Huawei AR550 Series - Page 98

Table 3-4 Example for assigning Host addresses in VLAN aggregation mode
VLA
N
Subnet Gateway
address
Number of
available
addresses
Number of
available hosts
Practical
requirements
2 10.1.1.0/24 10.1.1.1 10 10.1.1.2-10.1.1.11 10
3 5 10.1.1.12-10.1.1.1
6
5
4 1 10.1.1.17 1
In VLAN aggregation implementation, sub-VLANs are not divided according to the previous
subnet border. Instead, their addresses are flexibly assigned in the subnet corresponding to the
super-VLAN according to the required host number.
As the Table 3-4 shows that VLAN 2, VLAN 3, and VLAN 4 share a subnet (10.1.1.0/24), a
default gateway address of the subnet (10.1.1.1), and a directed broadcast address of the subnet
(10.1.1.255). In this manner, the subnet ID (10.1.1.16, 10.1.1.24), the default gateway of the
subnet (10.1.1.17, 10.1.1.25), and the directed broadcast address of the subnet (10.1.1.15,
10.1.1.23, and 10.1.1.27) can be used as IP addresses of hosts.
Totally, 16 addresses (10 + 5 + 1 = 16) are required for the three VLANs. In practice, in this
subnet, a total of 16 addresses are assigned to the three VLANs (10.1.1.2 to 10.1.1.17). A total
of 19 IP addresses are used, that is, the 16 host addresses together with the subnet ID (10.1.1.0),
the default gateway of the subnet (10.1.1.1), and the directed broadcast address of the subnet
(10.1.1.255). In the network segment, 236 addresses (255 - 19 = 236) are available, which can
be used by any host in the sub-VLAN.
Communications Between VLANs
l Introduction
VLAN aggregation ensures that different VLANs use the IP addresses in the same subnet
segment. This, however, leads to the problem of Layer 3 forwarding between sub-VLANs.
In common VLAN mode, the hosts of different VLANs can communicate with each other
based on the Layer 3 forwarding through their respective gateways. In VLAN aggregation
mode, the hosts in a super-VLAN uses the IP addresses in the same network segment and
share the same gateway address. The hosts in different sub-VLANs belong to the same
subnet. Therefore, they communicate with each other based on the Layer 2 forwarding,
rather than the Layer 3 forwarding through a gateway. In practice, hosts in different sub-
VLANs are separated in Layer 2. As a result, sub-VLANs fails to communicate with each
other.
To solve the preceding problem, you can use Proxy ARP.
NOTE
For details of Proxy ARP , refer to the chapter ARP in the IP Services.
l Layer 3 Communications Between Different Sub-VLANs
As shown in Figure 3-12, the super-VLAN, namely, VLAN 10, contains the sub-VLANs,
namely, VLAN 2 and VLAN 3.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
3 VLAN Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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