OSPF – Sequence numbers are sooo negative

OSPF Sequence Numbers

Image: Numbers by eye/see – some rights reserved

When an OSPF router originates an LSA for the first time, it will choose the sequence number 0x80000001. The 0x prefix means it’s a hexadecimal number, where each hex character represents a four bit binary word.  This post discusses why the OSPF sequence number begins with 0x8, and some quirks when counting with signed numbers.

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OSPF – Immediate hellos

Unicast Hello packets

I was debugging an issue recently and ran across something unusual. I had thought that all OSPF hellos on BCAST and P2P intefaces were sent as multicasts.  However as you can see from the wireshark capture below, packet 10 is clearly sent as a unicast.

#   Time       Source	      Dest             Proto Info
04  15.503997        OSPF  Hello Packet
09  23.507483        OSPF  Hello Packet
10  23.518430     OSPF  Hello Packet 
14  25.400245	       OSPF  Hello Packet

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OSPF – BROADCAST interfaces and Type-2 LSAs

A test – What LSAs will broadcast interfaces generate?


You have just added a new workgroup switch, switch_1 to your network. You have the following conditions.

  • Router_A and Router_B were already fully adjacent over an OSPF point-to-point link.
  • Network is immediately advertised by Router_A to Router_B.
  • There are no other OSPF routers connected to Switch_1.
  • Router_A interface Gi1/0 is up/up, has OSPF network-type BCAST and has assumed the role of DR for that segment.

My question to you is, “What LSA(s) will Router_A  send to Router_B to describe this new OSPF link?”.  Your choices are:

  1. Add a ‘Stub’ type-3 link to Router LSA
  2. Add a ‘Transit’ type-2 link to Router LSA
  3. A new network-LSA
  4. BOTH a new Network-LSA, AND add a ‘Transit’ type-2 link to Router LSA

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OSPF – Lingering LSAs from unreachable routers

When a single-homed router is isolated by link failure, the LSAs it had previously originated can live for up to 60 minutes in the OSPF LSDB of the surviving routers. This may not be what you were expecting, and can cause a lot of confusion when troubleshooting OSPF. In this post we’ll look at why LSAs from an isolated router linger and how OSPF still knows how to ‘do the right thing’.


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OSPF – Type-5 E1 path cost calculation

How does a Type-5 E1 path calculation work?  Does the E1 auto-magically have it’s cost incremented is passes from the ASBR to the receiving router?  If not, how does it work? If you read this blog long enough you’ll notice that I get stuck easily and often. When that happens I like to lab stuff up and share my lessons learned.


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