OSPF: Difference between revisions
→Troubleshooting
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= R&S Quick Notes =
= Concept Recap =
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Full: Database was sync-ed.
Have Interface MTU settings.
Initial(I) bit, More (M) bit and Master/Slave (MS) bit
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Neighbor sends DBD with I/M/MS = 1/1/1 with seq = y
After master selection, slave send DBD with I/M/MS = 0/1/0 with seq = master seq + 1.
* Retransmission packets are always unicast.
Use stub concept and summarization to reduce memory and CPU utilization.▼
Sequence number, checksum and age uniquely defines as LSA:▼
▲* Use stub concept and summarization to reduce memory and CPU utilization.
▲* Sequence number, checksum and age uniquely defines as LSA:
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.
* Virtual Links:
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.
* LSA types:
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.
* Areas:
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’.
* Path types:
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.
* Authentication:
No authentication: Type-0
Simple password: Type-1
MD5 password: Type-2
* Few points:
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.
* Troubleshooting point of view:
Few Neighborship not coming UP:
Check for Hello packet parameter match
Network ID in correct area??
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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.
* Observations:
ABR with area-1 and area-2 only.
Routes from area-1 are not leaked into area-2 using type-3 network summary LSA.
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*On a shared/ethernet network, only 2 router, DR & BDR will form full relationship, all others will stay in 2-way state.<ref>www.wipro.com</ref>
*If OSPF is stuck in ExStart State, MTU mismatch may be the cause, as it is requirement to successfully pass DBD Packets. Matching MTU is not a Adjacency Requirement, but is required to successfully pass Database Descriptor Packets
*In OSPF, MTU Mismatch causes neighbors swinging between ExStart state to Down state
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