> What is Link State Update (LSU) in OSPF....? - Sarada Hettiarachchi




Friday, May 20, 2016

What is Link State Update (LSU) in OSPF....?

Multiple LSAs can be found in a one LSU (Link State Update). LSU is having its won header and each LSA is having its own one. LSA type 01 is Router LSA and it contains all the links of the router (No individual LSA for each link). it will flood to all the routes in the same area. you can not summarize or filter the type 01 LSA because that will help to build the SPF tree of the particular area. LSA has LSID and LS Data. LSID is a 32 bit address.
LSID = Router ID
Link State Data may change base on the type of the network in the link.
If the network is a Stub network (No hellos coming from the other networks or no other networks) you will see only the subnet & mask of the network. (All most all the networks will be identified as stub networks immediately you configure OSPF because there won't be any other learned networks by the OSPF).
If there is a broadcast or non-broadcast network then you will get transit networks (hello messages will send and receive). In this type of networks, only the IP address of the DR will be included in the LS Data in the type 1 LSA. (Separate LSA will be used to send subnet and the mask of the link)
If the link is a point-to-point link then in the Type 1 LSA will include the neighbor Router ID in the LS Data section of the LSA and then the link will identify as a stub network. So, the same link will represent twice because, there is no DR and no mechanism to share subnet and mask. re-introducing the link as stub is the only way to include subnet and the mask in the Type 1 LSA.
If there are loopback interfaces advertise in the OSPF. Normally, other routers will identify it as /32 address because type 1 LSA will define the subnet mask as If you configure the looback with;
ip ospf network point-to-point
then you will see the original subnet mask in neighbor router's routing table.
You can use following command to see type 1 LSAs in the route;
Show ip ospf database router [router-id]

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