Transcript Configuring InterVLAN Routing

Configuring InterVlan Routing
Presented By
Brian, Kevin, and John
Understanding How InterVLAN
Routing Works
Network devices in different VLANs cannot
communicate with one another without a
router to route traffic between the VLANs.
In most network environments, VLANs are
associated with individual networks or
subnetworks.
Configuring VLANs helps control the size
of the broadcast domain and keeps local
traffic local. However, when an end station
in one VLAN needs to communicate with
an end station in another VLAN,
interVLAN communication is required. This
communication is supported by interVLAN
routing. You configure one or more routers
to route traffic to the appropriate
destination VLAN.
However, the real power of virtual networking
comes from its ability to affect VLAN topologies
that extend beyond single sites to combine
multiple LANs across an organization's
backbone network. Cisco Systems now offers a
comprehensive VLAN solution that can bring
together geographically dispersed users across
an enterprise network to form VLAN workgroup
topologies. Regardless of whether the network
comprises Asynchronous Transfer Mode (ATM),
Fiber Distributed Data Interface (FDDI),
Ethernet/Fast Ethernet, Token Ring, or serial
links, the Cisco product line now offers the
advantages of virtualization.
The degree of flexibility and control that
virtual networking offers is unprecedented.
Regardless of physical location or
interface type, network managers can
define workgroups based on logical
function rather than physical location
through simple port configuration. Using
switches and routers that have embedded
VLAN intelligence obviates the need for
expensive, time-consuming recabling to
extend connectivity in switched LAN
environments.
New Cisco IOS VLAN Services
Make "Virtual" a Reality
Virtual networking has rapidly become one of the
major new areas in the internetworking industry.
Virtual networking refers to the ability of switches
and routers to configure logical topologies on top
of the physical network infrastructure, allowing
any arbitrary collection of LAN segments within a
network to be combined into an autonomous
user group, appearing as a single LAN.
Virtual LANs (VLANs) offer significant benefits in terms of efficient
use of bandwidth, flexibility, performance, and security. VLAN
technology functions by logically segmenting the network into
different broadcast domains so that packets are only switched
between ports that are designated for the same VLAN. Thus, by
containing traffic originating on a particular LAN only to other LANs
within the same VLAN, switched virtual networks avoid wasting
bandwidth, a drawback inherent in traditional bridged/switched
networks where packets are often forwarded to LANs that do not
require them. This approach also improves scalability, particularly in
LAN environments that support broadcast- or multicast-intensive
protocols and applications that flood packets throughout the
network. Figure 1 depicts a typical VLAN, where traffic is only
switched between LAN interfaces that belong to the same VLAN.
Here, the criteria for VLAN membership is departmental function;
however, users could also be combined in VLAN topologies based
upon a common protocol or subnet address.
• Figure 1: A Typical VLAN
Understanding VLANs
A VLAN is a switched network that is logically segmented
by function, project team, or application, without regard
to the physical locations of the users. VLANs have the
same attributes as physical LANs, but you can group
end stations even if they are not physically located on
the same LAN segment. Any switch port can belong to a
VLAN, and unicast, broadcast, and multicast packets are
forwarded and flooded only to end stations in the VLAN.
Each VLAN is considered a logical network, and packets
destined for stations that do not belong to the VLAN
must be forwarded through a router or bridge.
Because a VLAN is considered a separate
logical network, it contains its own bridge
Management Information Base (MIB)
information and can support its own
implementation of spanning tree.
Understanding How InterVLAN
Routing Works
Network devices in different VLANs cannot
communicate with one another without a
router to route traffic between the VLANs.
In most network environments, VLANs are
associated with individual networks or
subnetworks.
Subnets and VLANs
Cisco recommends that you maintain a one-toone relationship between subnets and VLANs.
This means that all stations residing in or ports
configured on the same VLAN are assigned
network addresses with the same subnet.
If you wish to configure your VLAN differently
from the existing subnets, you must reassign the
IP addresses on the subnets to match your
intended VLAN configuration.
In order to create VLANs, you must
decide how to configure the
following items:
• What VLAN Trunking Protocol (VTP) domain
name and VTP mode will be used on this
switch?
• What ports on the switch will belong to which
VLAN?
• Will you need to have communication between
VLANs, or will they be isolated? If you require
communication between VLANs, you will need to
use a L3 routing device, such as an external
Cisco router or an internal router module such
as a Route Switch Module (RSM) or a Multilayer
Switch Feature Card (MSFC).
Recording the Plan
The table should contain the following
information:
• VLAN name
• Switch type, name, slot, port number and
port type of the proposed VLAN
• Subnet of each VLAN assignment
• Location where you plan to connect a
router(s)
• User name and user location
Number of VLANs and Users
• The maximum number of users that you
can define per known network is 1000.
• Cisco recommends that a VLAN contain
no more than 150 to 200 users.
Maximum Number of Supported
VLANs
Switch Model
•
•
•
•
•
•
•
•
Catalyst 2950-12
Catalyst 2950-24
Catalyst 2950C-24
Catalyst 2950G-12-EI
Catalyst 2950G-24-EI
Catalyst 2950G-48-EI
Catalyst 2950G-24-EI-DC
Catalyst 2950T-24
Number of Supported
VLANs
64
64
250
250
250
250
250
250
Configuring VTP and VLANs on
the Switch
To successfully configure a router for
interVLAN routing, you must configure
VTP and create and configure VLANs on
the switch.
