OSPF LSA's Quick Fact Guide

Today I am going to discuss about the OSPF LSA's and i hope this article will help CCNA engineers to understand the OSPF LSA in details. If we start with the OSPF, LSA is one of the most important topic to understand which will give you fair idea how OSPF works in the different Areas. 

The question is now, What is OSPF LSA's ?
So LSA stands for Link State Advertisements and LSA is a basic communication means of the OSPF routing protocol for the Internet Protocol (IP). It communicates the router's local routing topology to all other local routers in the same OSPF area. 

OSPF is designed for scalability, so some LSAs are not flooded out on all interfaces, but only on those that belong to the appropriate area. In this way detailed information can be kept localized, while summary information is flooded to the rest of the network. The original IPv4-only OSPFv2 and the newer IPv6-compatible OSPFv3 have broadly similar LSA types.

How many types of OSPF LSA exists ?
Well If you are studying CCNA, i will recommend to study the LSA from LSA 1 to LSA 7 except LSA 6. Although we have following LSA's in OSPF domain

  • LSA Type 1
  • LSA Type 2
  • LSA Type 3
  • LSA Type 4
  • LSA Type 5
  • LSA Type 6
  • LSA Type 7
  • LSA Type 8
  • LSA Type 9
  • LSA Type 10
  • LSA Type 11
What is the main functions of all these OSPF LSA's ?
Lets talk about LSA's one by one in details to understand 

Fig 1.1- Demo Topology having LSA's

  • LSA Type 1(Router LSA) -The Type 1 LSA is that where the router announces its presence and lists the links to other routers or networks in the same area, together with the metrics to them. Type 1 LSAs are flooded across their own area only. The link-state ID of the type 1 LSA is the originating router ID.
  • LSA Type 2 (Network LSA) -The Type 2 LSA is that where the designated router (DR) on a broadcast segment (e.g. Ethernet) lists which routers are joined together by the segment. Type 2 LSAs are flooded across their own area only. The link-state ID of the type 2 LSA is the IP interface address of the DR.
  • LSA Type 3 (Summary LSA) - Here in the Type 3 LSA, the Area Border Router (ABR) takes information it has learned on one of its attached areas and summaries it before sending it out on other areas it is connected to. This summarization helps provide scalability by removing detailed topology information for other areas, because their routing information is summarized into just an address prefix and metric. The summarization process can also be configured to remove a lot of detailed address prefixes and replace them with a single summary prefix, helping scalability. The link-state ID is the destination network number for type 3 LSAs.
  • LSA Type 4 (ASBR-Summary LSA) -Here in the type LSA 5, External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas because it may be using a different routing protocol. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs.
  • LSA Type 5  (External LSA) - In the case of Type LSA 5 as named as external LSA and the  LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas unchanged (except stub and NSSA areas). For "External Metric Type 1" LSAs routing decisions are made using the Type 1 metric cost sent, as the total cost to get to the external destination and includes the cost to the ASBR; while for "External Type 2" LSAs the metric sent is the cost from the ASBR to the External destination network and must be added to the OSPF cost to the ASBR advertising the Type 5. The link-state ID of the type 5 LSA is the external network number.
  • LSA Type 6  (Group Membership LSA): (Only supported on a few routers) - this was defined for Multicast extensions to OSPF (MOSPF), a multicast OSPF routing protocol which was not in general use. MOSPF has been deprecated since OSPFv3 and is not currently used. It may be reassigned in the future.
  • LSA Type 7 - Routers in a Not-so-stubby-area (NSSA) do not receive external LSAs from Area Border Routers, but are allowed to send external routing information for redistribution. They use type 7 LSAs to tell the ABRs about these external routes, which the Area Border Router then translates to type 5 external LSAs and floods as normal to the rest of the OSPF network.
  • LSA Type 8 - A link-local only LSA for OSPFv3. A Type 8 LSA is used to give information about link-local addresses and a list of IPv6 addresses on the link. In OSPFv2, however, the Type 8 was originally intended to be used as a so-called External-Attributes-LSA for transit autonomous systems where OSPFv2 could replace the internal Border Gateway Protocol (iBGP). In these networks, the BGP destinations would be carried in LSA Type 5 while their BGP attributes would be inserted into LSA Type 8. Most OSPFv2 implementations never supported this feature.
  • LSA Type 9 - a link-local "opaque" LSA in OSPFv2 and the Intra-Area-Prefix LSA in OSPFv3. It is the OSPFv3 LSA that contains prefixes for stub and transit networks in the link-state ID.
  • LSA Type 10 - The Type LSA 10 is an area-local "opaque" LSA. Opaque LSAs contain information which should be flooded by other routers even if the router is not able to understand the extended information itself. Typically type 10 LSAs are used for traffic engineering extensions to OSPF, flooding extra information about links beyond just their metric, such as link bandwidth and colour.
  • LSA Type 11 - LSA type 11 is an AS "opaque" LSA , which is flooded everywhere except stub areas. This is the opaque equivalent of the type 5 external LSA