Layer 3 Redundancy
Specifying a default gateway leads to a single point of failure. Proxy Address
Resolution Protocol (ARP) is one method for hosts to dynamically discover
gateways, but it has issues in a highly-available environment. With Proxy
ARP:
■ Hosts ARP for all destinations, even remote.
■ Router responds with its MAC.
■ Problem: Slow failover because ARP entries take minutes to timeout.
Instead of making the host responsible for choosing a new gateway, Layer 3
redundancy protocols allow two or more routers to support a shared MAC
address. If the primary router is lost, the backup router assumes control of
traffic forwarded to that MAC. This section refers to routers, but includes
those Layer 3 switches that can also implement Layer 3 redundancy.