ospfrouting
In an OSPF router LSA (LS Type 1), the Link State ID is the Router ID of the originating router. The Advertising Router used to identify the sender is also the Router ID.
So does it mean that the two parameters will always be same for every type 1 LSA.
Can you point a use case where it is not?
When using Cisco equipment, OSPF calls several things a "stub"... it gets a bit confusing....
The meanings are very different for each term; even more confusing is the reality that you can potentially apply multiple stub terms to a single OSPF interface. I'll elaborate more below.
Let's clear up some terminology... The question starts off asking about what Cisco is calling a "Stub Host"... OSPF automatically assigns Loopback interfaces to this category...
CORE01.PUB.DAL02#sh ip ospf interface lo0
Loopback0 is up, line protocol is up
Internet Address 128.66.62.5/32, Area 0.0.0.0
Process ID 100, Router ID 128.66.62.5, Network Type LOOPBACK, Cost: 1
Loopback interface is treated as a stub Host
^^^^^^^^^
CORE01.PUB.DAL02#
All this is not to be confused with what OSPF refers to as "Stub Network"... which merely means a network without an OSPF neighbor
CORE01.PUB.DAL02#sh ip ospf data router internal
...
Link connected to: a Transit Network <------- Broadcast Intf w/ Neighbor
(Link ID) Designated Router address: 128.66.62.75 <-------
(Link Data) Router Interface address: 128.66.62.75
Number of TOS metrics: 0
TOS 0 Metrics: 10
...
Link connected to: a Stub Network <------ Broadcast Intf w/o Neighbor
(Link ID) Network/subnet number: 128.66.62.32 <------
(Link Data) Network Mask: 255.255.255.248
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: a Stub Network <------ Loopback interface
(Link ID) Network/subnet number: 128.66.62.5
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Then, of course, we have an OSPF Stub Area. Let's consider where all this comes from...
Transit and Stub networks are discussed in RFC 2328: OSPFv2 - Section 2.1:
2. The Link-state Database: organization and calculations
The following subsections describe the organization of OSPF's link-
state database, and the routing calculations that are performed on
the database in order to produce a router's routing table.
2.1. Representation of routers and networks ... Networks can be either transit or stub networks. Transit networks are those capable of carrying
data traffic that is neither locally originated nor locally
destined. A transit network is represented by a graph vertex
having both incoming and outgoing edges. A stub network's vertex
has only incoming edges.
The whole point of this distinction between Transit Network and Stub Network is to save resources in the router.
When we start scaling to potentially thousands of LSAs, even small memory savings are important. Note that OSPF hellos are still sent on a stub networks, and they are capable of forming neighbors / transitioning to a transit network unless the interface has been administratively marked as a passive interface.
So now we still have the original discussion about the Stub Host, which also happens to be an OSPF Stub Network. This concept comes from RFC 2328: OSPFv2 - Section 9.1, where it discusses various interface states...
9.1. Interface states The various states that router interfaces may attain is
documented in this section.
Down
...
Loopback
... For this reason, IP packets may
still be addressed to an interface in Loopback state. To
facilitate this, such interfaces are advertised in router-
LSAs as single host routes, whose destination is the IP
interface address.[4]
...
Waiting
...
Point-to-point
...
The RFC requires Loopback network types to be advertised as a /32 host route. What Cisco calls a Stub Host in show ip ospf interface output is really just the Loopback interface type, which OSPF allocates automatically as a /32 host route within OSPF.
Stub areas are covered in RFC 2328: OSPFv2 - Section 3.6. There are several categories of OSPF stub areas...
ExternalRoutingCapability), which is cleared when an area is configured as a Stub AreaIn summary, we talked about...
If you really want to wrap your mind around all this, consider an OSPF router, which has a loopback interface; this router is completely contained in a single OSPF stub area. Technically, you could apply all three of the OSPF "stub" terms to this Loopback (Stub Host, Stub Network, Stub Area).
I think this is the exact situation addressed by the OSPF "Sham Link" feature. It's hard to know if that's the exact issue, without knowing what your MPLS provider is doing with the routes you exchange with them. It's possible that even though the MPLS link is up, it's considered a Inter-area link, while your "backdoor" VPN tunnels are Intra-area, and thus, always preferred over Inter-area links.
I don't know what routing platform you use, but Juniper's docs contain a good explanation of the issue: http://www.juniper.net/techpubs/en_US/junos12.3/topics/topic-map/ospfv2-sham-links.html
Best Answer
The Link State ID identifies what is being advertised by the LSA. A router LSA is describing what a router claims is directly connected to it, so the Link State ID is the Router ID of the originating router.
The Advertising Router is the Router ID of the router that originated the LSA.
With LS Type 1 LSAs, these two pieces of information happen to be identical, but that is not the case for other LS Types. An LS Type 1 LSA with different values in those two fields would be damaged and invalid.