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xStack DES-6500 Modular Layer 3 Chassis Ethernet Switch User Manual
This process is referred to as ingress filtering and is used to conserve bandwidth within the Switch by dropping packets that are
not on the same VLAN as the ingress port at the point of reception. This eliminates the subsequent processing of packets that will
just be dropped by the destination port.
Default VLANs
The Switch initially configures one VLAN, VID = 1, called "default." The factory default setting assigns all ports on the Switch to
the "default." As new VLANs are configured in Port-based mode, their respective member ports are removed from the "default."
Packets cannot cross VLANs. If a member of one VLAN wants to connect to another VLAN, the link must be through an external
router.
NOTE: If no VLANs are configured on the Switch, then all packets will be forwarded to
any destination port. Packets with unknown source addresses will be flooded to all
ports. Broadcast and multicast packets will also be flooded to all ports.
An example is presented below:
VLAN Name VID Switch Ports
System (default) 1 5, 6, 7, 8, 21, 22, 23, 24
Engineering 2 9, 10, 11, 12
Marketing 3 13, 14, 15, 16
Finance 4 17, 18, 19, 20
Sales 5 1, 2, 3, 4
Table 6- 2. VLAN Example - Assigned Ports
Port-based VLANs
Port-based VLANs limit traffic that flows into and out of switch ports. Thus, all devices connected to a port are members of the
VLAN(s) the port belongs to, whether there is a single computer directly connected to a switch, or an entire department.
On port-based VLANs, NICs do not need to be able to identify 802.1Q tags in packet headers. NICs send and receive normal
Ethernet packets. If the packet's destination lies on the same segment, communications take place using normal Ethernet protocols.
Even though this is always the case, when the destination for a packet lies on another switch port, VLAN considerations come into
play to decide if the packet is dropped by the Switch or delivered.
VLAN Segmentation
Take for example a packet that is transmitted by a machine on Port 1 that is a member of VLAN 2. If the destination lies on
another port (found through a normal forwarding table lookup), the Switch then looks to see if the other port (Port 10) is a member
of VLAN 2 (and can therefore receive VLAN 2 packets). If Port 10 is not a member of VLAN 2, then the packet will be dropped
by the Switch and will not reach its destination. If Port 10 is a member of VLAN 2, the packet will go through. This selective
forwarding feature based on VLAN criteria is how VLANs segment networks. The key point being that Port 1 will only transmit
on VLAN 2.
Certain network resources can be shared across VLANs. This is achieved by setting up overlapping VLANs. That is ports can
belong to more than one VLAN group. For example, setting VLAN 1 members to ports 1, 2, 3, and 4 and VLAN 2 members to
ports 1, 5, 6, and 7. Port 1 belongs to two VLAN groups. Ports 8, 9, and 10 are not configured to any VLAN group. This means
ports 8, 9, and 10 are in the same VLAN group.
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