SPF Tree Calculation
127
Sequence Number: 2147483708
Checksum: 0x3946
Link State Length: 28
Link State Advertisement Header
Age: 3600 seconds
Options: %00100010
No AS External Link State Advertisements
Type: 3 Summary Link (IP Network)
ID: 0x90fb6400
Advertising Router: 131.31.193.1
Sequence Number: 2147483650
Checksum: 0x25c0
Link State Length: 28
Frame Check Sequence: 0x00000000
You can tell that this is a Link State Acknowledgement packet based on
the OSPF header information. You will see that it is a type 5 OSPF packet,
or a Link State Acknowledgement packet.
There are two methods by which an implicit acknowledgement may be made:
Direct method The acknowledgement, either explicit or implicit, is sent
immediately. The following criteria must be met before the Direct method
is used:
A duplicate flooded LSA is received.
LSA age equals MaxAge (one hour).
Delayed method The recipient waits to send the LSA acknowledgement
with other LSA acknowledgements that need to be sent.
Validation occurs through the use of the sequencing, checksum, and aging
data contained in the LSA update packet. This information is used to make
sure that the router possesses the most recent copy of the link-state database.
SPF Tree Calculation
S
hortest Path First (SPF) trees are paths through the network to any
given destination. A separate path exists for each known destination. There
are two destination types recognized by OSPF: network and router. Router
destinations are specifically for area border routers (ABRs) and autonomous
system boundary routers (ASBRs).
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