clear forwarding route

Sat Oct 19 04:12:07 UTC 2013

On Fri, Oct 18, 2013 at 10:32 AM, John Smith <jsmith4112003 at yahoo.co.uk>wrote:

> This is a hack that most vendors provide, just in case their code doesnt
> work as expected.
>
> Nobody in his sane mind will clear the FIB on a live router. This creates
> all sorts of problems. The router starts sending out ICMP errors
> (unreachables, etc), BFD times out, causing all hell to break lose within
> the domain.
>

Oh, plenty of people on the list here have had to do
exactly that on live routers.  Not sure whether or not
we would ever be accused of being of 'sane mind', but
that kinda comes with the territory of trying to move large
volumes of packets at high speeds around the planet.

When one or two linecards in a chassis have a bad entry
stuck in their copy of the forwarding table, and things are
getting screwy, it's better to flush and relearn the FIB
then continue black-holing traffic for a subset of the
network.  :/

> It might make some sense to do this on flow based routers where you clear
> the FIB so that newer flows can get established in case there are hash
> collisions or issues in flow caches. Even in that case its an issue as all
> live traffic starts hitting SW before the flow can get established.
>
> Customers, you can rest assured, will not appreciate you doing this. And
> its precisely for this that you never ever do this on a live router.
>

Unless of course the customer is being black-holed due to a bad
FIB entry;  in which case, the customer will most assuredly be
more appreciative of you doing it, than of you *not* doing it.

Matt

>
> On Friday, 18 October 2013, 21:31, Manav Bhatia <manavbhatia at gmail.com>
> wrote:
> Hi,
>
> I would like understand the circumstances under which an operator may want
> to clear all (or a subset of) the routes programmed in the forwarding table
> (FIB).
>
> I believe the command to do this on Cisco is
>
> clear forwarding {ipv4 | ipv6} route {* | prefix} [vrf vrf-name] module
> {slot| all}
>
> I ask this since doing this would result in the router dropping all transit
> traffic till the routes get reprogrammed in the FIB.
>
> Why would somebody ever want to do this? One scenario that i can think of
> is when because of a bug a route does not get programmed in the FIB and the
> operator uses this command to install this once again the FIB.
>
> Thanks, Manav
>
>
>