65
Switch(config)# user radius server1
192.180.3.1
Set the primary RADIUS server address to
192.180.3.1.
Switch(config)# user radius server2
192.180.3.2
Set the secondary RADIUS server address
to 192.180.3.2.
2.6.23 VLAN Command
A Virtual Local Area Network (VLAN) is a network topology configured according to a logical
scheme rather than the physical layout. VLAN can be used to combine any collections of LAN
segments into a group that appears as a single LAN. VLAN also logically segments the network
into different broadcast domains. All broadcast, multicast, and unknown packets entering the
Switch on a particular VLAN will only be forwarded to the stations or ports that are members of
that VLAN.
VLAN can enhance performance by conserving bandwidth and improve security by limiting traffic
to specific domains. A VLAN is a collection of end nodes grouped by logics instead of physical
locations. End nodes that frequently communicate with each other are assigned to the same VLAN,
no matter where they are physically located on the network. Another benefit of VLAN is that you
can change the network topology without physically moving stations or changing cable
connections. Stations can be ‘moved’ to another VLAN and thus communicate with its members
and share its resources, simply by changing the port VLAN settings from one VLAN to another.
This allows VLAN to accommodate network moves, changes and additions with the greatest
flexibility.
802.1Q VLAN Concept
Port-Based VLAN is simple to implement and use, but it cannot be deployed cross switches VLAN.
The 802.1Q protocol was developed in order to provide the solution to this problem. By tagging
VLAN membership information to Ethernet frames, the IEEE 802.1Q can help network
administrators break large switched networks into smaller segments so that broadcast and
multicast traffic will not occupy too much available bandwidth as well as provide a higher level
security between segments of internal networks.
Introduction to 802.1Q frame format:
Preamble
SFD
DA
SA
Type/LEN
PAYLOAD
FCS
Original frame
Preamble
SFD
DA
SA
TAG
TCI/P/C/VID
Type/LEN
PAYLOAD FCS
802.1q
frame
PRE Preamble
62 bits
Used to synchronize traffic
SFD Start Frame Delimiter
2 bits
Marks the beginning of the header
DA Destination Address
6 bytes
The MAC address of the destination
SA Source Address
6 bytes
The MAC address of the source
TCI Tag Control Info
2 bytes set to 8100 for 802.1p and Q tags
P Priority
3 bits
Indicates 802.1p priority level 0-7
C Canonical Indicator
1 bit
Indicates if the MAC addresses are in
Canonical format - Ethernet set to "0"
VID VLAN Identifier
12 bits
Indicates the VLAN (0-4095)
T/L Type/Length Field
2 bytes
Ethernet II "type" or 802.3 "length"
Payload < or = 1500 bytes User data
FCS Frame Check Sequence
4 bytes
Cyclical Redundancy Check
