15.1.1 MAC and IP addresses of the virtual routers

A virtual router has a virtual MAC address. When a physical router running VRRP (VRRP router) operates as the master of a virtual router, it uses the virtual MAC address instead of its own physical MAC address. A virtual MAC address is automatically generated from the virtual router identifier.

The following table describes the supported VRRP specifications and corresponding virtual MAC addresses.

Table 15-1 VRRP standards and virtual MAC addressing

Standards		Virtual MAC address
IPv4	RFC 3768	0000.5e00.01 {virtual-router-ID}
IPv6	draft-ietf-vrrp-ipv6-spec-02	0000.5e00.01 {virtual-router-ID}
	draft-ietf-vrrp-ipv6-spec-07	0000.5e00.02 {virtual-router-ID}

When the master router receives an Ethernet frame for the virtual MAC address, it forwards the packet. Backup routers do not receive frames for the virtual MAC address. VRRP selects the virtual router that receives Ethernet frames for the virtual MAC address based on the status of the virtual routers. When the master router receives a frame for the virtual MAC address, it forwards the IP packet based on its routing table. Because terminals send frames to the virtual MAC address, they can continue communication even if the master and backup routers are switched over. The following figure shows how a frame for the virtual MAC address is received.

Figure 15-1: Receiving frames addressed to a virtual MAC

A virtual router also has a virtual IP address. When the master router receives an ARP request packet or an NDP request packet sent to the virtual IP address, it always uses the virtual MAC address to send an ARP reply or NDP reply. The following figure shows an example of an ARP reply and NDP reply with a virtual MAC address.

Figure 15-2: ARP replies and NDP replies by virtual MAC addressing

A host (such as a PC) that uses a virtual router as its default router receives the virtual MAC address when it receives an ARP reply from the virtual router. When the host obtains the virtual MAC address, it records the virtual IP address-virtual MAC address combination in its ARP cache. Thereafter, the host always specifies the virtual MAC address as the destination when it sends frames to the virtual router. This way, the host can continue communication even if the VRRP master and backup routers are switched over.