CCNA RnS Article #24: VLAN Tagging

In this article, we are going to talk about VLAN tagging. In a one switch environment, things are simple. You need to just configure the interface VLAN assignment, and you are good. But when we talk about a large campus network, we need VLAN tagging over the trunk connection (the connection between two switches). It is suggested you to please take 3 minutes to read the last article to well connect with this VLAN Tagging topic.

When we configure multiple VLANs in a multi-switch environment, we need to configure the trunking interface between the switches. Trunk configuration between switches requires VLAN tagging over this connection. It is required to tell the remote switch which VLAN a frame belongs to. When a host sends a frame to the switch [local], this switch adds additional information (VLAN ID field) to the frame and sends over it to the trunk interface. Receiving switch [remote] reads the tagging information and comes to know which VLAN should receive this traffic.

There are two different tagging protocols that exist –

Inter-Switch Link (ISL)
IEEE 802.1Q (Dot 1Q)

Both are supported by Cisco switches. However, IEEE 802.1Q (.1q) is the standard protocol and is supported by all OEM switches. In addition to the proprietary [ISL is Cisco proprietary protocol, only supported by Cisco switches] and standard protocol, there is another difference in how the frame is tagged or how the VLAN information is associated with the frame.

The IEEE 802.1Q protocol inserts an extra 4-byte 802.1Q VLAN header into the original frame’s Ethernet header. After adding the additional header information, the original frame looks like this –

Figure 1: 802.1Q Trunking

Out of this 4-byte additional information, only 12-bits are used for VLAN identification. It means it can support VLAN range from 0 – 4094 (2¹² = 4096 [0 and 4095 both are reserved]).

Cisco has two ranges for VLANs – the normal range and the extended range. All switches use a normal range from 1 – 1005. Advanced switches support extended-range VLAN IDs from 1006 to 4094.

Native VLAN

IEEE 802.1Q also defines a special VLAN ID on the trunk port that is known as Native VLAN. Frames that originated in the native VLANs are not tagged – I mean when a host that is part of the native VLAN, switches while sending a frame over the trunk port, is not tagged [no VLAN information is shared with receiving switch].

At the receiving switch end, as it is not tagged switch assumes it is part of Native VLAN. That’s the reason when configuring a trunk port between switches both should be configured with the same native VLAN ID.

One of the reasons behind using native VLAN is to make connections to the switches that do not understand the 802.1Q trunk. This means two switches can be connected and has the capability to send at least one VLAN traffic [native VLAN] over the trunk connection. That’s the reason network administrators use Native VLAN for network management subnet – so that they can remotely manage devices that are just a kind of hub or dump devices.

ISL vs IEEE 802.1Q

Feature	ISL	Dot 1Q
Standard	Cisco proprietary	IEEE standard/industry standard
VLAN Support	Up to 1000 VLANs	Up to 4096 VLANs
VLAN ID information	Encapsulate original frame	Add additional field to original frame
Processing	More intensive as new header and FCS to be checked	Less intensive as no change in original header or FCS
Configuration Example	Switch(config)#interface Fa1/1 Switch(config-if)#switchport trunk encapsulation isl Switch(config-if)#switchport mode trunk	Switch(config)#interface Fa 1/1 Switch(config-if)#switchport trunk encapsulation 802.1q Switch(config-if)#switchport mode trunk
Supported	Supported only on Cisco – legacy switches only	All Cisco and non-Cisco switches

I hope you find this informative!

Continue Reading...

Latest

CCNA RnS Article #24: VLAN Tagging