OSPF Options

[Balajee Muggalla]

Hi,

  OSPF options are carried in all types of ospf packets like hello,Database Description and LS-Update. What is the need for carrying this in all the above? Isnt it duplicate?

Thanks

[andre.ortega]

It is not true.
Each type of packet carry only some informations, necessary to the operation that it belongs.
You can capture a hello and a LSA and see on Wireshark that they are different.

[Balajee Muggalla]

Hi,
I captured the packets and options are there in all types of packets.

[Georg Pauwen]

Hello,

as far as I recall, only the Hello and Database Descriptor packets contain the 1-byte options field.

Check the link below which gives a pretty good explanation:

OSPF Packet Types

https://sites.google.com/site/amitsciscozone/home/important-tips/ospf/ospf-packet-types

[Balajee Muggalla]

Hi,
Thanks for your time. I am copying the LS-Update and Database
Description packets I captured from my switch. You can see the options in
the highlighted.

06:41:05.052355 00:1c:73:3c:e3:79 > 01:00:5e:00:00:05, ethertype IPv4
(0x0800), length 110: (tos 0xc0, ttl 1, id 2452, offset 0, flags [DF],
proto OSPF (89), length 96)
20.1.2.1 > 224.0.0.5: OSPFv2, LS-Update, length 76
Router-ID 1.1.1.1, Backbone Area, Authentication Type: none (0), 1 LSA
LSA #1
Advertising Router 1.1.1.1, seq 0x8000002a, age 1s, length 28
Router LSA (1), LSA-ID: 1.1.1.1
Options: [External, Demand Circuit] <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Router LSA Options: [none]
Neighbor Network-ID: 20.1.1.2, Interface Address: 20.1.1.1
topology default (0), metric 10
Neighbor Network-ID: 20.1.2.2, Interface Address: 20.1.2.1
topology default (0), metric 10
0x0000: 0000 0002 1401 0102 1401 0101 0200 000a
0x0010: 1401 0202 1401 0201 0200 000a


06:41:05.051840 00:1c:73:3c:e3:79 > 00:1c:73:c7:e8:57, ethertype IPv4
(0x0800), length 146: (tos 0xc0, ttl 1, id 23648, offset 0, flags [DF],
proto OSPF (89), length 132)
20.1.2.1 > 20.1.2.2: OSPFv2, Database Description, length 112
Router-ID 1.1.1.1, Backbone Area, Authentication Type: none (0)
Options [External, Opaque], DD Flags [none], MTU: 1500, Sequence: 0x000d7b66
Advertising Router 2.2.2.2, seq 0x80000001, age 507s, length 8
Summary LSA (3), LSA-ID: 2.2.2.2
Options: [External, Demand Circuit] <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Advertising Router 2.2.2.2, seq 0x80000001, age 73s, length 12
Network LSA (2), LSA-ID: 20.1.1.2
Options: [External, Demand Circuit]
Advertising Router 1.1.1.1, seq 0x80000029, age 46s, length 28
Router LSA (1), LSA-ID: 1.1.1.1
Options: [External, Demand Circuit]
Advertising Router 2.2.2.2, seq 0x80000027, age 47s, length 28
Router LSA (1), LSA-ID: 2.2.2.2
Options: [External, Demand Circuit]

[andre.ortega]

Well, you can see they are different, though in this specific case they may have some similarity.

Link-state routing protocol must have the link-state databases for all routers synchronized and OSPF uses Database Descriptor (DBD) packets for this purpose.

DBD are used to exchange LSA headers during the initial topology exchange, so that a router knows a list of that neighbor’s LSAs including their versions.
It is like a index.

Then the router can ask the complete LSAs it doesn´t have to its neighboor . The router uses Link-State Request (LSR) for that.
The neighboor answer the LSR with a LSU - Link-State Update, A packet that contains fully detailed LSAs.
Here (LSU) are the content, not only the headers.

So again, it is not the same.

[andre.ortega]

Sorry, only now I saw that you was talking about just about the Options and not about the full content of the packets.