i am a Technical manager in ISP company.
i am always working winking cisco router ( 8xx , 2xxx , 7200 &t 7609 ) , cisco switches ( 1900 , 2900 & 3500 ) , firewall ( ckeckpoint , fortinet & cisco pix firewalls ).
you must have alot of knowledges with :
1. IP ( layer 3 network )
3. know how to troubleshoot your network.
you must know all the network that you working with , if u know the network you can solve the problem so quickly.
something else , you must be familiar with IP , RIP v1+2 , EIGRP , OSPF.
try to learn how to sniffer your network.