Скачать или смотреть OSPF Database Summary and LSA Header

Hello everyone and welcome to Network Playroom!

In this video, we're going to have a quick overview of the OSPF link-state database and a short discussion about the LSA types. The link-state database is really the bread and butter of OSPF. All valid LSAs received by a router are stored in the link-state database. The LSAs describe the full topology of the area. Because each router in an area calculates its shortest path tree from this database, it is imperative that all databases are identical in an area.

There are several different types of LSAs.
1 - Router LSA: describes its own interfaces and attached networks.
2 - Network LSA: generated by the Designated Router (DR) to describe all routers connected to the network segment
3 - Network Summary LSA: generated by Area Border Routers (ABR) to summarize inter-area information
4 - ASBR Summary LSA: used to advertise the presence of an Autonomous System Border Router (ASBR) to other areas.
5 - AS External LSA: generated by the ASBR to advertise external redistributed routes into the OSPF domain
6 - Group Membership LSA: The Group Membership LSA is used with Multicast OSPF, which is not supported by Cisco.
7 - NSSA External LSA: used for some special area types that do not allow external distributed routes.
8 - External Attributes LSA: External Attributes LSAs were proposed as an alternative to running Internal iBGP, to transport BGP information across an OSPF domain. This LSA has never been deployed on a wide scale, and is not supported in IOS.
9 - Opaque LSA (link-local scope)
10 - Opaque LSA (area-local scope)
11 - Opaque LSA (AS scope)
*Opaque LSAs are a class of LSAs that contain application-specific information. The Information field can be used directly by OSPF or indirectly by other applications to distribute information throughout the OSPF domain. Opaque LSAs have been used to add various extensions to OSPF, such as traffic engineering parameters for (MPLS) networks.

For all types of LSAs, there is a 20-byte header. The LSA contains the following fields:

The first field is LSA Age. This is the time that has elapsed since the LSA was created. This field actually has an important function in ensuring the link-state database stays up-to-date and uncorrupted. LSAs are aged as they reside in the link-state database. If they reach MaxAge (1 hour), they are flushed from the OSPF domain. Therefore, there must be a mechanism for preventing legitimate LSAs from reaching MaxAge and being flushed. This mechanism is the link-state refresh. Every 30 minutes, known as the LSRefreshTime, the router that originated the LSA floods a new copy of the LSA with an incremented sequence number and an age of zero. When the other routers receive the new LSA, they replace the old copy of the LSA and begin aging the new copy.

So the link-state refresh process can be thought of as a keepalive for each LSA. An additional benefit is that any LSAs that might have become corrupted in a router’s link-state database are replaced with the fresh copy of the legitimate LSA.

The Options field indicates which optional capabilities the router supports. We’ve discussed this in another video, so I will not go over it again here.

The LS Type field indicates the type of LSA.

Link State ID field depends on the LSA type but it could be the router ID, the IP address or the network the link represents.

Advertising Router field is the router ID of the router who originated the LSA.

LS Sequence Number field is the sequence number used to detect old or duplicate LSA information.

LS Checksum field contains the checksum of the LSA, for data corruption protection.

Length field is the length of the LSA, including the 20-byte header.

The Do Not Age bit is set in On Demand circuits. An On Demand circuit is a type of link that is not up all the time but only comes up when you need and you’re billed for the usage. That’s why you don’t want a routing protocol to keep the link up all the time so the LSA refresh function, which we discussed earlier, is effectively disabled.

The Do Not Age bit is also set with virtual links. That is because there could be several routers between the endpoints and they could use these on demand circuits. This is essentially a kind of safety mechanism. Better not send LSAs “unnecessarily” rather than risk high billing costs.

Thank you for watching!

LSA Header: https://en.wikipedia.org/wiki/Link-st...

OSPF Packet Capture: https://www.cloudshark.org/captures/0...