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New Member

Could we establish OSPF Adjacent where our neighbours Undirectly connected

Dear All,

Could we establish OSPF Adjacent where our neighbour is in Undirectly connected condition (like BGP cmiiw ),

without any tunneling protocol like this picture below :

untitled.PNG

(R1 want to establish Adjacent with R3 but R2 only running static routing)

Thank You,

Everyone's tags (1)
3 ACCEPTED SOLUTIONS

Accepted Solutions
New Member

Could we establish OSPF Adjacent where our neighbours Undirectly

Hey Siswanto,

In OSPF if u want to creat adj betwwen two router, they must be directly connected. In order to understand this you need to go through the OSPF states ( down , attempt , init , 2way , ex-start , exchange, loading, full )

--->>>When router reached 2 way state , they complete the process of neighbourship . For that hello packets are exchanged which include the parameter, that both the interface should be in same subnet.

--->>>When router reaches Full state, then thet are known to be adj. Because in order to exchange the LSA the two router should be adjcent to each other.

Hope this helps you.

Thanks.

Silver

Could we establish OSPF Adjacent where our neighbours Undirectly

You can't use neighbor to IP that is not on connected interface and even if you could the TTL is still 1.

The solution here if you really want to use IGP is to tunnel it through GRE.

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib CCIE #37149 Please rate helpful posts.
Cisco Employee

Could we establish OSPF Adjacent where our neighbours Undirectly

Siswanto and Vishaw,

OSPF itself is an Interior Gateway Protocol, and in these protocols, routers communicate on per-directly-connected neighbor basis. OSPF in its basic operation communicates only to directly connected neighbors, and therefore, it is not possible to establish an adjacency from R1 directly to R3. Even if it was, what would be the point? R1 and R3 would know about OSPF routes, R2 would not, so any transit traffic would be dropped and lost at R2. Exactly this is at the core that all IGP protocols inlcuding OSPF talk neighbor-to-neighbor: to allow each and every router to know about all networks, because at any time, any router can become a transit router for any data flow.

Siswanto, what you may have heard about so-called targeted sessions in OSPF may have been related to two special provisions in OSPF: virtual links, and sham links. Both these special features cause two distant routers to establish a full OSPF adjacency but they are used in very specific scenarios and are not intended to be used in a network like yours.

Another solution would be to configure a tunnel between R1 and R3, and run OSPF over that tunnel. That would indeed create a session between physically distant (but logically neighboring) routers. However, this solution is not OSPF-specific. Any routing protocol could run that way.

Best regards,

Peter

5 REPLIES
New Member

Could we establish OSPF Adjacent where our neighbours Undirectly

Hey Siswanto,

In OSPF if u want to creat adj betwwen two router, they must be directly connected. In order to understand this you need to go through the OSPF states ( down , attempt , init , 2way , ex-start , exchange, loading, full )

--->>>When router reached 2 way state , they complete the process of neighbourship . For that hello packets are exchanged which include the parameter, that both the interface should be in same subnet.

--->>>When router reaches Full state, then thet are known to be adj. Because in order to exchange the LSA the two router should be adjcent to each other.

Hope this helps you.

Thanks.

New Member

Could we establish OSPF Adjacent where our neighbours Undirectly

Thanks Mr. Vishaw, but i ever heard that ospf can use unicast address and have a fitur to define ip router neighbor to establish adjacent, i think it looks like BGP when establish peering (  correct me if i wrong   )

Silver

Could we establish OSPF Adjacent where our neighbours Undirectly

You can't use neighbor to IP that is not on connected interface and even if you could the TTL is still 1.

The solution here if you really want to use IGP is to tunnel it through GRE.

Daniel Dib
CCIE #37149

Please rate helpful posts.

Daniel Dib CCIE #37149 Please rate helpful posts.
Cisco Employee

Could we establish OSPF Adjacent where our neighbours Undirectly

Siswanto and Vishaw,

OSPF itself is an Interior Gateway Protocol, and in these protocols, routers communicate on per-directly-connected neighbor basis. OSPF in its basic operation communicates only to directly connected neighbors, and therefore, it is not possible to establish an adjacency from R1 directly to R3. Even if it was, what would be the point? R1 and R3 would know about OSPF routes, R2 would not, so any transit traffic would be dropped and lost at R2. Exactly this is at the core that all IGP protocols inlcuding OSPF talk neighbor-to-neighbor: to allow each and every router to know about all networks, because at any time, any router can become a transit router for any data flow.

Siswanto, what you may have heard about so-called targeted sessions in OSPF may have been related to two special provisions in OSPF: virtual links, and sham links. Both these special features cause two distant routers to establish a full OSPF adjacency but they are used in very specific scenarios and are not intended to be used in a network like yours.

Another solution would be to configure a tunnel between R1 and R3, and run OSPF over that tunnel. That would indeed create a session between physically distant (but logically neighboring) routers. However, this solution is not OSPF-specific. Any routing protocol could run that way.

Best regards,

Peter

New Member

Could we establish OSPF Adjacent where our neighbours Undirectly

Oke, Thanks All... it help me to understand more about OSPF behaviour 

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