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beef410's version from 2018-05-20 17:28


Question Answer
SPFDijkstra Algorithm
sh ip ospf databaseOSPF LSA list 1-5 showing for all processes
Neighbor formationArea number, hello/dead timers, stub area, netmask (of port) must all match to form adjacency. MTU size also matters but can be disabled.
Hello packetEthernet 10s Serial 30s broadcast Contains info for adjacency formation
debug ip ospf helloShow's received vs local Hello packet info settings
Dead timerDefault 4x hello, will automatically adjust based on hello timer changes
Adj StatesDown, Attempt, Init, 2WAY, Exstart, Exchange, FULL
Top update sequenceRouter updates DR/BDR's then DR/BDR's relay change out
(config-if)#ip ospf PID area #manually put a port into the specified OPSF process.
DesignNo router should be in more than 3 areas; No area should contain more than 50 routers; No router should have more than 60 neighbors; No more than one OSPF process on an ABR

OSPF States

Question Answer
DownNothing happening
AttemptSerial Only NBMA Non-Broadcast Multi-Access only
InitHello's sent
2WAYReceiver of Init Hello unicasts Hello back
ExstartFollowing DR/BDR election highest RID will start and increment initial sequence number for DBD exchanges
ExchangeDatabase Descriptors DBD's are exchanged (LSA table hashes to see if they're the same) if not LSR -> LSU -> LSACK occurs
FULLSync'd DB's adjacency formed


Question Answer
Type 1Router Link Advertisement - Lists all routers in our area.
Type 2Network Link Advert. - Generated by DR's - Same area has DR/BDR info
Type 3Summary Link Advert. - Generated by ABR's every 30min - Sends to area external to where they were generated. Provides inter-area routes to backbone. Summary of LSA's 1 and 2
Type 4Summ. Link to ASBR's - Generated by ABR's informs how to reach ASBR
Type 5AS External Link - Sent by ASBR's to advertise routes external to the OSPF AS, one per external network, flooded everywhere. Redist routes. It is implied here that routers listed have redistributed routes or are connected to a NSSA with redist routes.
Seq NumSame new LSA is ignored, if an older one arrives recipient will send its newer revision to original sender

Section 3

Question Answer
All DR/BDR's224.0.0.6. One router can be the DR/BDR for multiple areas, watch for taxed cpu
All rotuers224.0.0.5
DROtherNon-BDR/DR router, maintains 2WAY with other DROthers. Only forms full adjacency with DR/BDR
DR/BDR electionDefault priority 1, 0disqualifies from election. Highest int priority wins -> highest RID
RIDRouter ID, highest loopback IP -> highest int IP. can be manually assigned. OSPF won't start without RID. Change requires router reload or 'clear ip ospf process'
ABRArea Border Router, at least one interface physical or logical in area 0. Should only have one OSPF process
ASBRAutonomous System Boundary Router
(config-router)# default-information originateAllows router to advertise local default route, <always> allows it to advertise a default route even if it doesn't have a local one. Sends it everywhere
StubAllows for route pruning
sh ip ospf border-routersShows routes to ABR/ASBR's their area. SPF cost interface and ASBR/ABR


Question Answer
Area 0Backbone, all other areas must have routes or direct links to area 0
Multi-AreaSmaller individual databases and shorter SPF runtimes
sh ip ospf neighborNeighbor relevant info. RID/Priority/State/Deadtimere/Addr/Interface
sh ip ospf intInterface info, including timers and neighbor count
sh ip ospfSPF values and execution count, area info, number of areas the router is in, ABR/DR/BDR status
sh ip route ospfOSPF discovered routes, won't show directly connected
(config-if)#ip ospf priority 1 is default, 0 disables DR/BDR election
Hub and Spoke/NBMAmust use neighbor command on hub for adjacencies to form over cloud/frame relay(can't broadcast through a router). Disable DR/BDR election for spokes w/ priority 0
OSPF Virtual Link(config)#router ospf <processID> -> (config-router)# area <not-zero-area#> virtual-link <RID>. Must apply to stub and intermediary routers.
Virutal Link tshootWrong RID, trying to use stub area as transit area, lack of authentication config if auth is used in area 0
Passive InterfaceStops sending of hello's. (config)# router ospf <PID> -> (config-router)# passive-interface fast 0/0. Can use default instead of int to turn off hellos for all ports
sh ip protocolWill show OSPF details including listing passive interfaces
sh ip ospf intBasic sh int info with OSPF passive/hello/neighbor info
network commandDetermines what interfaces OSPF will use. Most accurate is better, actual network advertised is determined by network of the port included in the network statement
Stub(config-router)# area x stub. Removes external routes from the table
Total stubConfigure on ABR bordering stub. (config-router)# area x stub no-summary

OSPF Costs

Question Answer
auto-cost reference-bandwidth(config-router)# auto-cost reference-bandwidth #, Mbits per second. Useful for giving Gig lines priority. Must apply to all routers
(config-if)# ip ospf cost #Port level cost config, manual config has highest precedence
Serial links to cloudAssumed to be T1 speed
(config-if)# bandwidth kbitsconfigure speeds manually, does not actually limit/change port speed only informs router. Router setting, not routing-protocol setting
Load BalancingOSPF will only do equal cost load balancing. Set costs to be equal to force load balancing
(config-router)# maximum-paths #Maximum paths for OSPF to use for load balancing. 4 by default can be set up to 16.

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