Friday, February 24, 2006


Stub Networks in Router LSAs. (Question #24)

Consider two routers connected back-to-back to each other via their serial interfaces. Both routers are running OSPF on their serial interfaces which are in the subnet. OSPF is not running on any other interfaces and in fact no interface other than the serial interface is even up. In particular, no loopback interfaces are configured. The router-ids for the routers are and

The OSPF database for the routers shows the following -

Router1#show ip ospf database router

OSPF Router with ID ( (Process ID 100)

Router Link States (Area 0)

LS age: 32
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID:
Advertising Router:
LS Seq Number: 8000000A
Checksum: 0xFD50
Length: 48
Number of Links: 2

Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID:
(Link Data) Router Interface address:
Number of TOS metrics: 0
TOS 0 Metrics: 64

Link connected to: a Stub Network
(Link ID) Network/subnet number:
(Link Data) Network Mask:
Number of TOS metrics: 0
TOS 0 Metrics: 64


Why is a "stub network" showing up in the Router LSA in addition to the point-to-point link corresponding to the serial interface? What purpose is this stub network entry in the router LSA serving?

OSPF SPF calculations are run first on "only the links between routers and transit networks". The router (point-to-point) entry is used here. Next, OSPF runs the SPF calculation on links to stub networks, at which point it considers the stub network.

Also - it fits in with point-to-multipoint networks, where there is still 1 stub network listed (which has the network address & subnet) and 1 point-to-point entry per connected host.
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