No.
Graceful Routing Engine switchover (GRES): the PFE / the cards don't reboot (so the ports stay up) when there's a switchover between both REs. All the routing protocols are restarted from scratch, all the neighbours see that the peer felt down and restarted (so they drop the peer routes too), all the routes must be relearned (in both directions), in the meanwhile the PFE FIB are emptied (and stop forwarding) and repopulated from scratch. So, while the ports stay up, the forwarding stops while all the routing relearns the routes and the PFE is re-programmed. This is nice but it should be used with either Graceful Restart (prehistoric way) or NonStopRouting / NonStopBridging (modern way) to keep the routing information alive:
- GRES + Graceful Restart: the routing protocols discuss with the neighbours with a GR option (they must be aware of it). After the switchover the PFEs keep temporarily their actual FIB, and keep forwarding with this old FIB. The new RE restarts from scratch the routing protocols, but the neighbours aware of the GR option know that this is not a real restart from scratch (because of the GR option), so they don't drop their former routing information about the peer for a few minutes, which allows the GR peer to resend its routes. In the meanwhile the RE relearns the routes from the neighbours, and updates the old PFE FIB with the new information.
- GRES + NSR: the routing protocols are synchronised between both REs, including RIB/FIB/states, TCP sequence numbers, md5 counters and so on. When the RE switchover, the neighbours are not even aware that something happened (except maybe a few lost packets but not enough to trigger a routing protocol session flap). The PFEs keep forwarding with their old FIB. The new RE refresh its RIB and updates the FIB with its current RIB. This is mostly invisible from the outside.
- NSB (nonstop-bridging): same that NSR, but for layer2 protocols (spanningtree, switching, layer2 stuffs, and so on).
On routers, today, usually one should use GRES + NSR/NSB.
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Olivier Benghozi
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Message d'origine:
Envoyé: 07-07-2025 04:32
Depuis: djadhav
Sujet: Question on Graceful RE Switchover
Hi Sabbir.
Thanks for your reply.
So, the backup RE maintains table information even without NSR configured?
Regards,
Deepak
Original Message:
Sent: 07-07-2025 01:39
From: Shabbir Ahmed
Subject: Question on Graceful RE Switchover
Hi Deepak,
All the state information is populated or synced between REs in real time. once the backup RE takes over it already has all it's tables populated.
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Shabbir Ahmed
Original Message:
Sent: 07-06-2025 08:54
From: djadhav
Subject: Question on Graceful RE Switchover
Hi all.
Assume Graceful RE Switchover is configured, but no Graceful Restart.
When the master RE fails, and the backup RE takes over, how does the new master RE populate its forwarding information? Does it get it from the PFE?
Regards,
Deepak