TCP/IP Tutorial and Technical Overview - IBM Redbooks

Figure 24-4 on page 918 shows a configuration with two virtual routers with the
hosts splitting their traffic between them. This example is expected to be very
common in actual practice. In this configuration, half of the hosts install a default
route to virtual router #1 (IP address of 9.180.20.3), and the other half of the
hosts install a default route to virtual router #2 (IP address o f 9.180.20.4). This
has the effect of load balancing the traffic from the hosts through the routers,
while also providing full redundancy.
24.6.5 VRRP packet format
The purpose of the VRRP packet is to communicate to all VRRP routers the
priority and the state of the master router associated with the virtual router ID.
VRRP packets are sent encapsulated in IP packets. They are sent to the IPv4
multicast address assigned to VRRP. The IP address, as assigned by the IANA
for VRRP, is 224.0.0.18. This is a link local scope multicast address. Routers
must not forward a datagram with this destination address regardless of its TTL
(see 3.1, “Internet Protocol (IP)” on page 68). The TTL must be set to 255. A
VRRP router receiving a packet with the TTL not equal to 255 must discard the
packet. Figure 24-5 shows the VRRP packet format.
0 4 8 16 24 31
vers type
Figure 24-5 VRRP packet format
virtual
router ID
priority
auth type advert int checksum
IP address (1)
. . .
authentication data (1)
authentication data (2)
count IP
addrs
Chapter 24. Availability, scalability, and load balancing 919