If you are running your xenserver on Dell servers you need to install dell_ft_install repo
If you want firmware updates, just install the glibc.i686 from CentOS along with dell_ft_install:
yum –-enablerepo=base,updates install glibc.i686 dell_ft_install
yum install $(bootstrap_firmware)
First you need to run update_firmware to check if there is firmware to be updated
update_firmware
If it finds firmware to be updated you should use the –yes switch
update_firmware –-yes