OSPF routers in numbered serial link

Unanswered Question
Sep 9th, 2007
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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


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etienne.basset Sun, 09/09/2007 - 23:46
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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]


GillieLucent Mon, 09/10/2007 - 00:22
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Hi,


Thanks for the info.

But whats the use of it ?



Thanks,

Vijaybabu

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