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Overlapping subnet

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

An overlapping subnet is a subnet whose address range overlaps or duplicates the range used by another interface or network.

An overlapping subnet shares part or all of its address range with another interface or network. On a normal router, IOS rejects overlapping interface subnets because interfaces must connect different networks. section 17.2

Virtual Routing and Forwarding (VRF) removes this limitation by isolating interfaces into separate virtual routers:

  • Interfaces in different VRFs may use overlapping subnets or identical IP addresses. section 17.2
  • Assign an interface with ip vrf forwarding vrf-name; IOS removes any existing IPv4 address, which must then be reconfigured. section 17.2
  • Verify each routing table with show ip route vrf vrf-name; show ip route alone does not display the VRF routes. section 17.2

IOS rejects an overlapping interface subnet

Configuring 192.168.1.1/30 on G0/0 and again on G0/2 produces “% 192.168.1.0 overlaps with GigabitEthernet0/0.” Placing the interfaces in different VRFs permits the same addressing scheme. section 17.2

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Demonstrates rejection, VRF assignment, address reconfiguration, route-table verification, and VRF-specific ping testing.
Examples
Uses repeated 192.168.1.0/30 interfaces, customer VRFs, show ip route vrf, and VRF-qualified ping commands.
What it assumes
Assumes familiarity with router interfaces, IPv4 addresses, subnets, and basic Cisco IOS configuration.
What you can do after
Configure and verify overlapping customer subnets through Cisco IOS VRF assignment and VRF-specific routing commands.
How it connects
Connects overlapping subnets with VRF, interfaces, IP addressing, routing tables, and connected and local routes.
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