BGP and soft reconfiguration inbound

[Fernando Bello]

Hello all,

I was just reading about soft reconfiguration inbound benefits and disadvantages and it came to my mind one question.

Why does the router need to create a 2nd copy of the table and store it in memory to apply filters any time I want instead of having only one table and mark as filtered or unused the the routes that are matched by the new filter?

If I need to do add another filter, then I rerun the filter on the whole table, the whole routes that are active and the routes that were previously filtered. And change the states of the routes in case the filter matches the prefix.

In that way I would be saving memory on my router.

THanks in advance,

Fernando

[Peter Paluch · Accepted solution]

Hi Fernando,

Wonderful and completely correct thinking! While many textbooks say that soft-reconfiguration inbound causes your router to keep a totally separate copy of all routes received from the partical neighbor for which this command is configured, that is not necessarily how this feature is implemented. This is the classic dichotomy between a principle (its idea) and its implementation.

It is completely true that for plain route filtering, you would be totally fine with having just a single copy of all routes received from a neighbor and just flagging them as "would be filtered" or "would be accepted" by your inbound policy. However, keep in mind that apart from filtering or accepting routes, you can also modify their BGP attributes. With a single copy of all routes, you would need to keep both the original and the modified attribute. Of course this can be done fairly easily, it's just a hint to you that simply keeping routes and flagging them as accepted or filtered would not be enough - whenever you modify an attribute of a path, you need to keep both the original and modified values stored in memory.

This way or the other, the soft-reconfiguration inbound principially causes your router to consume more memory than necessary. Its functionality has largely been superseded by the ROUTE REFRESH capability. It is surprising to see how many new routers in new deployments are still being configured with this feature. I guess the distinction between this soft reconfiguration and the route refresh feature is not sufficiently clear.

Best regards,

Peter

[Fernando Bello]

Thanks for the answer! very helpful!!!

thanks again!!!