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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)

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 12 '17 at 18:54
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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.

  • Thanks Ron, I have a few follow on questions: For the sake of these questions, lets assume 10.0.0.1 is the Physical address and VIP address of the Master VRRP router, and 10.0.0.2 is the Physical address of the Standby VRRP router. If the Master router fails, will the Standby VRRP take over answering for 10.0.0.1? Will the 10.0.0.1 address still get its own virtual MAC address? If so, will the Master (un-failed) respond to all ARP Queries for 10.0.0.1 with the Virtual MAC? Even if the traffic is intended to go directly to the Master Router itself (think routing protocol, or some such)? – Eddie Dec 13 '15 at 17:51
  • @Eddie, that is a little more tricky because I have heard that aspects of your questions have changed with IOS versions. The new master will respond to the virtual IP and MAC, but, and this is where is could vary, pinging the VIP will no longer work. The new master will respond to ARP and basically act like HSRP, but there are some undocumented aspects of how it may work with ICMP, and I can't find anything about how it may act with a routing protocol. I guess that would be up to lab testing and another reason to use router IDs and loopbacks. – Ron Maupin Dec 13 '15 at 19:08
  • I'm going to set some time aside to do some lab tests, then I'll come back and post the result. If no one else provides an answer by then, I'll give you the checkmark. Thanks Ron. – Eddie Dec 13 '15 at 22:13
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    When you test in the lab, be sure that you use the IP address for the neighbor. RIP, OSPF, and EIGRP normally use multicasts, but they can have specific neighbors set to use unicasts. – Ron Maupin Dec 13 '15 at 22:17

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