Subject: Re: Should linux send netlink message as it is deleting that routing entry?

From: Stephen Hemminger <[email-masked]>
Date: Fri, 11 Apr 2014 11:30:42 -0700

On Fri, 11 Apr 2014 17:15:48 +0800
zhuyj <[email-masked]> wrote:

> > With ubuntu 12.04, I run the following to reproduce this defect.
> >
> > 1) Configure an interface
> > ifconfig eth1 150.0.0.1/24 up
> >
> > 2) Add routing entry via that interface address
> > route add -net 200.0.0.0/24 gw 150.0.0.1
> >
> > 3) Change the ip address on that interface as shown below.
> > ifconfig eth1 151.0.0.1/24 up
> >
> > 4) Check netlink messages with "ip monitor all". There is no route 
> > delete netlink message.
> >

With IPv4 there are several cases where there is a non-notified
implicit route withdrawal. This is not something that can be fixed.
There are two issues:
 1. with large backbone size route tables (ie 1M routes), the number
    of notification messages becomes a bottleneck and would be unreliable
 2. the existing routing daemons (quagga, bird, etc) all understand/expect
    the existing semantics