Where is the network type field in ospf packet capture

[rookie R]

Hi!

I can't find the ‘network type’ field in the OSPF Hello packet, How can received router determine which network type interface of his neighbor are? Can someone please tell me?

Thanks!

[Martin L · Accepted solution]

no, there are no fields for that in Hello packets but OSPF does have several LSA packet types (or codes); we care mostly about Router LSA type 1, Network LSA is 2, Network Summary is 3, 4 , external 5 and 7.

Hello Packets have necessary info to discover OSPF routers such Router and Area IDs, hello and dead timers, DR/BDR, network mask, and others but no ‘network type’.  Timers and DR election (or not) are based on link media type and encapsulation.

After Hellos are exchange, LSA headers are exchanged before each router exchanges all LSA types he knows.

Regards, ML
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[Martin L]

no fields for that in Hello packets is why you can mix some types of networks together as long as DR election (or not) matches and if you edit timers to match. i.e. you can set up OSPF on non-broadcast with Broadcast networks after editing timers to mach each others. Or point-to-point with point-to-multipoint.

[omz]

Network type is determined from the physical interface type .. e.g. ethernet = broadcast, serial = non-broadcast 

ospf behaviour changes when physical interface type changes e.g. ospf will advertise subnet configured on loopback interface as /32 by default we change that by changing the interface type to point-to-point .. 

[balaji.bandi]

If I understand correctly you looking network types - like p2p or p2m scenario. if so here some of the Wireshark capture for your reference, you can view for reference.

here is a good explanation :

https://ccieblog.co.uk/ospf/ospf-hello-protocol

Wireshark samples.

https://packetlife.net/captures/protocol/ospf/