[166344] in North American Network Operators' Group
Re: clear forwarding route
daemon@ATHENA.MIT.EDU (John Smith)
Fri Oct 18 13:35:52 2013
Date: Fri, 18 Oct 2013 18:32:57 +0100 (BST)
From: John Smith <jsmith4112003@yahoo.co.uk>
To: Manav Bhatia <manavbhatia@gmail.com>, "nanog@nanog.org" <nanog@nanog.org>
In-Reply-To: <CAG1kdogOyEVeKhhDm3oOsvuHiJEnCEz=uYWjZY+DwKvKpzBLeg@mail.gmail.com>
Reply-To: John Smith <jsmith4112003@yahoo.co.uk>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
This is a hack that most vendors provide, just in case their code doesnt wo=
rk as expected.=A0=0A=0ANobody in his sane mind will clear the FIB on a liv=
e 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.=0A=0AIt might make some sense to do this on flow ba=
sed 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 c=
ase its an issue as all live traffic starts hitting SW before the flow can =
get established.=0A=0ACustomers, 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.=0A=0A=0AOn Friday, 18 October 2013, 21:31, Manav Bhatia <mana=
vbhatia@gmail.com> wrote:=0AHi,=0A=0AI would like understand the circumstan=
ces under which an operator may want=0Ato clear all (or a subset of) the ro=
utes programmed in the forwarding table=0A(FIB).=0A=0AI believe the command=
to do this on Cisco is=0A=0Aclear forwarding {ipv4 | ipv6} route {* | pref=
ix} [vrf vrf-name] module=0A{slot| all}=0A=0AI ask this since doing this wo=
uld result in the router dropping all transit=0Atraffic till the routes get=
reprogrammed in the FIB.=0A=0AWhy would somebody ever want to do this? One=
scenario that i can think of=0Ais when because of a bug a route does not g=
et programmed in the FIB and the=0Aoperator uses this command to install th=
is once again the FIB.=0A=0AThanks, Manav=0A
