Some takeaway notes:
- IGP has micro loop during convergence, because FIB update from near to far，EIGRP is an exception - explicit route due to diffusing computation.
- Convergence = Detection + Propogation + SpfComp + FibUpdTime
- 1. Detection:
- Don't use IGP KA, instead use hw failure detection, like p2p Ethernet.
- No debounce timers. SONET is 例外，有50 ms protection, can set 50ms+
- If P2P, OSPF does not need DR elections to save time. And no type2 LSA, reduces LSDB size
- If no hw failure detection, have to use fast protocol hello, like ospf fast timer - "ip ospf dead-interval minimal hello-multiplier 5" = 5 hello/sec
- Can do 1-sec failure detection, but cpu load is high
- Use BFD, workload is off to LC.
- 2. Propagation:
- Out: ISIS/OPSF, using LSA flood. IGP has throttling mechanism, like Cisco has exceptional backoff algorithm. so set initial as low as 5-10 ms. don't use 0, because multiple link down event better in ONE LSA.
- In: should be ok
- Processing:
- ISIS has fast-flood, flood before SPF processing.
- Timer pacing flood, the interval between LSA.
- OutIfQueue: Need Qos to reserve b/w for CP pkts
- Packet propagation: small not serious
- 3. SPF Computation
- Used to be a big impact before, but not now thanks for fast hw and some algorithm improvement like iSPF (incremental)
- 4. Fib update
- This stage contributes the MOST to convergence time
- 2 ways to speed up: 1) less prefixes; 2) sequence important first
- Edge routers don't need core transit prefixes. So,
- (1) use ip unumbered so to put loopback0;
- (2) ospf 2 LSA for p2p links, type-1 router; and type-3 network. "ip ospf prefix-suppression", only process type 1, no type-3
- (3) ISIS has priority driven installation
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