I'll disagree with dan here.
There are 2 reasons to have your management network segregated from your production network.
- security. Typically you pass traffic over the mgmt network that's a bit more sensitive and trusted than the traffic in the production network. I'm not 100% convinced this is a sufficient reason to set up a management network. Say, for instance, if you're doing SNMPv2, it helps to have this stuff on a totally independent link / network.
- your management network shouldn't be routed and you should have out-of-band access to it. That way, if you do something to your production network that causes it to not work or work very poorly, you can still access all of your devices. Very handy if you screw up a routerID or a firewall rule or are dealing with a broadcast storm...
In my opinion, the reliability aspect of the management network (reason 2) is the primary justification.