how does IP addressing work for routers with redundant connections

[red red]

I'm trying to build the following topology in GNS3 using HSRP (the switches are unmanaged):

I created this topology without enabling R3's f0/1 and R4's f0/0 interfaces and it works, but I'm confused about what IP addresses I should assign these 2 interfaces. A router can't have 2 interfaces in the same segment right? How is this usually handled? I always see diagrams like this, but I'm confused about how the addressing in this kind of setup would work.

[vmiller]

A router can have multiple addresses in the same subnet.

Start digging in on the following :

First Hop Redundancy Protocol (FHRP)

http://www.cisco.com/c/en/us/products/ios-nx-os-software/first-hop-redundancy-protocol-fhrp/index.html

that should get you sufficient links to various FHRP's available.

[vmiller]

 

i should have been more explicit. as in the example above.

[Hassan Chalabi]

A router cannot have multiple addresses in the same subnet, it will rejected, however, if you are trying to setup FHRP, you will need to create virtual gateway, that gateway should be configured in the interface level that has to have an address in the same subnet of physical interface or sub-interface. Hope this helps.

[Peter Paluch · Accepted solution]

Hi,

In addition to other friends' responses here, the topologies you are often seeing use different devices: In place of routers, there are multilayer switches capable of both routing and switching, and in place of Sw2 and Sw1, there are managed access layer switches, usually not configured for routing themselves. The multilayer switches would indeed be connected to the access layer switches in a redundant fashion, however, the links are not routed but rather switched - they belong to the same VLAN or they are all trunks - and they do not have their own IP addresses. Therefore, the problem of a router having multiple routed interfaces in the same network never occurs. A multilayer switch would have just a single routed interface called interface Vlan vlan-id configured for a particular VLAN, and that's it.

Best regards,
Peter