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VRRP
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14.2.1. Advertisement Interval
A master indicates that it still 'alive' by periodically sending an advertisement multicast packet to the group
members. A failure to receive a multicast packet from the master router for a period longer than three times the
advertisement interval timer causes the backup routers to assume that the master router is down.
The interval is specified in multiples of 10ms, so a value of 100 represents one second. The default value, if not
specified, is one second. If you set lower than one second then VRRP3 is used by default (see below). VRRP2
only does whole seconds, and must have the same interval for all devices. VRRP3 can have different intervals
on different devices, but typically you would set them all the same.
The shorter the advertisement interval, the shorter the 'black hole' period, but there will be more (multicast)
traffic in the network.
Note
For IPv6 VRRP3 is used by default, whereas for IPv4 VRRP2 is used by default. Devices have to
be using the same version. IPv4 and IPv6 can co-exist with one using VRRP2 and the other VRRP3.
Setting the same config (apart from priority) on all devices ensures they have the same version.
14.2.2. Priority
Each device is assigned a priority, which determines which device becomes the master, and which devices
remain as backups. The (working) device with the highest priority becomes the master.
If using the real IP of the master, then the master should have priority 255. Otherwise pick priorities from 1
to 254. It is usually sensible to space these out, e.g. using 100 and 200. We suggest not setting priority 1 (see
profiles and test, below).
14.3. Using a virtual router
A virtual router is used by another device simply by specifying the virtual-router's virtual IP address as the
gateway in a route, rather than using a router's real IP address. From an IP point-of-view, the upstream device is
completely unaware that the IP address is associated with a group of physical devices, and will forward traffic
to the virtual IP address as required, exactly as it would with a single physical gateway.
14.4. VRRP versions
14.4.1. VRRP version 2
VRRP version 2 works with IPv4 addresses only (i.e. does not support IPv6) and whole second advertisement
intervals only. The normal interval is one second - since the timeout is three times that, this means the fastest a
backup can take over is just over 3 seconds. You should configure all devices in a VRRP group with the same
settings (apart from their priority).
14.4.2. VRRP version 3
VRRP version 3 works in much the same way, but allows the advertisement interval to be any multiple of 10ms
(1/100th of a second). The default interval is still 1 second, but it can now be set much faster - so although the
timeout is still 3 times the interval, this means the backup could take over in as little as 30ms.
VRRP3 also works with IPv6. Whilst IPv4 and IPv6 VRRP are completely independent, you can configure
both at once in a single
vrrp
object by listing one or more IPv4 addresses and one or more IPv6 addresses.
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