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Types of link-state advertisements (LSAs) used in Open Shortest Path First (OSPF)

John Burke explains different types of link-state advertisements (LSAs) that would be used for Open Shortest Path First (OSPF).

Open Shortest Path First (OSPF) is a routing protocol for IP networks. It is used inside an organization’s routing domain, making it an interior protocol, as opposed to being used between the organization’s network and other networks (edge routing, done with edge protocols).  Other interior routing protocols include Intermediate System-to-Intermediate System (IS-IS) and Enhanced Interior Gateway Routing Protocol (EIGRP).

 

OSPF routers create a routing table showing the shortest-path routes through the network.  When a component fails, the routers automatically reroute around it.  Routers build these tables based on their link state database, which they populate based on link state advertisements (LSAs) from other routers in the network.  LSAs are the means by which the network maintains its current knowledge about routing traffic.

 

There are 11 types of LSA:

 

Type #

LSA Name

Description

1

Router

A router sends this to describe neighbors and its own interfaces.

2

Network

For broadcast networks only; the designated router for the broadcast segment uses this to list all the routers connected to the segment directly.

3

Summary

A router touching multiple areas of the network uses this to summarize information about one of those areas to update the other areas.

4

ASBR-summary

Sent by a border router that has sent a Type 5 LSA to describe how to get to that border router via the internal network.

5

AS-external

Sent by a border router to describe external routes through itself. Can be used to advertise "this is the way to (all or part of) the Internet."

6

Group membership

Used in a now obsolete multicast version of Multicast Open Shortest Path First (MOSPF).  Most routers no longer support MOSPF; this LSA may be reassigned in the future.

7

NSSA External

OSPF stub areas are those without a direct path to the outside; they do not allow Type 5 LSAs in, and all externally bound traffic is just routed out through the stub area’s boundary router.  A “not so stubby area” (NSSA) has some limited external routes available; they are advertised within the NSSA using Type 7 LSAs.  The NSSA’s boundary routers translated Type 7 LSAs into Type 5 LSAs to flood into adjacent areas.

8

External Attributes

In OSPFv2, this was used to convey BGP link attributes into an OSPF network, but it was rarely implemented.  In OSPFv3, it has been repurposed to send information about all of the IPv6 address information associated with the local link.

9

Intra-Area-Prefix

(was Link-local Opaque)

Communicates prefixes for stub areas and for transit networks, in OSPFv3.  (In OSPFv2, it was a link-local “opaque” field, conveying information passed on by OSPF routers but not used for OSPF routing.)

10

Area-Local Opaque

Flooded to area routers even if the router doesn’t understand the information within.  Used by applications to extend OSPF functionality, e.g. for MPLS traffic engineering.

11

AS Opaque

Like a Type 10, is not flooded into stubs, as with Type 5 LSAs.

 

This was first published in October 2014
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