Specifications

1-8
Software Configuration Guide For Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series Routers
OL-1957-03
Chapter 1 Understanding Interface Numbering and Cisco IOS Software Basics
Understanding Interface Numbering
Some modules have two small slots, labeled W0 and W1, for WAN interface cards. For example,
Figure 1-8 shows the W0 and W1 slots of the 2 Ethernet 2 WAN card slot (2E 2-slot) module. You can
install WAN interface cards into the small module slots (W0 and W1). Integrated Services Digital
Network (ISDN) Basic Rate Interface (BRI) WAN interface cards are keyed so that you can install them
into slot W1 only. Serial WAN interface cards can be installed into either slot, W0 or W1.
Figure 1-8 WAN Interface Card Slots
Cisco 3600 Series Router Unit Numbering
Cisco 3600 series routers unit numbers identify the interfaces on the modules and WAN interface cards
installed in the router. Unit numbers begin at 0 for each interface type, and continue from right to left
and (if necessary) from bottom to top. Modules and WAN interface cards are identified by interface type,
slot number, followed by a forward slash
(/), and then the unit number; for example, Ethernet 0/0.
Note In the Cisco 3660 router, the fixed FastEthernet ports are located in chassis slot 0, and are identified
by:
interface type chassis slot/ unit number
For example: FastEthernet 0/0
Figure 1-9 shows a router with a 2E 2-slot module in slots 0 and 1. Two serial WAN interface cards are
installed in the module in slot 0. One serial and one ISDN BRI WAN interface card are installed in the
module in slot 1.
As shown in Figure 1-9, the unit numbers are as follows:
• Slot 0, Ethernet interface 0, referred to as Ethernet 0/0
• Slot 0, Ethernet interface 1, referred to as Ethernet 0/1
• Slot 0, serial interface 0, referred to as serial 0/0
• Slot 0, serial interface 1, referred to as serial 0/1
• Slot 1, Ethernet interface 0, referred to as Ethernet 1/0
• Slot 1, Ethernet interface 1, referred to as Ethernet 1/1
• Slot 1, serial interface 0, referred to as serial 1/0
• Slot 1, BRI interface 0, referred to as BRI 1/0
Note The 2E 2-slot module described in this example provides both an attachment unit interface (AUI) and
10BASE-T port. Only one of these ports can be used at a time. The module automatically detects
which port, AUI or 10BASE-T, is in use.
2E
2W
W1
STP
ETHERNET 0ETHERNET 1
AUI
EN
ACT LNK
WO
ACT
ILNK
Slot W1 Slot W0
H8603