particular address type. The Route Refresh capability is negotiated dynamically and you do not configure it.

I am not sure what was the first IOS version that implemented the Route Refresh capability, but the RFC 2918 is from September 2000, and Cisco must have had this feature implemented before that because they used a different capability code before the Route Refresh capability code was defined by IANA:

https://supportforums.cisco.com/thread/2123459

In any case, if the neighbor supports some kind of soft refresh, be it either Soft Reconfig or Route Refresh, you do not specifically need to write the soft keyword in the clear ip bgp command.

I am not sure if this covers your WS-C3750X-48PF-S Price question - please feel welcome to ask further.