Lecture 2 Agenda –Finish with OSPF, Areas, DR/BDR –Convergence, Cost –Fast Convergence –Tools to troubleshoot –Tools to measure convergence –Intro to implementation: scheduling Readings –Sub-milisecond IGP convergence –uLoop elimination

To READ Sub-millisecond convergence Threads vs. events

HWks Study the scheduling or Quagga Fast convergence in quagga OSPF LSA generator Implement BFD We will take it slow…

Some complications EXAMPLE In a broadcast network can not have n^2 adjacencies Use a fake centralized router Designated router, one is elected as such –Setup adjacencies only with it The DR advertises the network LS for the network Backup-DR to ensure that if DR dies I recover quickly

Scalability concerns EXAMPLE: flooding cost and number of routes AREAS –Limit the scope of flooding –Limit the number of routes If allocated hierarchically/properly Area border router (ABR) between two areas ΕΧΑΜPLE: how to compute areas through ABR Area 0 is special and is the backbone Stub areas that do not have through traffic

Periodic LSA refresh To catch some rate memory corruptions Necessary to make the protocol really robust In OSPF refresh each 30 minutes Synchronization of updates –If not careful all routers will refresh all their LSAs at the same time –Randomization

What matters Convergence speed –How quickly all routers will have consistent information after a change in the network uLoops cause me to loose traffic –How quickly new routes start to being used So traffic flows properly again Stability –How much protocol control traffic –How much CPU I burn –How things work when CPU is overloaded The above may be conflicting goals

Important Times in IGP EXAMPLE timeline –Link fails, router detects failure, sends update, computes SPF, updates RIB, updates FIB Failure detection time –Depends on link technology –May need to rely on the HELLO protocol Flooding time –Depends on the CPU load and network load, interfaces SPF time –Depends on how loaded is the CPU and how many routes I have –Depends on the algorithm RIB/FIB update –How fast I change forwarding plane –depends on number of routes

Network wide timeline Routers next to the failed node will detect the failure, originate LSA LSA will travel the diameter of the network Last router will compute SPF Last router will install routes in FIB Done

How to be faster Faster SPF –Better algorithms –Incremental SPF Faster detection –Faster HELLOs –BFD!!! In the line card instead of the control plane many protocols can share Faster FIB download –Download “important” prefixes first Do things faster –Trigger SPF immediately –Trigger LSA origination immediately

How to be stable SPF may be expensive –Can not do SPF all the time something minor changes, may be better to do one SPF for all changes Avoid extra FIB downloads –Dot not want to do SPF all the time if there is network churn, will overload CPU Do not want to sent too many updates at once –Receiver may get overloaded Do not want to send updates too quickly –Link may be flapping When CPU/links are loaded ensure that –Do not miss HELLOs, will make things worse

Configuration: Timers Hello timer, dead timer LSA update delay LSA pacing LSA retransmission pacing SPF delay –Wait for this time before you do SPF SPF hold-time –Do not do another SPF before this time passes Can have dynamic timers –Be fast when CPU is idle –Be slow when CPU is loaded

How to measure performance Black box vs. white box –White box is near impossible for commercial products Black box needs tricks –Without knowledge of the internal-structure –See paper