1.4d Operations

There are a number of different packet types used by OSPF these are typically Hello packets, and the different LSA types; 

LSA Type 1	Router LSA	The router will announce its presence and list the links to other routers/networks in the same area along with metrics to them. Type 1 LSAs do not cross into a different area and stay within their area of origin.
LSA Type 2	Network LSA	The designated router on a broadcast segment will list which routers are joined together by that segment.  Similar to Type 1, Type 2 LSAs do not cross into a different area.
LSA Type 3	Summary LSA	An Area Border Router (ABR) takes information it has learned on one of its attached areas, summarizes it, and then sends it out to other areas it is connected to.
LSA Type 4	ASBR Summary LSA	This is needed as Type 5 LSAs are flooded to all areas with the source as the ASBR router ID, but router ID’s are not advertised between areas.
LSA Type 5	External LSA	ASBR’s use Type 5 LSAs to describe routes imported by redistribution from another process or protocol. Type 5 LSAs are flooded to all areas except stub and NSSA areas.
LSA Type 6	Group Membership LSA	Defined for a multicast extension of OSPF (MOSPF). Otherwise this LSA Type is not in current use.
LSA Type 7	NSSA External LSA	Used to advertise external routes in a NSSA to an ABR, which will then convert the Type 7 LSA to a Type 5 LSA.
LSA Type 8	Link-local LSA	In OSPFv3, Type 8 LSAs are used to give information about link-local addresses and a list of IPv6 addresses on the link.
LSA Type 9	Link-local Opaque LSA	OSPFv3 LSA for prefixes of stub and transit networks.  Also used for NSF (Non-stop forwarding).
LSA Type 10	Area Local Opaque LSA	Opaque LSAs are LSAs that should be flooded to all routers even if that router doesn’t understand the content.  Typically used for traffic engineering on MPLS-TE by carrying information such as link bandwidth and colour.
LSA Type 11	AS Opaque LSA	Flooded everywhere except stub areas.  The opaque equivalent of a Type 5 LSA.

Route Types

LSAs are grouped together by 3 route types…

• Intra-Area Routes (O)
  • LSA Types 1 & 2
• Inter-Area Routes (O IA)
  • LSA Types 3 & 4
• External Routes
  • E1/E2
    • LSA Type 5
  • N1/N2
    • LSA Type 7

A great visualization from ipcisco.com:

1.4d ii Graceful Shutdown 

Graceful shutdown is a feature that improves convergence in the network.  It allows an interface to be shutdown without affecting the network traffic.  When graceful shutdown is enabled, the router will advertise all the networks with the max-metric for a particular time period, which allows other routers to choose an alternative path.  Use command (under OSPF process): max-metric router-lsa. 

1.4d iii GTSM (Generic TTL Security Mechanism) 

During reconvergence following an LSA flood, network attacks can occur.  GTSM provides a security mechanism for OSPF, that allows you to configure an allowed TTL over which OSPF LSA’s can be sent and received, the premise being that if a router advertises an LSA with a TTL of 10, but the maximum TTL on your network is 8, then those LSA’s are potentially malicious.