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 does packet get analyzed by CPU or control plane. If the CPU never sees the actual packet how does asic know where to forward the packet and does that mean the packets stay within asic itself and is that what is meant to be hardware forwarding. Is Asic = dataplane. Tx
I like this question because it captures a lot of my early assumptions and concerns about data and control plane separation. My response assumes a single-stage modern switch-on-chip ASIC without backplane or fabric. Continue reading
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 192.168.12.2 126.96.36.199 OSPF Hello Packet
09 23.507483 192.168.12.1 188.8.131.52 OSPF Hello Packet
10 23.518430 192.168.12.2 192.168.12.1 OSPF Hello Packet
14 25.400245 192.168.12.2 184.108.40.206 OSPF Hello Packet
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 this post I show you where I got stuck and what I learned about the type-4.