Category: protocols

  • Does your Wave2 AP need NBase-T?

    Cisco recently launched the 2800 and 3800 series 802.11ac wave-2 access points. The 3800 Datasheet quotes a theoretical maximum throughput of 5.2Gbps when operating in Dual 5GHz radio mode (2 x 2.6Gbps). If you ran two cables to your AP you could use the second ethernet port to create a 2 x 1Gbps LAG. However there is…

  • Redistribution of named and tagged static routes

    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.…

  • Link Utilisation Varies By Packet Size

    I said to a colleague recently, “you can’t get 100% link utilisation on an Ethernet link”. When I tried to explain myself I wished I could link to a simple blog post with a nice graph. So here’s a quick blog post with a nice graph. I have talked a little about link speed in…

  • Is CPU or ASIC responsible for forwarding?

    I received the question below from reader Ned as a comment on my 24-port ASIC post and thought that the discussion was worth a post of it’s own. …Would you be able to speak a bit about the actual physical path or packet flow a packet takes inside the switch itself and how does the hardware…

  • A quiz on OSPF

    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!).…

  • OSPF – Setting MTU values for Cisco and Juniper

    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…

  • OSPF – Sequence numbers are sooo negative

    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…

  • 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…

  • 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 192.168.12.0/24 is immediately advertised by Router_A to Router_B. There are no other OSPF routers connected to…

  • 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…