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 forwarding take place within the switch and asic. When does packet get sent to the Asic. Is it happen on ingress or on egress? When…
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…
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’.
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.
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.
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…
It is a common myth that OSPF ABR generates a type-4 ASBR summary ‘when it sees a type-5’ from an ASBR. The ABR does generate the type-4, but it isn’t the type-5 that triggers the ABR to originate it. I’ve setup a quick lab in GNS3 to help track down the true trigger.
When you configure an OSPF interface with a cost, you can do it directly using ip ospf cost or have the cost calculated for you using auto-cost reference bandwidth. Whichever method you choose, the OSPF RFC 2328 calls it “the cost of sending a packet out this interface”. The Router LSA, or type-1 LSA, has a 16-bit field (65535 in decimal) to represent the “interface output cost”. An interface cost of 65535 is also known as “LSInfinity”.
Todays post will cover how OSPF withdraws a route after a link failure or de-configuration. There are some fantastic resources out there describing how to originate and advertise links and prefixes, but I haven’t found a good summary of OSPF link failure handling.
I earned my CCNA a good few years back, but I still remember all the pencilled notes in the OSPF LSA section of my Todd Lammle study guide. I found the Router LSA information hard to learn, mainly because I didn’t understand why it was so complex. In this post we’re going to take a closer look at the Router LSA, specifically the point-to-point (P2P) link. On completion, you should better understand why the Router LSA seems complex and you will…