A default gateway plays a crucial role in enabling communication between devices on different networks or subnets. When a device needs to communicate with a destination outside of its network, it sends the data packet to the default gateway, which then forwards it to the appropriate network.
If there is only one default gateway and it fails, all traffic will be unable to leave or enter the network. This can result in downtime and lost productivity. With default gateway redundancy, multiple default gateways are configured on a network, and they work together to ensure that traffic can continue to flow even if one of them fails.
Additionally, default gateway redundancy can also improve network performance by balancing the traffic load across multiple gateways, resulting in faster and more efficient traffic flow. Overall, default gateway redundancy is an important aspect of network design and can help ensure the high availability and reliability of network services.
However, redundant default gateways alone do not guarantee failover. Let's look at the image above. The topology consists of two routers, R1 and R2, both aware of the local network where PC1 belongs, and how to reach the network's core network and the internet.
R1 operates as the primary default gateway for the endpoints, so when PC1 needs to send data outside its local network, it sends it to R1. However, if R1 fails, all traffic cannot leave the network. The most interesting part is that, even though there is a redundant link through R2, it cannot be used, and the reason for that behavior is quite simple.
Each endpoint is configured with a single default gateway, and this information does not dynamically update when the network topology changes. Because there is no dynamic method for devices to update the default gateway IP, they stay isolated from the rest of the network, even though a redundant device like R2 in the example is available and can take over the role of the default gateway when properly configured.
For that reason, default gateway redundancy is extremely important. When implemented on Layer 3 devices that should function as default gateways in the network, it provides network redundancy and ensures that data traffic continues to flow in the event of a failure in one of the default gateways.