why do some routers have int VLAN 1 configured by default?

[ccnaluna93]

I thought that VLAN interfaces were only used by switches, but I have seen this ina router:

Router>show ip int br

Interface IP-Address OK? Method Status Protocol

GigabitEthernet0/0/0 unassigned YES unset administratively down down

GigabitEthernet0/0/1 unassigned YES unset administratively down down

GigabitEthernet0/0/2 unassigned YES unset administratively down down

Vlan1 unassigned YES unset administratively down down

[balaji.bandi · Accepted solution]

Cisco routers can have built-in or optional, add-on switch modules. The interfaces for this module are switch interfaces, not router interfaces. You need to create VLAN interfaces to link the layer-2 switch to the layer-3 router.

[ccnaluna93]

Thanks for your reply. At work I have a Cisco 4331 which has a SM-X-ES3-24-P sw module, it works as another device, but when I checked the

sh ip int br

command I don't see any created vlan interface, the only virtual LAN purpose interface used is a BDI50, it's a bit strange because I've seen other routers like this that have vlan interfaces

[balaji.bandi]

yes that is cisco default vlan part of IOS code, but there is no use until you have switch module in it.

[ccnaluna93]

Ok I got it, so if we don't have any switch module in use, then that vlan interface is useless, right? 

[balaji.bandi]

yes you got it right !

[Richard Burts]

This is an interesting aspect to think about. I believe that for most of the traditional IOS routers (such as the 4300 series) I have never seen them by default have an interface vlan 1, with switch module or without switch module. I am thinking about some other router models (perhaps the 800 series, or the Small Business series) and believe that I have seen the default vlan 1 interface on them. Perhaps the original poster can post the show version of the router from which he got the output in the original post so that we can understand what model of router this was?

Perhaps the bottom line here is that while we tend to think of Cisco as a single entity that there are multiple product lines (some appealing to different markets, and different levels of sophistication) and different implementations of their OS, and that we do not always see the same behavior when we look across these product lines or OS variants.

[ccnaluna93]

Sure, I've seen this in a Cisco4331, here we have an active switch module but interface vlan is not being used...

01>sh ver

Cisco IOS XE Software, Version 16.06.08
Cisco IOS Software [Everest], ISR Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 16.6.8, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2020 by Cisco Systems, Inc.
Compiled Thu 23-Apr-20 19:06 by mcpre


Cisco IOS-XE software, Copyright (c) 2005-2020 by cisco Systems, Inc.
All rights reserved. Certain components of Cisco IOS-XE software are
licensed under the GNU General Public License ("GPL") Version 2.0. The
software code licensed under GPL Version 2.0 is free software that comes
with ABSOLUTELY NO WARRANTY. You can redistribute and/or modify such
GPL code under the terms of GPL Version 2.0. For more details, see the
documentation or "License Notice" file accompanying the IOS-XE software,
or the applicable URL provided on the flyer accompanying the IOS-XE
software.


ROM: IOS-XE ROMMON

01 uptime is 3 weeks, 6 days, 2 hours, 41 minutes
Uptime for this control processor is 3 weeks, 6 days, 2 hours, 43 minutes
System returned to ROM by PowerOn
System restarted at 19:38:10 GMT Tue Dec 22 2020
System image file is "bootflash:isr4300-universalk9.16.06.08.SPA.bin"
Last reload reason: PowerOn


This product contains cryptographic features and is subject to United
States and local country laws governing import, export, transfer and
use. Delivery of Cisco cryptographic products does not imply
third-party authority to import, export, distribute or use encryption.
Importers, exporters, distributors and users are responsible for
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agree to comply with applicable laws and regulations. If you are unable
to comply with U.S. and local laws, return this product immediately.

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If you require further assistance please contact us by sending email to
export@cisco.com.




Suite License Information for Module:'esg'

--------------------------------------------------------------------------------
Suite Suite Current Type Suite Next reboot
--------------------------------------------------------------------------------
FoundationSuiteK9 FoundationSuiteK9 Permanent FoundationSuiteK9
securityk9
appxk9

AdvUCSuiteK9 AdvUCSuiteK9 Permanent AdvUCSuiteK9
uck9
cme-srst
cube


Technology Package License Information:

-----------------------------------------------------------------
Technology Technology-package Technology-package
Current Type Next reboot
------------------------------------------------------------------
appxk9 None None appxk9
uck9 None None uck9
securityk9 None None securityk9
ipbase ipbasek9 Permanent ipbasek9

cisco ISR4331/K9 (1RU) processor with 3843991K/6147K bytes of memory.
Processor board ID FLM235012D
5 Virtual Ethernet interfaces
3 Gigabit Ethernet interfaces
2 Serial interfaces
4 Voice FXS interfaces
32768K bytes of non-volatile configuration memory.
8388608K bytes of physical memory.
6598655K bytes of flash memory at bootflash:.
0K bytes of WebUI ODM Files at webui:.

Configuration register is 0x2102




01>Show cdp nei det

-------------------------
Device ID: sm1
Entry address(es):
IP address: 11.x.x.x
Platform: cisco SM-X-ES3-24-P, Capabilities: Router Switch IGMP
Interface: Ethernet-Internal1/0/0, Port ID (outgoing port): GigabitEthernet0/26
Holdtime : 167 sec

Version :
Cisco IOS Software, C3560E Software (C3560E-UNIVERSALK9-M), Version 15.2(4)E7, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2018 by Cisco Systems, Inc.

..........




01>Show ip int brief

Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0/0 unassigned YES NVRAM administratively down down
GigabitEthernet0/0/1 unassigned YES NVRAM administratively down down
GigabitEthernet0/0/2 unassigned YES NVRAM administratively down down
Serial0/1/0 201.x.x.x YES NVRAM up up
Serial0/1/1 unassigned YES NVRAM administratively down down
Service-Engine0/2/0 unassigned YES unset up up
Service-Engine0/4/0 unassigned YES unset up up
Ethernet-Internal1/0/0 unassigned YES unset up up
Ethernet-Internal1/0/1 unassigned YES unset up up
GigabitEthernet0 unassigned YES NVRAM administratively down down
Loopback0 11.x.x.x YES NVRAM up up
Vlan1 unassigned YES unset administratively down down 
Vlan50 11.x.x.x YES manual up up
Vlan100 11.x.x.x YES NVRAM up up
Vlan500 11.x.x.x YES NVRAM up up
Vlan900 11.x.x.x YES NVRAM up up

Now I have another question, why do we use segment 11.x.x.x for all LAN segments, we should use private segments, shouldn't we? haha it is kind of weird

[Richard Burts]

Thank you for the output. And +5 for showing me an aspect of IOS XE that I had not noticed before. Looks like it is the case that when you have a switch module that it does generate the vlan 1 default interface.