Loop switching

[abdelmoulamarhabi1]

Hi ,

I have the following problems :

Jan  5 21:13:53.921: %IP-SW1-3-LOOPPAK: Looping packet detected and dropped -src=10.0.11.97, dst=10.0.11.254, hl=20, tl=78, prot=17, sport=137, dport=137

in=Vlan3, nexthop=10.0.11.254, out=Vlan3.

Can you please help me for resolve this problems.

thank you,

[paul driver]

Hello

the src/dst/next hop addressing is on the same subnet -

do you have:
hrsp enabled?
Any span ports being utilised by end hosts
Is Nat being used?
ip redirects enabled on SVI of vlan 3?
incorrect static routes?

res

Paul

 

[Peter Paluch]

Hi Abdel and Paul,

I was not able to find the description of this particular syslog message using the Cisco Error Message Decoder Tool - but this thread provides an interesting info:

https://supportforums.cisco.com/discussion/10960711/ip-3-looppak-looping-packet-detected-and-dropped

In essence, this thread talks about a situation where the ip helper-address command was configured using the IP address of the router itself.

In any case, I wonder how Cisco routers can determine that IP packets are looping. I would understand if the "looping" was occurring in a single device. However, I do not believe that they can detect IP packets looping between multiple routers. In my opinion, this has to do something with a router sending packets to itself all over.After all, the prot=17 refers to UDP and port 137 refers to NetBIOS Name Service, indeed one of messages handled by the helper mechanism.

Best regards,
Peter

[abdelmoulamarhabi1]

Hi ,

Thank you for your reply ,so i'm not use hsrp i use  vss (virtual switching system) ,i use so ip helper-adress of my router ip  because it's dhcp server for vlan 3 .

thank you in advance.

[Peter Paluch]

Hi,

You wrote:

i use so ip helper-adress of my router ip  because it's dhcp server for vlan 3

Does this mean you have configured the ip helper-address command on a router and used one of that router's own IP addresses in this command?

If yes then that is an incorrect use of the ip helper-address. If the router is a DHCP server for one or more of its directly connected networks or VLANs, there is no need for this command. The router can hear DHCP client messages and respond to them directly. The ip helper-address is used only when the DHCP server is running on a different device that is not directly attached to the network where the clients are.

So if you have configured an ip helper-address command on a router pointing to that same router's address, please remove it.

Best regards,
Peter

[abdelmoulamarhabi1]

Thank you for your help.