before router change

[Mary]

we  have two routers R1 and R2, R1 is BGP backup link and R2 is primary link. both are online, we want to replace R1 during non-business hours, because R1 has intermittent error last month., interface G0/0/0 is connected to telecom provider.

we prepare new router R3 and try to replace R1, R3 is already preconfigured. before we  re-connect the cable into R3, . do we need to shutdown interface G0/0/0 and BGP neighbor in R3. we are afraid that if interface g0/0/0 is not down, the time we plug in the cable, it will cause network flap, is this true? any other things we need to take care when we replace a online router, thanks

[Francesco Molino]

Hi

If R1 is your backup link, then in terms of BGP the route decision will be always made towards R2 and not R1. If that's the case, shutting down or not the g0/0/0 won't affect your network, only the BGP peering with your Provider goes down unless there're some networks that are preferred over R1.

Are there other things:
- Are you using certificates? If yes, don't forget to regenerate them on new router.
- make a backup in case there're some preshared keys (routing password, crypto psk...)

Except that, you just copy/paste the config, everything should be fine

[Joseph W. Doherty]

Generally you cause a network flap (across a routed link) anytime the link (or routing adjacency/peering) goes up or down regardless of whether it's caused by a physical link change (e.g. removing/inserting cable) or a logical change (e.g. shutting port or protocol).

However, the effect of a network flap will vary depending on whether what's changed is the preferred path, or not. If it's the preferred path, you might have micro routing loops, or black holes, while your network detects the lost path and reconveges to an alternate path.

The impact of a non-preferred path flap generally is limited to just your devices processing the change (which is usually not an issue unless such flaps keep rapidly reoccurring).