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Chapter 28: Internet Protocol Version 4 Packet Routing
310 Section VII: Routing
Note
Routing interfaces can be configured from either the command line
interface or the menus interface.
The following subsections describe the three main components of a
routing interface:
 VLAN ID (VID)
 Interface number
 IP address and subnet mask
VLAN ID (VID) An interface must be assigned to the VLAN on the switch where its
network or subnet resides. The VLAN is identified by its VLAN
identification (VID) number or VLAN name. The sequence of operations is
to create the VLAN first and then the routing interface. Creating the
interface before the VLAN is not permitted.
A VLAN can have more than one interface in circumstances where a
virtual LAN contains more than one subnet. The maximum number is
sixteen routing interfaces per VLAN, making sixteen the maximum number
of subnets you can have in an VLAN and still support packet routing on all
of them.
Interface
Numbers
An interface must be assigned an interface number in the range of 0 to 15.
This range corresponds to the maximum number of interfaces permitted in
a VLAN. Interfaces in different VLANs on the same switch can have the
same interface number, but interfaces in the same VLAN must have
different numbers.
For instance, if a switch has four local subnets and each is in a different
VLAN, all of the interfaces could have the same interface number, such as
0. However, if two or more of the subnets reside in the same VLAN, the
routing interfaces for the subnets in the VLAN must be assigned different
interface numbers.
Interfaces numbers are only used for interface identification when there is
more than one subnet and routing interface in a VLAN. Consequently, the
sequence in which the interface numbers are used is not important.
IP Address and
Subnet Mask
An interface must be a member of the local network or subnet where it will
function as the logical connection for routing IPv4 packets. As such, it
must be assigned a unique IP address and a subnet mask appropriate to
the network or subnet.
The IP address and subnet mask of an interface can be assigned
manually or supplied by a DHCP or BOOTP server on the network. When
a VLAN contains more than one interface, only one of the interfaces can
obtain its IP address from a DHCP or BOOTP server. The IP addresses of