What is VRRP and what it is being used for?

What is VRRP and what it is being used for? I would like to know the use case. Appreciate any help in this. Thanks!
beer9Asked:
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JustInCaseCommented:
The Virtual Router Redundancy Protocol (VRRP) is almost the same as HSRP (if you are familiar with it), but it is open standard (HSRP is Cisco proprietary) so all vendors can use it. It is First Hop Redundancy Protocol (FHRP). Basically, VRRP provide virtual IP address for hosts to be used as default gateway, in case that L3 device that is in use as default gateway is unavailable, other L3 devices replace unavailable device to be default gateway.
Wiki article.
More theory and configuration on Cisco devices
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Don JohnstonInstructorCommented:
The primary function of VRRP is as a standards based First Hop Routing Protocol (FHRP).

Basically, if your network has two separate routers which provide connectivity to the same locations (the Internet, branch offices, etc.), when you configure the local PC's, you have to pick one of those two routers as the default gateway.  If the router you chose to use fails, that second router will not be used until you redefine the default gateway on each PC to use the other router.

With a FHRP, the two routers work together to provide redundancy for the default gateway service.  One of the routers becomes the "active" forwarder.  The other is in "standby" mode.  All traffic sent through the default gateway will be handled by the active router.  If the active router fails, the standby promotes itself to be the active router and the traffic continues to flow.  The PC's have no idea this happened.

There is a bit more going on behind the scenes but I didn't know how much detail you were looking for.
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beer9Author Commented:
Thanks for explaining, I have below details for my interface, and IP 10.4.12.20/32 is being used as VRRP via Keepalived (http://www.keepalived.org/) applications. I would like to understand what is happening here.

oot@localhost:~# ip addr show eth2
4: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 05:2b:21:2f:f2:65 brd ff:ff:ff:ff:ff:ff
    inet 10.4.12.58/24 brd 10.4.12.255 scope global eth2
    inet 10.4.12.20/32 scope global eth2
    inet6 fe20::27b:22ff:f1cf:7bc5/64 scope link
       valid_lft forever preferred_lft forever

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Don JohnstonInstructorCommented:
It looks like the 10.4.12.20 is the "virtual IP" address for VRRP.  This is the IP address that the hosts use as their default gateway.

When a host forwards traffic to that IP address, only the active router will forward it.

Edit: This implementation of VRRP uses "Master" and "Backup" instead of "Active" and "Standby".

Recommend reading the users guide, section V.
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beer9Author Commented:
Thanks for the explanation. What is the difference of "Master"/"Backup" and "Active"/"Standby".
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Don JohnstonInstructorCommented:
Please see my first post.
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