Troubleshooting Gateway Redundancy Protocols
Gateway redundancy

A network with High Availability provides alternate means by which all infrastructure paths and key servers can be accessed at all times. The Hot Standby Routing Protocol is one of those software features configured to provide Layer 3 redundancy to network hosts. HSRP optimization provides immediate or link specific failover as well as a recovery mechanism. Virtual Router Redundancy Protocol and Gateway Load Balancing Protocol are derivatives of HSRP providing additional Layer 3 redundancy features such as load balancing.

When router redundancy is configured, a virtual router is created. The IP address of the virtual router will be configured as the default gateway for the workstations on a specific IP segment. When frames are to be sent from the workstation to the default gateway, the workstation will use ARP to resolve the MAC address associated with the IP address of the default gateway. The ARP resolution will return the MAC address of the virtual router. Frames sent to the MAC address of the virtual router can then be physically processed by any active or standby router that is part of that virtual router group.

A protocol is used to identify two or more routers as the devices responsible for processing frames sent to the Mac or IP address of a single virtual router. Host devices send traffic to the address of the virtual router. The physical router that handles forwarding this traffic is transparent to the end stations. The redundancy protocol provides the mechanism for determining which router should take the active role in forwarding traffic, and for determining when that role must be taken over by a standby router. The transition from one forwarding router to another is transparent to the end devices.