OSPF routers in numbered serial link

Unanswered Question
Sep 9th, 2007

Hi all,

When OSPF routers are connected in numbered serial link, why the router LSA shows two links for that connection. One as 'Connected to another router (point-point)' and another as "Connected to stub network'. Is it same if they are in multi access network ?

Thanks,

Vijaybabu

I have this problem too.
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etienne.basset Sun, 09/09/2007 - 23:46

hello,

it's rfc (2328) expected behaviour :

basically, the point2point link is used for topology but doesn't have the IP/mask info that is in the stub network link

Etienne

quoting "http://www.ietf.org/rfc/rfc2328.txt"

For point-to-point interfaces, one or more link

descriptions are added to the router-LSA as follows:

o If the neighboring router is fully adjacent, add a

Type 1 link (point-to-point). The Link ID should be

set to the Router ID of the neighboring router. For

numbered point-to-point networks, the Link Data

should specify the IP interface address. For

unnumbered point-to-point networks, the Link Data

field should specify the interface's MIB-II [Ref8]

ifIndex value. The cost should be set to the output

cost of the point-to-point interface.

o In addition, as long as the state of the interface

is "Point-to-Point" (and regardless of the

neighboring router state), a Type 3 link (stub

network) should be added. There are two forms that

this stub link can take:

Option 1

Assuming that the neighboring router's IP

address is known, set the Link ID of the Type 3

link to the neighbor's IP address, the Link Data

to the mask 0xffffffff (indicating a host

route), and the cost to the interface's

configured output cost.[15]

Option 2

If a subnet has been assigned to the point-to-

point link, set the Link ID of the Type 3 link

to the subnet's IP address, the Link Data to the

subnet's mask, and the cost to the interface's

configured output cost.[16]

etienne.basset Mon, 09/10/2007 - 00:46

hello

that's because of the router lsa format :

for the p2p link type, linkid is set to the neighbor routerid (used for building the net topology). There's not enough room in the link data (32bits) to store IP+mask of the p2p link thus the need for another link (stub)

well, that's how i understand this ;)

regards

etienne

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