IP debug Commands

Debug commands can be used to provide you with a great deal of troubleshooting information. Using these commands comes at a price, namely the loss of processing power on the router. By using debug commands you can actually make the router so slow that no data is routed through the network. These commands should be used as a last-resort troubleshooting tool and not for monitoring day-to-day operations. The following output from a Cisco 1720 shows all of the syntaxes available to the debug ip command.

 RTR#  debug ip ?  audit       IDS audit events   auth-proxy  Authentication proxy debug   bgp         BGP information   cache       IP cache operations   cef         IP CEF operations   cgmp        CGMP protocol activity   dhcp        Dynamic Host Configuration Protocol   dvmrp       DVMRP protocol activity   eigrp       IP-EIGRP information   error       IP error debugging   flow        IP Flow switching operations   ftp         FTP dialogue   html        HTML connections   http        HTTP connections   icmp        ICMP transactions   igmp        IGMP protocol activity   igrp        IGRP information   inspect     Stateful inspection events   interface   IP interface configuration changes   mbgp        MBGP information   mcache      IP multicast cache operations   mhbeat      IP multicast heartbeat monitoring   mpacket     IP multicast packet debugging   mrm         IP Multicast Routing Monitor   mrouting    IP multicast routing table activity   msdp        Multicast Source Discovery Protocol (MSDP)   mtag        IP multicast tagswitching activity   nat         NAT events   nbar        StILE - traffic classification Engine   ospf        OSPF information   packet      General IP debugging and IPSO security transactions   peer        IP peer address activity   pgm         PGM Reliable Transport Protocol   pim         PIM protocol activity   policy      Policy routing   postoffice  PostOffice audit events   rgmp        RGMP protocol activity   rip         RIP protocol transactions   routing     Routing table events   rsvp        RSVP protocol activity   rtp         RTP information   scp         Secure Copy   sd          Session Directory (SD)   security    IP security options   socket      Socket event   ssh         Incoming ssh connections   tcp         TCP information   tempacl     IP temporary ACL   udp         UDP based transactions   urd         URL RenDezvous (URD)   wccp        WCCP information

Although there are many syntaxes for the debug ip command, the most important syntaxes used for troubleshooting TCP/IP are shown in Table 4.4.

The debug ip commands related to routing protocols will be discussed in Chapter 11.

Table 4.4. Important IP Debug Syntaxes to Know

Syntax	Description
arp	IP ARP and HP Probe transactions
cache	IP cache debugging
error	IP error debugging
flow	IP flow switching operations
ftp	FTP operations debugging
http	HTTP connection debugging
icmp	ICMP debugging
igmp	Internet Group Management Protocol debugging
igrp	Internet Gateway Routing Protocol debugging
mcache	IP multicast cache debugging
mds	IP distributed multicast debugging
mobile	Mobility protocols debugging
mpacket	IP multicast packet debugging
mrouting	IP multicast routing table activity
msdp	Multicast Source Discovery Protocol (MSDP) debugging
mtag	IP multicast tag switching activity
nat	Network Address Translation debugging
packet	IP debugging and security debugging
peer	IP peer address activity
pim	Protocol Independent Multicast debugging
policy	Policy routing debugging
sd	Session Directory (SD) debugging
security	IP security debugging
socket	Socket event debugging
tcp	TCP information debugging
udp	UDP information debugging

The debug ip packets command provides an option for an access list to narrow the scope of information this command provides. In order to properly use the debug ip packets command, the packets must be process-switched. All other switching types must be turned off.