Introduction to VLAN Aggregation

Definition

VLAN aggregation, also called super-VLAN, partitions a broadcast domain into multiple VLANs (sub-VLANs) on a physical network and aggregates the sub-VLANs into a single logical VLAN (super-VLAN). The sub-VLANs use the same IP subnet and default gateway address, so the number of IP addresses used is reduced.

Purpose

VLAN technology is commonly used on packet switching networks because it can flexibly control broadcast domains and is easy to deploy. Usually, a Layer 3 switch uses a Layer 3 logical interface in each VLAN to allow hosts in different broadcast domains to communicate. This wastes IP addresses. On a subnet corresponding to a VLAN, the subnet ID, directed broadcast address, and subnet default gateway address all cannot be used as IP addresses of hosts in the VLAN. In addition, IP addresses available in a subnet may exceed the number of hosts. These excess IP addresses cannot be used by other VLANs.

In Figure 5-1, VLAN 2 requires 10 host addresses. The subnet 10.1.1.0/28 with a 28-bit mask is assigned to VLAN 2, where 10.1.1.0 is the subnet ID, 10.1.1.15 is the directed broadcast address, and 10.1.1.1 is the default gateway address. Hosts cannot use these three addresses, but the other 13 addresses ranging from 10.1.1.2 to 10.1.1.14 are available to them.

At least three IP addresses are wasted for VLAN 2, and at least nine IP addresses are wasted for three VLANs. Although VLAN 2 requires only 10 IP addresses, the remaining 3 IP addresses cannot be used by other VLANs and are wasted. If more VLANs are added, the problem is exacerbated.

Figure 5-1  Networking of a common VLAN

VLAN aggregation is used to solve the preceding problem. VLAN aggregation maps each sub-VLAN to a broadcast domain, associates a super-VLAN with multiple sub-VLANs, and then assigns just one IP subnet to the super-VLAN. This ensures that all sub-VLANs use the IP address of the associated super-VLAN as the gateway IP address to implement Layer 3 connectivity.

Sub-VLANs share one gateway address to reduce the number of subnet IDs, subnet default gateway addresses, and directed broadcast IP addresses used is reduced. The switch assigns IP addresses to hosts in sub-VLANs according to the number of hosts. This ensures that each sub-VLAN acts as an independent broadcast domain, conserves IP addresses, and implements flexible addressing.