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 10.10.10.0/24 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 1.1.1.1 and 2.2.2.2.

The OSPF database for the routers shows the following -

Router1#show ip ospf database router 1.1.1.1

OSPF Router with ID (1.1.1.1) (Process ID 100)

Router Link States (Area 0)

LS age: 32
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
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: 2.2.2.2
(Link Data) Router Interface address: 10.10.10.1
Number of TOS metrics: 0
TOS 0 Metrics: 64

Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.10.10.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 64


Router1#


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?

Comments:
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.
 
Post a Comment

Links to this post:

Create a Link



<< Home

This page is powered by Blogger. Isn't yours?