CCNP ROUTE 2.0: IP Helper Address, Debug IP Packet, Identifying Memory Issues & Core Dumps

IP Helper Address Command

Routers or Multilayer Switches cannot forward broadcasts, but enabling the IP Helper Address command allows it to forward UDP broadcasts and forward them as a unicast to the address specified. The command “ip helper-address” must be configured on the interface/VLAN receiving the broadcasts. On a MLS for all users in VLAN 10 then the IP Helper Address must be configured on the VLAN SVI.

interface vlan 10
ip helper-address

The command forwards 8 UDP broadcasts by default

  • Time (37)
  • TACACS (49)
  • DNS (53)
  • BOOTP (DHCP Server) (67)
  • BOOTP (DHCP Client) (68)
  • TFTP (69)
  • NetBIOS Name Service (137)
  • NetBIOS Datagram Service (138)

You can specify additional UDP protocols to forward using the command “ip forward-protocol udp“, you can remove protocols using the command “no ip forward-protocol udp

Debug IP Packet

Just issuing the command “debug ip packet” will debug everything and could potentially overload the router. You can create an ACL to filter the source/destination of the traffic eg

Create the ACL “access-list 100 permit ip any log
Enable debug utilising the ACL 100 “debug ip packet 100

Identifying Memory Issues
Signs of a memory problem with a router:

  • Router hanging when connecting via the console cable
  • If router rejects telnet sessions the router may have a memory issue
  • If the router displays memory issues on the console
  • If the router tells you it is low on memory
  • If there is no output from a show command

If the router is hanging Cisco recommends disconnecting the network cables to the router and try again.

Use the “show memory allocating-process totals” or “show memory summary

Core Dumps

A core dump is a copy of the router’s entire memory, not just the memory that is in use.
A core dump can disrupt the operation of the device, do not run the command during heavy load or run from direction from Cisco TAC.
The core dump can be created via either FTP, RCP (Remote Copy Protocol), TFTP or to flash disk.

Requires the use of the “exception core-file” and “exception region-size” commands

The “exception core-file” command changes the default name of the core dump, the default would be the router name followed by –core. E.g “CORE-ROUTER-core”. The “exception region-size” command reserves some memory to be used by the core dump in case the memory pool is corrupted, the default is 16384 bytes.

You specify which protocol to use using the command “exception protocol“, then select ftp, rcp or tftp.

Configure Core Dump using FTP

ip ftp username
ip ftp password
exception protocol ftp
exception region-size 65536
exception dump

Configure Core Dump using RCP

ip rcmd remote-username
exception protocol rcp
exception region-size 65536
exception dump

Core Dump using TFTP

exception protocol tftp
exception region-size 65536
exception dump

Configure Core Dump to Flash Disk
exception flash procmem|iomem|all device_name [:partition_number]

You can test a core dump by executing the command “write core


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