[j-nsp] graceful failover and software upgrades
daniel at claudius.demon.nl
Fri Mar 18 17:54:24 EST 2005
pls see inline.
On 20:41 Fri 18 Mar , Pekka Savola wrote:
> Thanks for the procedures. This is a nice start, because there isn't
> anything published on this.
> A few comments inline..
> On Fri, 18 Mar 2005, Jeff C. Strahl wrote:
> > 1. Enable Graceful restart to minimize/eliminate forwarding plane
> > disruption . You also need to enable graceful restart on peer nodes for it
> > to buy you anything.
> This brings up an interesting point which hasn't been clearly
> documented anywhere.
> What's the relation of graceful restart support (at your neighbor
> nodes) and graceful switchover?
> That is, if we have graceful switchover enabled, what happens if
> a) neighbor supports graceful restart
> b) neighbor does not support graceful restart
> (And the currently the router is also configured to do graceful
> My hunch is that the protocol sessions don't flap with a), and do flap
> with b). (The flap probably takes 2-10 seconds, the time it takes for
> the other RE to take control using the switchover..)
> Anyone have concrete ideas about the relation of graceful restart and
> swichover ?
actually; in both scenario's the sessions flap. Here's what happens with
GRES (graceful RE switchover) enabled:
- during normal operation the active RE synchronizes state with the
backup. This is things like interfaces, routes etc.
- when the active RE fails; the standby takes over. During this time
the PFE (i.e. forwarding complex) continues to forward with the state
of the time of the failure.
- GRES does not mirror TCP sessions or any other routing protocol
neighborships. So they will go down.
- This is where graceful restart comes into play. The original master
had sessions with it's neighboring routers. When graceful restart is
enabled, you effectively tell your neighbor: if i disappear don't
worry i'll be back (tm) and while i'm gone assume i can still forward
traffic (i.e. don't withdraw my routes from the rest of the network).
- when the new master comes up; it establishes new sessions/adjacencies
and indicates it came back after a graceful restart. The neighbors will
now resend all their routing info as it's possible that right at the
time of the failure a routing update was missed.
- the router now has all current info again and can update the PFE if
there are any discrepancies between the PFE state and the RE's current
view on the world.
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