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How does a Tagged VLAN work?
If the ingress filtering is enabled and when a packet is received, the VLAN
bridge will first check if the VID of the packet presents.
1). If the packet has a non-zero VID, the VLAN bridge will apply this VID as the
VLAN
ID of the packet in the network.
2). For a packet with a null tag or no VLAN tag, if the VLAN bridge provides rules to
decide its VID, then this VID is applied to the packet.
If the VLAN bridge does not support any rule for VID, then apply the PVID of
the port to the packet which came from that port. The VLAN bridge checks to see if
the ingress port and the received packet are on the same VLAN. If not, drop the
packet. If yes, forwards it to the associated ports. Meanwhile, this VLAN must be
applied to the egress port, or the packet will be dropped.
If ingress filtering is disabled, the VLAN bridge will only check the MAC
address table to see if the destination VLAN exists. If VLAN does not exist, then
drop the packet; and if both DA and VLAN do not exist, forward the packet. If VLAN
is only known to exist, then floods the packet to all the ports the VLAN covers.
If we plan to deploy four VLANs in an office and use a switch to partition
them, we need to check the ports/VLAN assignment first. Assuming a 24-port switch
is applied.
Name VID Port Members
Marketing 2 1,2,3,4,5
Service 3 6,7,20,21,22
Sales 4 8,9,10,11,12,13,14,15,16
Administration 1 17,18,19,23,24
Table 3-6
Next, assign IP address to each VLAN. Practically, we use 10.x.x.x as local
IP block. Because there are total four VLANs in the network, we must assign 4 IP
blocks to each of them.
Name VID Network Address
Marketing 2 10.1.2.0/24
Service 3 10.1.3.0/24
Sales 4 10.1.4.0/24
Administration 1 10.1.1.0/24
Table 3-7
Here we apply the subnet mask 255.255.255, and each VLAN is capable of
supporting 254 nodes.