OSPF LSA type 1, link ID meaning

Answered Question
Jun 15th, 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 TypeDescriptionLink ID
1Point-to-point connection to another router Neighbor router ID
2Connection to a transit network DR's interface address
3Connection to a stub network IP network/subnet number
4Virtual LinkNeighbor 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 !

Correct Answer by Giuseppe Larosa about 6 years 8 months ago

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

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Edison Ortiz Tue, 06/15/2010 - 09:54

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.

Correct Answer
Giuseppe Larosa Tue, 06/15/2010 - 09:56

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

Actions

This Discussion