Gateway High Availability

Hi,

In this article I will explain the different techniques used for gateway high availability.

So first let’s explain what is a HA gateway ?
A high available gateway is a first-hope router (layer 3) that is available 100% (or almost) of time. This can be achieved using many strategies.

The most used, well known and reliable strategy is the first-hop redundancy where two or more routers act as a unique virtual gateway.
This strategy is divided into two techniques : Failover and Loadbalancing.

The failover technique defines a router as the master of the group and the others routers as the backups. When the master router goes down a backup router (depending on his priority) becomes the master.
The loadbalancing technique uses all the routers (all of them are masters). Every router is used, for example, in round robin manner.

The HSRP protocol (proprietary and no more supported by Cisco) and the VRRP protocol (Virtual Router Redundancy Protocol) are two protocols used for failover redundancy.

GLPB (Gateway Load Balancing Protocol) is a Cisco proprietary protocol that can be used for load balancing traffic between many gateways.

Here is a very nice cheat sheet from stretch