One reason can be broken or malfunctioning hardware. If the hardware is destroyed and thus not discovered by IOS, then the interfaces will not show up in the running-config. Be aware, "show config" shows you the startup-config, which is a stored file used during the boot process.
In case the interface is not detected and reference to it with configuration commends will result in a syntax error or "illegal command".
So one thing you could do is to try to reseat the hardware, which requires to power off the router, depending on the model.
If that does not work you might have to RMA the part.
[toc:faq]The ProblemOn traditional switches whenever we have a trunk
interface we use the VLAN tag to demultiplex the VLANs. The switch needs
to determine which MAC Address table to look in for a forwarding
decision. To do this we require the switch to do...
[toc:faq]Introduction:Netdr is a tool available on a RSP720, Sup720 or
Sup32 that allows one to capture packets on the RP or SP inband. The
netdr command can be used to capture both Tx and Rx packets in the
software switching path. This is not a substitut...
IntroductionOSPF, being a link-state protocol, allows for every router
in the network to know of every link and OSPF speaker in the entire
network. From this picture each router independently runs the Shortest
Path First (SPF) algorithm to determine the b...