Interface Naming Conventions

Unanswered Question
Feb 8th, 2010

First off, I apologize for posting on a subject I'm sure has been beaten to death -- but reading the Cisco documentation has managed to confuse me entirely. I am not seeming to find an explanation that drives it home for me. So - regarding the following excerpt from a 3rd party I have a question:


(1)Now let’s talk about the “slot#/port#” designation. On the old 2500 series routers, they had fixed ports so there was no slot numbers. Thus, if a 2500 series router had two Ethernet interfaces, they were called Ethernet0 and Ethernet1.  It is important to point out that Cisco slots and ports always start with zero first, then one.

(2)On the newer model routers with slots, any interface built onto the router (a fixed port) is considered to be in slot 0 (zero).  This is even true for WAN interface cards (WIC) slots that are on the router.  Any WIC installed in a router is in slot 0.  So, the first WIC installed in the router will always be WIC 0, even if it is in slot 1. This can be confusing sometimes.  So if Slot 0 has a 2 port serial WIC and Slot 1 had a BRI interface, you have Serial 0/0, Serial 0/1, and BRI0/0.

(3)Say that you have a Cisco 2610 router.  If you put a Serial WIC card in slot zero (called W0 on the router), that module would be called Serial0/0. If there was a two port Serial WIC card in the other slot (slot W1), it would be called Serial0/1 and Serial0/2.

(4)Say that you had a Cisco 3640 router. That router has 4 NM slots and no built in network interfaces.  The network module numbering on a 3640 starts with zero and goes to three.  This number starts on the bottom right of the router (there are four slots, two and two, each two on top of the other two) with zero. So, the bottom right slot is slot zero. The bottom left slot is one, the top right slot is two, and the top left slot is three. If you put a module with two WIC cards in the top right slot, your WIC cards would be called Ethernet2/0 and Serial2/0.

I was fine all the way up to paragraph 4. I understand the right to left, bottom to top numbering, but I thought all WIC interfaces regardless of slot, based on the previous discussion would start with 0? Any clarification is greatly appreciated.


I have this problem too.
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Giuseppe Larosa Mon, 02/08/2010 - 23:55

Hello Kelly,

in modular IOS routers there is the concept of slot for additional modules that can be present or not.

a Network module can be installed in a C3640 as a way to add interfaces.

The same happens on C7200.

Actually there are devices that use three numbers slot#/subslot#/port#:

old C7500 VIP linecards

current ISR routers 2800,3800 and ISR G2 2900,3900.

Then 4 numbers is used on IOS XR high end routers chassis#/slot#/subslot#/port#:

In any case you can easily find the interface list with

sh ip interface brief

once you have access to the device

Example: this is taken from a C3640:

sh ip int br
Interface                  IP-Address      OK? Method Status                Protocol
Ethernet0/0          YES NVRAM  up                    up     
Serial0/0            YES NVRAM  administratively down down   
TokenRing0/0               unassigned      YES NVRAM  up                    up     
Ethernet1/0                unassigned      YES NVRAM  administratively down down   
Serial1/0                  unassigned      YES manual up                    up     
TokenRing1/0               unassigned      YES NVRAM  administratively down down   
Serial1/1                  unassigned      YES NVRAM  up                    up     
Multilink1           YES manual up                    up     
Loopback0            YES NVRAM  up                    up     
Loopback8              YES NVRAM  up                    up   

there is a network module in slot 1 that hosts an ethernet port a TR and a serial WIC.

all of them are numbered type 1/0

example with 4 numbers indexes:

sh ip int br
Tue Feb  9 08:53:17.565 MET MEST

Interface                      IP-Address      Status                Protocol
Bundle-Ether1              Down                  Down   
Loopback5                 Up                    Up     
MgmtEth0/8/CPU0/0              unassigned      Shutdown              Down   
MgmtEth0/8/CPU0/1              unassigned      Shutdown              Down   
MgmtEth0/8/CPU0/2              unassigned      Shutdown              Down   
TenGigE0/0/0/0                 unassigned      Shutdown              Down   
GigabitEthernet0/0/1/0         unassigned      Down                  Down   
GigabitEthernet0/0/1/1      Down                  Down   
GigabitEthernet0/0/1/2         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/3         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/4         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/5         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/6         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/7         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/8         unassigned      Shutdown              Down   
GigabitEthernet0/0/1/9      Down                  Down   
TenGigE0/1/0/0                 unassigned      Shutdown              Down   
GigabitEthernet0/1/1/0         unassigned      Down                  Down   
GigabitEthernet0/1/1/1      Up                    Up     
GigabitEthernet0/1/1/2         unassigned      Shutdown              Down   
GigabitEthernet0/1/1/3         unassigned      Shutdown              Down   
GigabitEthernet0/1/1/4         unassigned      Shutdown              Down   
GigabitEthernet0/1/1/5         unassigned      Shutdown              Down   
GigabitEthernet0/1/1/6         unassigned      Shutdown              Down   
GigabitEthernet0/1/1/7      Up                    Up     
GigabitEthernet0/1/1/8      Up                    Up     
GigabitEthernet0/1/1/9      Up                    Up     
POS0/2/0/0                Up                    Up

This is taken from a Cisco IOS XR 12000.

So as you see numbering varies widely on cisco platforms

Hope to help



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