Ospf Cheat Sheet
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1
2OSPF P
1
ART
packetlife.net
Protocol Header
Attributes
8
16
24
32
Type
Link-State
Version
Type
Length
Algorithm
Dijkstra
Router ID
Metric
Cost (Bandwidth)
Area ID
AD
110
Checksum
Instance ID
Reserved
Standard
RFC 2328, 2740
Data
Protocols
IP
Transport
IP/89
Link State Advertisements
Authentication
Plaintext, MD5
Router Link (Type 1)
AllSPF Address
224.0.0.5
Lists neighboring routers and the cost to each; flooded within an area
AllDR Address
224.0.0.6
Network Link (Type 2)
Generated by a DR; lists all routers on an adjacent segment; flooded
Metric Formula
within an area
100,000 Kbps*
Network Summary (Type 3)
cost =
Generated by an ABR and advertised among areas
link speed
ASBR Summary (Type 4)
* modifiable with
Injected by an ABR into the backbone to advertise the presence of an
ospf auto-cost reference-bandwidth
ASBR within an area
External Link (Type 5)
Adjacency States
Generated by an ASBR and flooded throughout the AS to advertise a
1
Down
5
Exstart
route external to OSPF
2
Attempt
6
Exchange
NSSA External Link (Type 7)
Generated by an ASBR in a not-so-stubby area; converted into a
3
Init
7
Loading
type 5 LSA by the ABR when leaving the area
4
2-Way
8
Full
Router Types
Area Types
DR/BDR Election
Internal Router
Standard Area
The DR serves as a common point for
All interfaces reside within the
Default OSPF area type
all adjacencies on a multiaccess
same area
Stub Area
segment
Backbone Router
External link (type 5) LSAs are
A router with an interface in
replaced with a default route
The BDR also maintains adjacencies
area 0 (the backbone)
with all routers in case the DR fails
Totally Stubby Area
Area Border Router (ABR)
Type 3, 4, and 5 LSAs are
Election does not occur on point-to-
Connects two or more areas
replaced with a default route
point or multipoint links
AS Boundary Router (ASBR)
Not So Stubby Area (NSSA)
Default priority (0-255) is 1; highest
Connects to additional routing
A stub area containing an ASBR;
priority wins; 0 cannot be elected
domains; typically located in
type 5 LSAs are converted to type
the backbone
7 within the area
DR preemption will not occur unless
the current DR is reset
External Route Types
Virtual Links
E1
Cost to the advertising ASBR plus the external cost of the route
Tunnel formed to join two areas
E2 (Default)
Cost of the route as seen by the ASBR
across an intermediate
Troubleshooting
Both end routers must share a
common area
show ip [route | protocols]
show ip ospf border-routers
show ip ospf interface
show ip ospf virtual-links
At least one end must reside in area 0
show ip ospf neighbor
debug ip ospf […]
Cannot traverse stub areas
by Jeremy Stretch
v2.1
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