Quagga integrates the OSPF protocol, which allows routes to be dynamically advertised so that other machines know which path to use to reach internal networks.

Context :

We have two routers

• router-002 (BACKUP)
• router-003 (ACTIVE)

When a failover occurs:

• CARP moves the VIP to the new ACTIVE router
• The OVH script updates the MAC address T2276

Remote machines (such as Windriver, Ysul, and CloudHugger) use a GRE tunnel to reach the network 172.27.27.0/27.
When the active router changes, the previous tunnel becomes unavailable and is removed, which also deletes the associated route.

As a result, if nothing is done, these machines no longer have a valid route to reach the internal network 172.27.27.0/27.

To solve this, the active router must advertise the network using OSPF, so that remote machines automatically learn the correct route and can continue to reach the network.

References :

https://fr.wikipedia.org/wiki/Quagga_(logiciel)
https://enotepaper.wordpress.com/2014/01/08/installing-quagaa-on-freebsd/