2811 default router picking up a public ip address via dhcp behind a nat

[sandman2036]

I have a 2811 that I’m getting some odd behavior from.  Just to be clear the startup config is blank and the only file on the flash is the IOS image"c2800nm-advipservicesk9-mz.150-1.M10.bin".  If I connect it to my network via F0/0 and turn it on ill enable the port and grab a DHCP address.  But its not the class C that’s offered by the router but one the addresses out of my public /29. When I look at the running config after boot, the only difference is that F0/0 has some configuration applied to it. I have three other 2811’s in the exact same condition and they start up with all the ports shutdown regardless of what I have them connected to. I did have a peek at the NAT router that should be supplying the class C address to it but can’t really find anything there, and the diagnostics that it offers are rather poor.

[Georg Pauwen]

Hello,

--> But its not the class C that’s offered by the router but one the addresses out of my public /29

Which router is offering the class C address to your 2811 ? When you enable the interface for DHCP, the interface will accept the first lease offered, which usually means from the first DHCP server it can find.

Just to be sure, you don't mean the 2811 itself is the DHCP server ?

[sandman2036]

The class c address is offered by the CPE provided from AT&T. The 2811 is a client and is grabbing an inside global address of the CPE. The only other DHCP servers are isolated behind another router and shouldn’t be advertised on this segment. This is the only device in this segment that does this. I did call my service provider and the only solution they had was to replace the CPE. There is a section that offers static DHCP, but I don’t have anything configured in there. For the moment I’m using another router for the lab I’m working on right now.

The other thing is why is it enabling the interface on boot. I was under the impression that Cisco routers by default have their interfaces shutdown by default.

[paul driver]

Hello

show version 

show boot

dir flash;

[sandman2036]

---

@paul driver wrote:

Hello

show version 

show boot

dir flash;

---

Show version

Cisco IOS Software, 2800 Software (C2800NM-ADVIPSERVICESK9-M), Version 15.0(1)M10, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2013 by Cisco Systems, Inc.
Compiled Tue 26-Feb-13 13:25 by prod_rel_team

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

Router uptime is 5 minutes
System returned to ROM by power-on
System image file is "flash:c2800nm-advipservicesk9-mz.150-1.M10.bin"
Last reload type: Normal Reload

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Cisco 2811 (revision 53.51) with 245760K/16384K bytes of memory.
Processor board ID FTX1152A40U
2 FastEthernet interfaces
1 Serial interface
2 Channelized (E1 or T1)/PRI ports
1 Virtual Private Network (VPN) Module
4 Voice FXS interfaces
DRAM configuration is 64 bits wide with parity enabled.
239K bytes of non-volatile configuration memory.
62720K bytes of ATA CompactFlash (Read/Write)

License Info:

License UDI:

-------------------------------------------------
Device# PID SN
-------------------------------------------------
*0 CISCO2811 FTX1152A40U

Configuration register is 0x2102

dir flash:

Directory of flash:/

1 -rw- 60251444 May 5 2019 17:40:50 +00:00 c2800nm-advipservicesk9-mz.150-1.M10.bin
6 -rw- 4279 Nov 20 2019 04:38:12 +00:00 2811_ACCESS_CONFIG_19NOV19_2156.ios

64000000 bytes total (3727360 bytes free)

dir nvram

Directory of nvram:/

238 -rw- 0 <no date> startup-config
239 ---- 0 <no date> private-config
240 -rw- 0 <no date> underlying-config
1 ---- 59 <no date> persistent-data
2 ---- 4 <no date> rf_cold_starts
3 -rw- 0 <no date> ifIndex-table
4 -rw- 2945 <no date> cwmp_inventory

245752 bytes total (239556 bytes free)

The .IOS file on the flash is a backup config to quickly convert the router to an access server quickly.

EDIT: I just got a responce back form AT&T.  Aperantley the CPE doesnt display all the address allocations and i guess at some point I had mapped the mac address of that inteface to one of my inside global addresses.  

[paul driver]

Hello

To understand a little better can you post a topology of your network please.