For configuring FIFO queuing strategy the Cisco IOS does not provide any explicit commands. The main configuration of FIFO on specific interface is done when you will turn off any other queuing method. For example, default queuing method on serial interfaces running at E1 speed and slower is WFQ. To enable FIFO you must first disable WFQ using following command:
router(config-if)#no fair-queue
If other queuing tools is used, you should also disable these too.
Second configuration step to configure FIFO should be setting up the queue length by overriding default setting. Use following command:
router(config-if)#hold-queue #_of_packets out
for example following commad set up queue length on 50 packets (max. is 4096 on 12.4.11 IOS and 18xx and 28xx routers)
router(config-if)#hold-queue 50 out
To verify setting use:
show interface serial x/y/z