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New Member

port 9202 + Null interface

Hi,

I am facing a strange issue with OSPF configuration. i have a server in DMZ and ports 22 & 9202 are opened. when some one does a telnet on port 22 it works and the same can be seen in sh ip cache flow command, but when we do a telnet on port 9202, the dest int is showing as NULL. i have OSFP & BGP configured on the same router and redistribution is also enabled.

MPLS Router (BGP & OSPF running) --> FW (DMZ int is configured, OSPF running) --> LAN (Inside, OSPF running).

when i execute sh ip ospf command on MPLS router, i couldnt see any route with Null interface. from MPLS router i am able to telnet on port 22 but not on 9202. also no packets hit my FW and the dest int shows as Null in sh ip cache flow command. but when i do a telnet on someother port, i can see the packets hitting my FW. why only traffic to that particular server on port 9202 is getting blcoked? also why the packets are hitting NULL interfcace when it desnt exists?

Thanks,

Sridhar

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5 REPLIES
Cisco Employee

port 9202 + Null interface

Hello Sridhar,

The Null0 interface always exists, even if it is not visible in the configuration or a routing table. Referencing Null0 interface means that the packet should be dropped and possibly ICMP Unreachable message should be sent to the packet's sender.

I believe that the issue has nothing to do with OSPF. There must be some reason that the packets to port 9202 are being dropped. On which router do you see the packets being punted to Null0 - the MPLS router or the FW router? Are there any firewalls configured? Perhaps posting the configuration of that router would be helpful.

Best regards,

Peter

New Member

port 9202 + Null interface

I can see the Null0 on MPLS router. Our local vendor advised us to upgrade the IOS. but when we rebooted the router (prior to upgrading IOS) the issue was resolved. but even after upgrading IOS, the issue still persists. it works for few days and starts pointing to Null all of a sudden without making any changes.

Cisco Employee

port 9202 + Null interface

Hi Sridhar,

I believe that at this point, we would need to see the outputs of the show commands you are using when diagnosing the issue, including an example of the communication that does not work. It may contain valuable information to help resolve your issue.

Best regards,

Peter

New Member

port 9202 + Null interface

hi,

what commands you want me to execute under working condition? the only command that is telling us abt the communication break is sh ip cache flow | in x.x.x.x and telnet x.x.x.x 9202 on  the MPLS router at the moment. if we see Null instead of detstination int, we assume that the communication has broken.

Thanks,

Sridhar

New Member

port 9202 + Null interface

hi,

the communictaion betweeen the servers broke. rebooting the router will fix the issue, but not sure why it is working only for 4 days only.

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