1.1 Explain the different design principles used in an enterprise network

What this section tests

This section covers enterprise network design (objective 1.1). The exam asks two things. Can you compare 2-tier, 3-tier, fabric, and cloud designs? Can you say what happens when a part fails?

A clean design answers four questions:

• Where do users and endpoints attach?
• Where does Layer 2 exist?
• Where does Layer 3 routing start?
• What happens when a link, switch, supervisor, or gateway fails?

Note: Words like scalable or resilient earn no points alone. Attach each word to a design choice and a failure domain.

2-tier, 3-tier, fabric, and cloud

In a 2-tier campus, access switches connect to a collapsed core pair. The collapsed pair is the Layer 3 boundary and the gateway. In a 3-tier campus, the distribution layer holds gateways, policy, and route summarization. The core only forwards.

In a fabric, the physical network is the underlay. The overlay controls identity, segmentation, and endpoint location. In cloud designs, a provider owns part of the infrastructure. You still control routing, segmentation, and inspection at the edge.

For a deeper comparison of the four designs, read High-level enterprise network design.

Lab: high-availability baseline

Build this small design. Two distribution switches give VLAN 10 a redundant gateway. One access switch dual-homes to both.

Topology:

          [DIST1]====[DIST2]
             \      /
              [ASW1]
                |
             VLAN 10 users
             10.10.10.0/24, gateway 10.10.10.1

Do these steps:

1. Create VLAN 10 on all three switches.
2. Configure the uplinks as an EtherChannel trunk.
3. Configure the VLAN 10 interface and HSRP on each distribution switch.
4. Verify the trunk, the bundle, and the gateway.

DIST1 configuration:

vlan 10
 name USERS

interface Port-channel10
 description ACCESS-SW1 dual-uplink
 switchport mode trunk
 switchport trunk allowed vlan 10

interface Vlan10
 description USERS gateway
 ip address 10.10.10.2 255.255.255.0
 standby version 2
 standby 10 ip 10.10.10.1
 standby 10 priority 110
 standby 10 preempt

DIST2 uses the same configuration with two changes: address 10.10.10.3 and priority 100.

Verify the design, not the syntax:

show interfaces trunk
show etherchannel summary
show standby brief
show ip route 10.10.10.0

The lab passes when show standby brief shows DIST1 active and DIST2 standby. Shut down DIST1. A user ping to 10.10.10.1 must continue.

Exam traps

• A redundant diagram can still have one failure point. Check power, uplinks, and gateway state.
• A switch stack hides member failure from the access layer. The stack is still one control unit.
• A fabric does not remove routing. It changes where the network learns endpoint locations.
• Cloud does not remove design work. Failures now cross ownership boundaries.

Pass check

You are ready when you can do these things:

• Answer the four design questions for any campus diagram.
• Name the Layer 3 boundary and the gateway control point in each design.
• Prove the redundant gateway with show standby brief.