VRRP Process
Typically, end hosts are connected to the enterprise network through a single router (first-hop router) that is in the same Local Area Network (LAN) segment. The most popular method of configuration is for the end hosts to configure statically this router as their default gateway. This minimizes configuration and processing overhead. As shown in Figure 1-172, the problem with this configuration is that it produces a single point of failure if this first-hop router fails.
Figure 1-172: VRRP Process - First-Hop Not Reachable
The Virtual Router Redundancy Protocol attempts to solve this problem by introducing the concept of a virtual router, composed of two or more VRRP routers on the same subnet as shown in Figure 1-173. The concept of a virtual IP address is also introduced, which is the address that end hosts configure as their default gateway. One of the routers called the “Master” forwards packets on behalf of this IP address.
Figure 1-173: VRRP Process - Master and Backup VR
As shown in Figure 1-174, if the Master router fails, one of the other routers (Backup) assumes forwarding responsibility for it.
Figure 1-174: VRRP Process - Master Down and Backup Takeover
At first glance, the configuration in might not seem very useful, as it doubles the cost, and leaves one router idle at all times. This, however, can be avoided by creating two virtual routers and splitting the traffic between them.
Note: Adding a default route in the kernel on the interface that is used for VRRP might cause loss of network connectivity. According to the VRRP guidelines, when the VRRP session changes, the MAC address for the machine that attains the master state also changes. The change causes the default route from the kernel to disappear and leads to loss of connectivity. To avoid this situation, add the default route in the NSM and not in the kernel. This ensures that the default route remains on the machine across changes in the VRRP state.
To add default route through NSM, run the following command in NSM:
ip route 0.0.0.0/0 <IPADDRESS>
where <IPADDRESS> is the IP address of the default gateway.
Last modified date: 08/28/2023