VRRP enables hosts on a LAN to make use of
redundant routing platforms on that LAN without requiring more than the static
configuration of a single default route on the hosts
The VRRP routing platforms share the IP address
corresponding to the default route configured on the hosts. At any time, one of
the VRRP routing platforms is the master (active) and the others are backups
Implementation
A
virtual router must use 00-00-5E-00-01-XX as its Media Access Control (MAC)
address. The last byte of the address (XX) is the Virtual Router IDentifier
(VRID), which is different for each virtual router in the network. This address
is used by only one physical router at a time, and it will reply with this MAC
address when an ARP request is sent for the virtual router's IP address.
Physical
routers within the virtual router must communicate within themselves using
packets with multicast IP address 224.0.0.18 and IP protocol number 112.
Routers
have a priority of between 1-255 and the router with the highest priority will
become the master. The default priority is 100, for address owner the priority
is always 255.
Lab
Below is given a simple lab that anyone can try at home with GNS3 to effectively understand the working of VRRP.
The configurations and outputs after configs on the Routers are as below:
Configs and Outputs when VIP is
set to nonphysical interface IP
Configs When VIP is owner IP
One more thing to remember is that VRRP uses Gratious ARP packets to advertise itself as master initially inorder to populate the host arp entries.
Also GARP packets are used by backup when it transitions from Backup to master.
The packet format is as below.
Also to understand Better , have a look at the packet captures from the lab.
VRRP Control Packet
VRRP Data Packet
