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DES-3010F/G / DES-3018 / DES-3026 Fast Ethernet Switch Manual

Ingress Filtering

A port on a switch where packets are flowing into the Switch and VLAN decisions must be made is referred to as an

ingress port. If ingress filtering is enabled for a port, the Switch will examine the VLAN information in the packet header

(if present) and decide whether or not to forward the packet.

If the packet is tagged with VLAN information, the ingress port will first determine if the ingress port itself is a member of

the tagged VLAN. If it is not, the packet will be dropped. If the ingress port is a member of the 802.1Q VLAN, the Switch

then determines if the destination port is a member of the 802.1Q VLAN. If it is not, the packet is dropped. If the

destination port is a member of the 802.1Q VLAN, the packet is forwarded and the destination port transmits it to its

attached network segment.

If the packet is not tagged with VLAN information, the ingress port will tag the packet with its own PVID as a VID (if the

port is a tagging port). The switch then determines if the destination port is a member of the same VLAN (has the same

VID) as the ingress port. If it does not, the packet is dropped. If it has the same VID, the packet is forwarded and the

destination port transmits it on its attached network segment.

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 7- 1. VLAN Example - Assigned Ports

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.

Network resources such as printers and servers however, 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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