Cisco – VRRP Sharing Master Router’s IP

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This set of question stems from a part of the answer to this question: What are the differences between VRRP and HSRP?

With HSRP, each interface must have an IP address that is separate from the HSRP group address. VRRP lets you share the Master's interface IP address.

I'm trying to understand this point in more detail.

  • How are the individual routers sharing the master router's interface IP address? (AKA, how does it work at the ARP/Switch layers)
  • Are the individual (non-master) routers simply not accessible via this IP address until the master fails (and a new master is elected)?
  • Is there a benefit to this configuration other than simply conserving a few more IP addresses?

(so its clear, I'm not referring to the virtual address created by VRRP that becomes the end hosts's default-gateway. I'm talking about the physical router's physical interface address)

Best Answer

The individual routers' physical interfaces do not share the master router's IP address. Each router must have different addresses configured on the physical interface. What that statement means is that the virtual interface may be assigned to the IP address of one of the physical interfaces. The physical interface which shares its address with the virtual interface becomes the master. IOS will assign the interface with same address as the virtual interface to a priority of 255 to always make it the master because the maximum priority which can be configured is 254.

There is no problem with pinging or reaching each router since each router, except the master, actually has a different assigned address which is different than the virtual address.

This feature really only conserves IP addresses, and only one per subnet.

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