ripngd: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/ripngd - from package: quagga

You may install the required package with command yum install {package}
Error in other languages:
ripngd: bevel nie gevind nie
ripngd: comandă negăsită
ripngd: comando não encontrado
ripngd: commande introuvable
ripngd: command not found
ripngd: käsku ei ole
ripngd: không tìm thấy lệnh
ripngd: komanda nerasta
ripngd: Kommando nicht gefunden.
ripngd: kommandot finns inte
ripngd: komut yok
ripngd: nie znaleziono polecenia
ripngd: níor aimsíodh an t-ordú
ripngd: no se encontró la orden
ripngd: no s'ha trobat l'ordre
ripngd: opdracht niet gevonden
ripngd: parancs nem található
ripngd: perintah tidak ditemukan
ripngd: príkaz nenájdený
ripngd: příkaz nenalezen
ripngd: команда не найдена
ripngd: командата не е открита
ripngd: 命令找不到
ripngd: コマンドが見つかりません