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Tag: OSPF

Redistribution of named and tagged static routes

Redistribution of named and tagged static routes

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I always name my IOS static routes as a best practise. However I hit a syntax issue last week when I tried to combine the named static with a tag, then redistributing that tagged static route into OSPF. If you have issues redistributing a ‘named and tagged static’ then this may be the post for you. The simplified config snippet below is configured on SW1 (cisco 3750X). This config will match all static routes tagged with ‘200’ and redistribute them into…

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A quiz on OSPF

A quiz on OSPF

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Hi All, I’m experimenting with a new quiz engine and thought I’d write an OSPF Quiz.  It’s only 8 questions and covers some of the weird and wonderful OSPF topics I have covered in the past. You’ll find them pretty easy if you’re a regular reader, or just don’t get out that much (like me!). I’d love it if you could have a go and give me some feedback. http://thenetworksherpa.com/ospf-quiz/ Regards, John Harrington.

OSPF – Setting MTU values for Cisco and Juniper

OSPF – Setting MTU values for Cisco and Juniper

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MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state.  After hellos are exchanged and the routers become neighbors, each OSPF speaker advertises the IP MTU of it’s local interface in a Data Base Description (DBD) LSA.  If there is a mismatch you’ll probably just adjust the configuration to be identical on both ends of the link and be done. However, when you try to peer two OSPF routers with different network operating systems, things…

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OSPF – Sequence numbers are sooo negative

OSPF – Sequence numbers are sooo negative

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OSPF Sequence Numbers 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.

OSPF – Immediate hellos

OSPF – Immediate hellos

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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 192.168.12.2 224.0.0.5 OSPF Hello Packet 09 23.507483 192.168.12.1 224.0.0.5 OSPF Hello Packet 10 23.518430 192.168.12.2 192.168.12.1 OSPF Hello Packet 14 25.400245 192.168.12.2 224.0.0.5 OSPF…

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OSPF – Lingering LSAs from unreachable routers

OSPF – Lingering LSAs from unreachable routers

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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’.

OSPF – Routing Bit set on this LSA

OSPF – Routing Bit set on this LSA

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Navigating the OSPF LSDB to find relevant LSAs can be tricky. Even when you find the right LSA, there is no guarantee that OSPF will include that LSA in it’s SPF calculation. Thankfully Cisco routers will tell you if the router advertising that LSA is reachable or not.

OSPF – Type-5 E1 path cost calculation

OSPF – Type-5 E1 path cost calculation

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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.  

OSPF – Why have a Type-4 LSA?

OSPF – Why have a Type-4 LSA?

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Type-4  LSAs always seemed like an ugly afterthought to me.  I know it’s irrational and ignorant but the type-4 seemed to disturb the symmetry of OSPF.  I cursed the type-5 for needing this kludgy type-4 helper. However, time was short, so I acknowledged my ignorance, rote-learned the type-4 and moved on. When I later revisited OSPF for a deeper understanding I got confused and questioned if a type-4 really was necessary.  [Hint: John Moy is considerably smarter than me! ]  In…

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