Debugging - IOS

debug errors                                                  -displays errors that are raised on the device

ICMP

debug ip icmp                                                -shows information about ICMP messages, to determine whether
                                                                      or not the router is sending/receiving ICMP packets (e.g. ping)

OSPF

debug ip ospf events                                      -displays information regarding OSPF events

debug ip ospf packet                                      -displays informtation about each OSPF packet received

RIP

debug ip rip                                                    -displays RIP transaction information, including routing updates received by and sent                                                                       from a router

debug ip routing                                              -displays detailed information about RIP routing table and cache updates, such as                                                                           added/deleted routes, metric changes, cache invalidations, etc. 

ARP

debug arp                                                       -displays ARP transactions, showing ARP packets received or sent by the router

IPsec

debug crypto ipsec                                           -shows IPsec events incuding Security Association transations

debug crypto isakmp                                        -shows information regarding IKE negotiations between peers

Neighbor Discovery

debug ipv6 nd                                                 -shows information regarding neighbor discovery that is
                                                                       sent or received

Border Gate Protocol (BGP)

debug ip bgp                                                  -shows information regarding bgp path updates