OSPF: Difference between revisions
→Concept Recap
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= Concept Recap =
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Protocol Number = 89, TTL=1, DES IP = 224.0.0.5 & 6
PTP:
No DR, BDR and ospf packets are sent to MC address.
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Virtual link:
Packets are unicast.
Hello – type 1
Database description – type 2
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Hello interval, Dead interval, area ID, network mask, option fields and authentication, if any
Neighborship will not form via secondary address. Other words, router will not generate hello packet with SRC IP = secondary IP.
Highest Priority
Tie, highest router ID
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Priority=0 means ineligible to become DR/BDR.
When interface comes up, it sets DR,BDR to 0.0.0.0 and wait for ‘wait timer’= router dead interval. Within that period, if it receives hello with DR/BDR filled, accept those. Else if the time period elapse, move to BDR and then to DR.
Default H=10 sec and D= 40 sec
InfTransDelay = 1 sec. change by “ip ospf transmit-delay’
RxmtInterval = 5 sec. Change by “ip ospf retramit-delay’
Down: at initial
Attempt: Only in NBMA
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Loading: Syn the LSD using LS request and LS update. In real scenario, both Exchange and loading occurs parallel.
Full: Database was sync-ed.
Have Interface MTU settings.
Initial(I) bit, More (M) bit and Master/Slave (MS) bit
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Retransmission packets are always unicast.
Use stub concept and summarization to reduce memory and CPU utilization.
Seq #:
From 0×80000001 to 0x7fffffff
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0 to 3600 (1 Hr-MaxAge)
Age increments when it resides in LSA and also incremented by ‘Infdelay’ value as it transits an interface.
Must be configured between 2 ABRs alone.
Transit area must have full routing information.
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Usually it is the worst design and needs to be changed.
Can temporarily deploy to avoid partitioned area.
Router LSA:
Type-1. Generated by all routers with all active networks which has ospf configured.
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Type- 9,10,11
Extension to OSPF. Used for TE in MPLS.
Backbone area:
Area-0. Need for inter-area communication.
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Can see Type-1,2,3,4 and type-7 LSAs.
Command: ‘area x nssa’.
Intra-area paths
Inter-area paths. Denoted by “IA” in show ip route.
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If there is only one ASBR in OSPF, there is no necessity of adding cost to ASBR. Hence E2 is default.
If there are more than on ASBR, enable E1.
No authentication: Type-0
Simple password: Type-1
MD5 password: Type-2
Router LSA of ABR will have “B-bit” set in router-lsa packet
Router LSA of ASBR will have “E-bit” set in router-lsa packet
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E – E bit=0 to indicate stub area.
MT – set to indicate Multi-topology OSPF support. Under development.
Neighborship not coming UP:
Check for Hello packet parameter match
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In case of external routes not installed in routing table, check whether the forwarding address is reachable. If not, configure ‘suppress-fa’ option at the NSSA ABR.
If an LSA has less age or high sequence number among others, we need to look for why this network was keep flapping and flooding updates. Might be reason for high CPU utilization.
ABR with area-1 and area-2 only
Routes from area-1 are not leaked into area-2 using type-3 network summary LSA.
Authentication:
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“%OSPF-4-ASBR_WITHOUT_VALID_AREA: Router is currently an ASBR while having only one area which is a stub area”
Summarization has no effect of external routes (E1/E2).
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