How VRRP Is Implemented in E-series Routers

VRRP is implemented in E-series routers to meet two goals. The first goal is to avoid the single point of failure inherent to hosts that have a single default gateway configured. The second goal is to keep the complexity of redundancy away from the hosts themselves. These goals comply with RFC 2338 and RFC 2787.

The association between VRIDs and IP addresses is coordinated among all participating VRRP routers. The following scenario can help you understand how VRRP is implemented in the router.

1. An E-series router assigns common VRIDs to the group of routers that are going to share IP addresses.
2. The E-series router sends VRRP advertisements to well-known multicast addresses. The router that owns the addresses automatically becomes the master and sends periodic VRRP advertisement messages. A VRRP advertisement consists of the IP addresses that the master router controls and the VRID.
3. If the master router stops advertising for a predetermined period of time, the remaining routers using the same VRID enter an election process to determine which router takes over the master router responsibilities.
4. Depending on the configuration, the master router that does not own the IP addresses might do one of the following:
   • Drop all packets that have destination addresses to these IP addresses (default)
   • Accept packets that have destination addresses to these IP addresses as if the addresses belonged to the master router (using the ip vrrp accept-data command).
5. If the elected master router fails, backup routers start the election process again.
6. When the original master router becomes operational again, it restarts broadcasting advertisements as long as preemption is enabled or the master router is the address owner. Packet forwarding responsibility then shifts back to the original master router.

Router Election Rules

If the master router becomes unavailable, the following rules govern election of the master router:

• The backup router assigned the highest priority for each VRID becomes the master router.
• If two backup routers were assigned the same priority, the router that has the highest primary address becomes the master router. For example, if several routers were all assigned the default priority of 100, the IP addresses must be compared.
• Router election on a VRRP router can also be determined by whether the preemption option is enabled.

  When a backup router detects a master router with a lower priority than the backup router has, the backup router might leave the current master router alone or take over the current master router and become the master router itself.

  When preemption is enabled, a backup router always preempts or takes over the responsibility of the master router. When preemption is disabled, the lower-priority backup is left in the master state.

  Note: Using VRRP can override the source address of the ICMP redirect. When a backup VRID functions as a master router on a given IP interface, its ICMP redirects must fake the source IP address of the IP address owner. The redirect must fake the IP address because hosts accept only an ICMP redirect that is sent by the current gateway of the host.

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