Solved: OSPF LSA type 1, link ID meaning

xine xine · ‎06-15-2010

Hi !

I remember I had already posted this question on news groups but the discussion as been removed (it's look like too older discussion)

I had read : Authorized Self-Study Guide Building Scalable Cisco Internetwork (BSCI) for my preparation to the exam. In the OSPF chapter the author as wrote on page 230 :

Each type 1 LSA is identified by the originating router's ID in the link-state ID field.

Each of the router's links (interfaces) is defined as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID field to identify what is on the other end of the link; depending on the link type, the link ID field has different meanings. Type 1 LSA link types and their link ID meanings are described in

Table 5-2.

Table 5-2. LSA Type 1 (Router LSA) Link Types

Link Type	Description	Link ID
1	Point-to-point connection to another router	Neighbor router ID
2	Connection to a transit network	DR's interface address
3	Connection to a stub network	IP network/subnet number
4	Virtual Link	Neighbor router ID

During my revision this part on the text let me confused, I try to find the same part of information in the OSPF Network Desing Solution book from Cisco Press, but I had find nothing about it.

Someone can give to me some more detailed about the LSA Type 1 and link ID meaning ?

Thanks a lot !

Giuseppe Larosa · ‎06-15-2010

Hello Christien,

a router LSA is a data structure that has an owner/ originator so it has a "title" = OSPF router-id of originating router.

The router LSA has some bit flags used as options for example to signal if the router is acting as ASBR (= injecting external routes into OSPF domain).

After the title, and the flags there is a list of links that are:

in the same area to which the Router LSA refers.

For each of these links there is a small data structure made of the fields described in the table

so there is a link type and a link-id

The objective of link state database is to provide an exact image of links within the area

So if the link is point-to-point to allow remote routers in the same area to figure who is on the other end of the link the link id is the OSPF router-id of neighbor.

For multiaccess segment in order to correctly draw the lan segment the remote router needs to know :

on non DR devices (state DRother) a pointer to DR IP address on segment is provided

on DR device a list of OSPF router-ids of devices connected to the LAN segment is provided and the IP subnet of the segment in corresponding network LSA

(edit: this is actually a type 2 network LSA on DR device not a router LSA sorry for this but it is part of the picture )

This allows to a remote device to know what routers are connected to a LAN segment!

if no OSPF neighbor is seen on the interface, this is a leaf and the router needs to advertise the IP subnet associated to it

for those very rare cases where a virtual link has to be used it is like a point-to-point link so the link-id is the other endpoint OSPF router-id

Hope to help

Giuseppe

View solution in original post

Edison Ortiz · ‎06-15-2010

LSA Type 1 is basically the router saying to everyone that its running OSPF and its OSPF interfaces. The Link ID is equivalent to the Router-ID which often is the loopback interface or if the loopback is missing, the highest IP address will be selected. You can also alter this process by manually entering the router-id information under the OSPF process.

Regards

Edison.

Giuseppe Larosa · ‎06-15-2010