Loopback Interface - Switch, Router management

[cisco_lite]

I have created a management Vlan. And I would like to access all the devices being the part of same subnet. I would like to understand how does it work?

If I telnet Cat6500 on loopback ip from the same subnet, how do the packets travel to loopback ? Is it through other ethernet for e.g. physical interface. So if the ethernet interface was down how would I reach the loopback.

And if the loopback ip is multiple hops away from the remote connecter, would the routes towards loopback interface need to be advertised.

Just interested in knowing how does loopback work for management purpose.

[cisco_lite]

If I am in network1 i.e. 30.1.1.0 /24 subnet which is the management VLAN. Now the router ethernet is outside this network i.e. network2 with 40.1.1.0 /24. If I create a loopback address on the router such as 30.1.1.2 how can I access it from network1.

Because network1 will assume that 30.1.1.2 should be available on the same VLAN. And it will not pass it on to the next hop if /32 route is defined for 30.1.1.2.

[Jon Marshall]

I can see the confusion.

If the device your are trying to manage is separated by a number of L3 hops then you cannot use the same subnet in 2 different locations because as you quite rightly point out this can't work. This is why i wrote this -

"But if you are in the same building or campus then advertising the subnet may not work and you may indeed to advertise each loopback as a /32"

What i meant by this is exactly what you are referring to ie. you may not be able to advertise as a subnet. Where i was talking about advertising a subnet was in the following example

Site 1 is your location

Site 2 is a remote site that is routed to from site 1.

So within Site 2 you can use a /24 or /25 etc.. subnet for all loopbacks. But within that site if you wanted all devices out of the same subnet they would need L2 adjacency precisely because of what you pointed out. Doesn't matter if you advertised them within that site as /24 addresses or /32's, they still need L2 adjacency if they are out of the same subnet.

But from outside the site you can advertise as a /24, /25 etc.. and route across your WAN to that site.

Hope that's cleared up some of the confusion.

Jon