Why Use Vrrp; Definition Of A Virtual Router - Alcatel-Lucent OmniSwitch 6800 Series Network Configuration Manual

Configuring VRRP
If OmniSwitch A becomes unavailable, OmniSwitch B becomes the master router. OmniSwitch B will
then respond to ARP requests for IP address A using the virtual router's MAC address
(00:00:5E:00:01:01). It will also forward packets for IP address B and respond to ARP requests for IP
address B using the OmniSwitch's physical MAC address.
OmniSwitch B uses IP address B to access the LAN. However, IP address B is not backed up. Therefore,
when OmniSwitch B becomes unavailable, IP address B also becomes unavailable.

Why Use VRRP?

An end host may use dynamic routing or router discovery protocols to determine its first hop toward a
particular IP destination. With dynamic routing, large timer values are required and may cause significant
delay in the detection of a dead neighbor.
If an end host uses a static route to its default gateway, this creates a single point of failure if the route
becomes unavailable. End hosts will not be able to detect alternate paths.
In either case, VRRP ensures that an alternate path is always available.

Definition of a Virtual Router

To backup an IP address or addresses using VRRP, a virtual router must be configured on VRRP routers
on a common LAN. A VRRP router is a physical router running VRRP. A virtual router is defined by a
virtual router identifier (VRID) and a set of associated IP addresses on the LAN.
Note. A limitation of the OmniSwitch is that a single VRID may be associated with a VLAN.
Each VRRP router may backup one or more virtual routers. The VRRP router containing the physical
interfaces to which the virtual router IP addresses are assigned is called the IP address owner. If it is avail-
able, the address owner will function as the master router. The master router assumes the responsibility
IP
of forwarding packets sent to the IP addresses associated with the virtual router and answering
requests for these addresses.
To minimize network traffic, only the master router sends VRRP advertisements on the LAN. The IP
address assigned to the physical interface on the current master router is used as the source address in
VRRP advertisements. The advertisements communicate the priority and state of the master router associ-
ated with the VRID to all VRRP routers. The advertisements are IP multicast datagrams sent to the VRRP
multicast address 224.0.0.18 (as determined by the Internet Assigned Numbers Authority).
If a master router becomes unavailable, it stops sending VRRP advertisements on the LAN. The backup
routers know that the master is unavailable based on the following algorithm:
Master Down Interval = (3 * Advertisement Interval) + Skew Time
where Advertisement Interval is the time interval between VRRP advertisements, and Skew Time is calcu-
lated based on the
VRRP
Skew Time = (256 - Priority) / 256
If the backup routers are configured with priority values that are close in value, there may be a timing
conflict, and the first backup to take over may not be the one with the highest priority; and a backup with a
higher priority will then preempt the new master. The virtual router may be configured to prohibit any
OmniSwitch 6800/6850/9000 Network Configuration Guide
router's priority value as follows:
March 2008
VRRP Overview
ARP
page 23-7