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Because a trunk link carries traffic, or frames, from
multiple VLANs, the switch must have a method of
identifying which VLAN a frame belongs to. Cisco
supports four methods of frame identification:
Cisco Inter-Switch Link (ISL)—The Cisco proprietary
trunking method used over Fast Ethernet, Gigabit
Ethernet, and EtherChannel
IEEE 802.1Q—The IEEE industry standard trunking
method, also used over Fast Ethernet, Gigabit Ethernet,
and EtherChannel
802.10—The Cisco proprietary method of trunking over
Fiber Distributed Data Interface (FDDI)
LAN Emulation (LANE)—The IEEE standard for
trunking over Asynchronous Transfer Mode (ATM)
networks
Virtual LAN Standardization IEEE 802.1Q
Cisco Systems pioneered the frame tagging technique
for virtual LANs with both the ISL protocol and the use of
the IEEE 802.10 Standard and has leveraged that
experience to take a leadership role in defining the
emerging, functionally equivalent IEEE 802.1Q virtual
LAN Standard. It is anticipated that this standard with be
ratified later in 1997 following which the Cisco IOS(tm)
will offer the same comprehensive capabilities for IEEE
802.1Q based vLANs as are currently available with ISL,
IEEE 802.10 and LAN Emulation based virtual LANs.
Support for IEEE 802.1Q will be delivered via a regular
software upgrade available on Cisco IOS(tm) router and
switch platforms.
While configuring 802.1Q trunking it is very
important to match the native VLAN across the
link. In the Cisco IOS software versions earlier
than 12.1(3)T, you cannot define the native
VLAN explicitly, as the encapsulation dot1Q 1
native command under the sub-interface is not
available.
In the earlier Cisco IOS versions, it is important
not to configure VLAN1 interface as a subinterface. The router then expects a tag dot1q
frame on VLAN1 and the switch is not expecting
a tag on VLAN1. As a result, no traffic will pass
between VLAN1 on the switch and the router.
Using the VLAN Trunk Protocol
VTP is a Layer 2 messaging protocol that
maintains VLAN configuration consistency
by managing the addition, deletion, and
renaming of VLANs on a network-wide
basis. VTP minimizes misconfigurations
and configuration inconsistencies that can
cause several problems, such as duplicate
VLAN names, incorrect VLAN-type
specifications, and security violations.
By default, a Catalyst 2950, 2900 XL, or
3500 XL switch is in the no-managementdomain state until it receives an
advertisement for a domain over a trunk
link (a link that carries the traffic of multiple
VLANs) or until you configure a domain
name. The default VTP mode is server
mode, but VLAN information is not
propagated over the network until a
domain name is specified or learned
VTP server
• In this mode, you can create, modify, and delete
VLANs and specify other configuration
parameters (such as VTP version) for the entire
VTP domain. VTP servers advertise their VLAN
configurations to other switches in the same
VTP domain and synchronize their VLAN
configurations with other switches based on
advertisements received over trunk links.
• In VTP server mode, VLAN configurations are
saved in nonvolatile RAM. VTP server is the
default mode.
VTP client
• In this mode, a VTP client behaves like a
VTP server, but you cannot create,
change, or delete VLANs on a VTP client.
• In VTP client mode, VLAN configurations
are saved in nonvolatile RAM.
VTP transparent
• In this mode, VTP transparent switches do not
participate in VTP. A VTP transparent switch
does not advertise its VLAN configuration and
does not synchronize its VLAN configuration
based on received advertisements. However,
transparent switches do forward VTP
advertisements that they receive from other
switches. You can create, modify, and delete
VLANs on a switch in VTP transparent mode.
• In VTP transparent mode, VLAN configurations
are saved in nonvolatile RAM, but they are not
advertised to other switches.
Communication Between VLANs
Communication between VLANs is
accomplished through routing, and the
traditional security and filtering functions of the
router can be used. Cisco IOS software provides
network services such as security filtering,
quality of service (QoS), and accounting on a
per VLAN basis. As switched networks evolve to
distributed VLANs, Cisco IOS provides key interVLAN communications and allows the network
to scale.
VLAN Colors
VLAN switching is accomplished through frame tagging
where traffic originating and contained within a particular
virtual topology carries a unique VLAN identifier (VLAN
ID) as it traverses a common backbone or trunk link. The
VLAN ID enables VLAN switching devices to make
intelligent forwarding decisions based on the embedded
VLAN ID. Each VLAN is differentiated by a color, or
VLAN identifier. The unique VLAN ID determines the
frame coloring for the VLAN. Packets originating and
contained within a particular VLAN carry the identifier
that uniquely defines that VLAN (by the VLAN ID).
The VLAN ID allows VLAN switches and routers
to selectively forward packets to ports with the
same VLAN ID. The switch that receives the
frame from the source station inserts the VLAN
ID and the packet is switched onto the shared
backbone network. When the frame exits the
switched LAN, a switch strips header and
forwards the frame to interfaces that match the
VLAN color. If you are using a Cisco network
management product such as VlanDirector, you
can actually color code the VLANs and monitor
VLAN graphically.
Adding a Tag Recomputes the
Frame Control Sequence
Why Implement VLANs?
Network managers can group logically networks that
span all major topologies, including high-speed
technologies such as, ATM, FDDI, and Fast Ethernet. By
creating virtual LANs, system and network administrators
can control traffic patterns and react quickly to
relocations and keep up with constant changes in the
network due to moving requirements and node relocation
just by changing the VLAN member list in the router
configuration. They can add, remove, or move devices or
make other changes to network configuration using
software to make the changes.
Issues regarding benefits of creating VLANs
should have been addressed when you
developed your network design. Issues to
consider include
• Scalability
• Performance improvements
• Security
• Network additions, moves, and changes