The following steps describe the process for
creating a broadcast domain within the Layer 3 device and then
mapping that to the VLAN through a trunk link:
|
1.
|
Create a bride
group
.
|
IOS
|
(global)
bridge
number
protocol ieee
|
In global configuration mode, the
bridge
command creates a bridge group. The
number
option specifies the group;
numbers
can range from 1 to 255. If your VLAN on the switch is
within this range, it is recommended to create a bridge group with
the same number as the VLAN. The
protocol
option should always be
ieee
when working with switches
because the switch does not understand the Digital Equipment
Corporation
Spanning-Tree Protocol
(STP), and bridge
loops
may occur.
|
|
2.
|
Enable routing and bridging for the
protocol.
In global configuration mode, the
bridge irb
command allows both routing and
bridging for configured protocols. By default a Layer 3 switch or
router will not bridge any routable protocol, such as IP, even if
an interface is configured for bridging. To make the interface
bridge the traffic, you could disable IP routing completely (not a
likely option) or enable the IRB function.
NOTE
Depending on the operating system (OS), you
might have to add another command when IRB is enabled. The command
no bridge
number
route
ip
might display in the configuration after you enable IRB.
This command disables IP routing on any interface that is in the
bridge group specified by the
number
option. If the command is not
automatically entered, you will have to manually enter the command
if you want to bridge between the interfaces.
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|
3.
|
Assign interfaces to the bridge group.
|
IOS
|
(interface)
bridge-group
number
|
In interface configuration mode, the
bridge-group
command adds a port to the
bridge group. When a port is a member of the bridge group, all the
interfaces in that group can communicate at Layer 2. Essentially
the interfaces are in the same Broadcast domain, just like
interfaces in the same VLAN on a Layer 2 switch. The
number
option specifies to which group an
interface belongs. When an interface becomes a member of the group,
it begins running the STP on that interface as well.
|
|
4.
|
Create a subinterface on a trunk link.
|
IOS
|
(global)
interface
type number.subintnumber
|
To extended the VLAN from a switched network into the router or
Layer 3 device, you must get traffic from the Layer 2 switch with
some indication of the VLAN associated with the traffic. You can do
so through a trunk link. On the interface attached to the switch
trunk link, you are going to create a subinterface. The
interface
command followed by
the
type
(FastEthernet or
GigabitEthernet) and the
number.subintnumber
option creates the
subinterface. Each VLAN that will be mapped to the Layer 3 device
will have its own subinterface. It is recommended that the
subinterface number (the one after the dot) be the same as the VLAN
that will map to that subinterface. For example, VLAN 3 connected
to interface FA 0/1 would have a subinterface FA 0/1.3.
|
|
5.
|
Specify the encapsulation type and VLAN
number.
|
IOS
|
[View full width]
(subinterface)
encapsulation
{
dot1q
isl
}
vlannumber
[
native
]
|
On the subinterface that was created in Step 4, you must specify
the trunking encapsulation and the VLAN number that will be
associated with this subinterface. The
encapsulation
command specifies this
information. The
dot1q
or
isl
command specifies the trunk
type, and the
vlannumber
specifies
the VLAN.
NOTE
802.1Q trunk links do not tag for the native
VLAN. On the Layer 3 switch, you must specify the native option for
the subinterface attached to the native 802.1Q VLAN to prevent that
VLAN from being tagged (typically VLAN 1).
|
|
6.
|
Add the VLAN to the bridge group.
|
IOS
|
(subinterfac)
bridge-group
number
|
By adding the bridge group to the subinterface, you have added the
VLAN from the switch to the bridge group on the router or Layer 3
device. This means all the interfaces on the Layer 3 device in the
bridge group, and all the ports on the switch in the VLAN are in
the same broadcast domain and effectively the same VLAN.
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|
7.
|
Routing for the broadcast domain.
-
a.
Enable routing for the bridge group.
|
IOS
|
(global)
bridge
groupnumber
route ip
|
In Step 2, either you or the router disabled the routing function
on the Layer 3 device for interfaces in the bridge group for the IP
protocol. If you want the Layer 3 device to route for all the
devices in this broadcast domain (including those in the switch
VLAN), you must first enable routing for this bridge group with the
bridge
groupnumber
route
ip
command.
-
b.
Create the virtual Layer 3 interface.
|
IOS
|
(global)
interface bvi
groupnumber
|
When you enable routing for these interfaces, bridging will
cease
to function until you create a virtual Layer 3 interface to be used
by the group. The
interface bvi
command creates that virtual interface for all the
members
of the
bridge group.
-
c.
Assign an IP address to the Layer 3 interface.
|
IOS
|
(bvi-interface)
ip address
address mask
|
After you create the interface, you must assign it a Layer 3
address. This address becomes the gateway for all the devices in
the bridge group and the VLAN.
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