Virtual Network Tags - Cisco Catalyst 4500 Series Software Configuration Manual

About Easy Virtual Network
In addition to utilizing VRFs to provide device-level separation, each virtual network has path isolation
from the other. Path isolation is achieved by tagging the traffic so it carries the same tag value throughout
the same virtual network. Each network device along the path uses the tags to provide separation among
different VRFs. A single tag number ties VRF red, for example, on one device to VRF red on another
device.

Virtual Network Tags

Each VPN and associated EVN has a tag value that you assign during configuration. The tag value is
global, meaning that on each device, the same EVN must be assigned the same numerical tag value. Tag
values range from 2 to 4094.
An EVN is allowed on any interface that supports 802.1q encapsulation, such as Fast Ethernet, Gigabit
Ethernet, and port channels. To allow for backward compatibility with the VRF-Lite solution, the vLAN
ID field in the 802.1q frame is used to carry the virtual network tag.
Traffic that carries a virtual network tag is called tagged traffic. Traffic that does not carry a virtual
network tag is called untagged traffic.
Tags are illustrated in the following configuration with two VRFs, red and green:
! Define two VRFs, red and green.
vrf definition red
vnet tag 101
!
address-family ipv4
exit-address-family
!
vrf definition green
vnet tag 102
!
address-family ipv4
exit-address-family
!
A virtual network is defined as a VRF instance with a virtual network tag assigned.
vnet Global
A predefined EVN known as vnet global is on the device. It refers to the global routing context and it
corresponds to the default RIB. In figure 2 and figure 3, vnet global is represented by a black line
connecting devices. The vnet global carries untagged traffic. By default, interfaces belong to the vnet
global. Furthermore, vnet global is always running on trunk interfaces. The vnet global is also known as
the default routing table.
Note IPv6 traffic is supported in vnet global only.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
83-4
Chapter 83 Configuring Easy Virtual Network