Hot Standby Router Protocol HSRP Tutorial

This tutorial covers Hot Standby Router Protocol (HSRP) — what it is, and why a network needs it.

Most companies connect to the Internet through a setup about as simple as this:

Getting a topology like this working just takes two things:

• Assign IP addresses to the router’s two interfaces — say, Fa0/0 (the interface facing the switch) gets 192.168.1.1.
• Assign IP addresses, default gateways, and DNS servers to every PC — with the default gateway pointing at the router’s Fa0/0 address, 192.168.1.1. This can be set manually or handed out automatically via DHCP.

Sooner or later, though, someone will want redundancy built in — so that if the router fails, every PC can keep reaching the Internet without anyone having to touch their configuration. That means adding a second router:

But now there’s a new problem: each host can only have one default gateway configured. If Router1 goes down and traffic needs to flow through Router2 instead, every host’s default gateway would need to be manually changed to 192.168.1.2 — and changed right back once Router1 comes back online. During that switch, nobody can reach the Internet at all. This is exactly the problem HSRP solves.