One of the major concepts that Cisco is pushing forward in BCMSN is that creating non-hierarchical networks is a big “No, No”.
So here are a quick few points as to why we should avoid them:
Issues with poorly designed networks
- Failure domains – If Layer 2 and Layer 3 boundaries are not clearly defined, reducing the impact of a failure in one area from affecting other areas can become difficult.
- Broadcast domains – Broadcasts exist in every network and are often required by applications to function correctly. Excessive broadcasts have a detrimental impact on network performance and can be minimized by segmenting the network into a number of different broadcast domains.
- Large amounts of unknown MAC unicast traffic – Remember from the CCNA days, switches keep a record of unicast mac-addresses in the mac-address-table. If this table becomes full the switch cannot keep a record of where to send the destined frame and hence the frame will be sent out of all switch ports. This is referred to as unknown MAC unicast flooding.
- Multicast traffic on ports where not intended – Multicast is a technique by which IP traffic from one source can be propagated to a multicast group identified by a single IP and MAC destination group address pair. In a poorly designed network multicast flooding can occur similarly to unicast and broadcast flooding.
- Difficult to manage and support – Lack of documentation, poorly designed network flows, time consuming problem resolution.
- Possible security vulnerabilities – Poorly designed security requirements at the access layer can compromise security.
Implementing a hierarchical addressing scheme addresses some of the issues with poorly designed networks. A hierarchical addressing scheme is a means of assigning IP addresses to network segments and VLANs in an orderly fashion whereby the entire network is considered.
Benefits of Hierarchical Addressing
- IP addresses are assigned on contiguous blocks. This leads to more efficient troubleshooting because devices are easier to locate.
- There is the reduced possibility of duplicate IP address assignment
- Routing protocols can perform route summarisation to reduce routing tables. This results in reduced CPU and memory requirements on routers, faster convergence times and easier troubleshooting
Guidelines for Hierarchical Addressing
- Design the IP addressing scheme so that blocks of 4, 8, 16, 32, 64 contiguous network numbers can be assigned to a switch block. This allows route summarisation to occur
- Assign network numbers contiguously from the distribution layer to the access layer
- Assign each VLAN a single IP subnet thereby creating a separate broadcast domain for each VLAN
- Subnet at the same binary value on all network numbers, avoiding VLSM where possible. This reduces confusion and eases troubleshooting.
Hierarchical Addressing Scheme Example
Company XYZ has 1,000 employees and the company can be divided into six separate business communities. According to Cisco’s ECNM each business unit should be assigned its own VLAN and each VLAN should be allocated a different IP address range.
The largest department at Company XYZ is the finance department with 150 employees. Therefore a subnet mask of 255.255.255.0 or /24 is chosen giving a maximum of 254 hosts per network.
The six VLANs that will be created for company XYZ are as follows:
- Sales Department – VLAN 10 – Building A
- IT Department – VLAN 20 – Building A
- Engineering Department – VLAN 30 – Building B
- Marketing Department – VLAN 40 – Building B
- Executive Department – VLAN 50 – Building C
- Finance Department – VLAN 60 – Building C
Due to the building design of Company XYZ it has been decided that each building block will be assigned addresses accordingly to accommodate future growth:
- Building A – 10.0.0.0/16
- Building B – 10.1.0.0/16
- Building C – 10.2.0.0/16
The allocation of VLANs and IP subnets for Building A will be:
- Sales Department – VLAN 10 – 10.0.0.0/24
- IT Department – VLAN 20 – 10.0.1.0/24
- Spare Addresses – 10.0.2.0 – 10.0.255.0
The allocation of VLANs and IP subnets for Building B will be:
- Engineering Department – VLAN 30 – 10.1.0.0/24
- Marketing Department – VLAN 40 – 10.1.1.0/24
- Spare Addresses – 10.1.2.0 – 10.1.255.0
The allocation of VLANs and IP subnets for Building C will be:
- Executive Department – VLAN 50 – 10.2.0.0/24
- Finance Department – VLAN 60 – 10.2.2.1/24
- Spare Addresses – 10.2.2.0 – 10.2.255.0
Company XYZ – Hierarchical Network Diagram

Once again notes from Cisco.com and image is a modified image from Cisco.NetAcad.Net
Deacon…