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OSPF Neighborship with duplicate addresses

‎05-30-2020 02:40 AM

Hi Team,

 

I am new with the JUNOS. i configured ospf between two connected routers and by mistake i configured the same ip address on both the interfaces .  but the ospf neighbourship stuck at Exstart state. In the other vendors (CISCO, Huawei) this is not the behaviour.

 

can anyone please explain me dispite of dupticate address how both the routers reached at Extart State? 

 

 

Thanks

5 REPLIES 5
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Re: OSPF Neighborship with duplicate addresses

[ Edited ]
‎05-30-2020 05:23 AM

Hi Sunil, 

 

Greetings, 

To answer you question, the routers use the interface IP's to establish OSPF and at this stage it is struck because both DR and BDR are having the same IP address which is a misconfiguration. Ideally this can be identified using the outputs of monitor traffic where you will observe that the IN and OUT packet have the same IP address.

This conflict is not letting OSPF transition to next state and would not allow the full adjacency to complete. 

r@R1_re> show ospf neighbor 
Address Interface State ID Pri Dead
10.10.10.2 ge-0/0/0.0 ExStart 128.93.23.183 128 33 >> Yes this is expected.

r@R1_re> show ospf neighbor detail Address Interface State ID Pri Dead 10.10.10.2 ge-0/0/0.0 ExStart 128.93.23.183 128 36 Area 0.0.0.0, opt 0x12, DR 10.10.10.2, BDR 10.10.10.2 >> Duplicate IP Up 00:08:42 master, seq 0x805fae4e, rexmit DBD in 0 sec lsreq-retransmit-time 0 r@R1_re> monitor traffic interface ge-0/0/0 verbose output suppressed, use <detail> or <extensive> for full protocol decode Address resolution is ON. Use <no-resolve> to avoid any reverse lookup delay. Address resolution timeout is 4s. Listening on ge-0/0/0, capture size 96 bytes Reverse lookup for 10.10.10.2 failed (check DNS reachability). Other reverse lookup failures will not be reported. Use <no-resolve> to avoid reverse lookups on IP addresses. 05:15:42.200997 Out IP truncated-ip - 20 bytes missing! 10.10.10.2 > ospf-all.mcast.net: OSPFv2, Hello, length 60 05:15:45.922755 In IP 10.10.10.2 > ospf-all.mcast.net: OSPFv2, Hello, length 60 05:15:51.631514 Out IP truncated-ip - 20 bytes missing! 10.10.10.2 > ospf-all.mcast.net: OSPFv2, Hello, length 60 05:15:55.573192 In IP 10.10.10.2 > ospf-all.mcast.net: OSPFv2, Hello, length 60

 

Please mark "Accept as solution" if this answers your query.   Kudos are appreciated too! 

 

Regards, 

Sharat Ainapur

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Re: OSPF Neighborship with duplicate addresses

‎05-30-2020 06:22 AM

what happen in case if network type is P2P on the interface and duplicate ip addresses ?

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Re: OSPF Neighborship with duplicate addresses

[ Edited ]
‎05-30-2020 07:51 AM

Hello,

 


@Sunil_Sandhu wrote:

 

can anyone please explain me dispite of dupticate address how both the routers reached at Extart State? 

 

 


 

Please open a JTAC case to have this investigated.

On the face of it, it looks like a non-supported feature contrary to what RFC 2328 section 8.2 says:

 

 Locally originated packets should not be passed on to OSPF.
 That is, the source IP address should be examined to make
 sure this is not a multicast packet that the router itself
 generated.

https://tools.ietf.org/html/rfc2328#section-8.2

 

Please pay attention to "SHOULD" language used in RFC, not "MUST". 

 

As to why DBD did not go through - OSPF Database Exchange is unicast, so in order to send unicast OSPF DB Exch packets, the router needs to find out first what peer MAC is. The router tries to resolve peer IP to a MAC, and of course finds out that IP is also own IP and own MAC! Hence, unicast OSPF DB Exchange is not attempted.

 

HTH

Thx

Alex

 

 

 

 

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Re: OSPF Neighborship with duplicate addresses

[ Edited ]
‎06-01-2020 05:20 AM
Hi aarseniev,

Thanks for your responce, i configured ospf between two routers with duplicate address but this time with P2P network type. in this case i think ospf detected duplicate addresses at its initial stages (down, init ) because when i checked with "show ospf neighbor" command it was showing nothing. In the P2P network type all the ospf messages are sent on multicast address(224.0.0.5) then how its being detected ?

Please help to clarify
Thanks
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Re: OSPF Neighborship with duplicate addresses

‎06-02-2020 02:21 PM

That's really interesting!

It seems that the packets are still arriving, and before the router drops them for being invalid, OSPF starts processing them.

That's enough to start the OSPF neighbourship, but not enough to get past EXSTART (the point you hit when there's a misconfiguration).

Nice brain-teaser to start the morning!