Hi.
I know I'm raising this from the dead, but I'm experiencing the same issue with both ISP interfaces on my ISR 4451X. Both are DHCP enabled, and both will at certain points, with random frequency, stop acquiring an IPv4 address from the provider. The only "fix" I have found for the code I am running, which is Fiji - Cisco IOS XE Software, Version 16.09.08, is to default the interface config and add the commands in this order, one at a time.
conf t
default interface gig x/y/z
(I usually wait a few seconds)
!
int gig x/y/z
ip address dhcp
no shut
!
(I usually wait for a few seconds and then it will grab an IP)
!
ip nat outside
!
This has always "fixed" the issue for me. until it happens again. It's mega annoying, but it's the only workaround I've found. I'm using traditional licensing, and am at the max code before I'd need to convert to Smart licensing. I'm sure the conversion would be fine, but I'm just not ready to commit to that yet.
From my situation, the issue revolves around the ip nat outside function. As to why, how, when... Still waiting for a fix. I love this router and it works perfectly for my home and lab network, and the upgrade to the 8200/8300 platform (which I believe also is listed as an affected platform for this bug) is cost prohibitive for me.
Maybe this will help someone who is in the same or similar situation.
