OSPF - linking two non-backbone area routers

[enyamada]

Hi,

Suppose I have a network with 3 areas (areas 0, 1 and 2).

Consider also that R1 is an area 1 interior router, and R2 the same for

area 2.

What would happen If I put a direct link between R1 and R2? If R1

wants to communicate with R2, it will have to pass through area 0

(even if it´s worse than simply cross the link) ?

Thanks

Edson

[donewald · Accepted solution]

Edson,

If a non area 0 adjacency was made from R1 to R2 no summary inter area LSAs would be installed into each of the others routing table. This is soley of function of Area 0. So, in short, your packets would have to traverse Area 0. There are ways around this, if this inter -area link is something you want to do. Another OSPF process between the two or another routing protocol all together just to allow reachability between the two sites.

Hope this helps you,

Don

[thisisshanky]

OSPF requires, that all routes traverse through the backbone (area0). IF you intend to connect the two routers, via link, not through the back bone, still that link has to be in some area ( say area X) Routes wont be exchanged, across this area.

[jmcoffey]

If there is a virtual link to each ABR in each area then the circuit between theses two routers would be used. For example: (view w/ fixed width font)

R2 (ABR for area 1)

/ |

/ |

area 0 R1 |

\ |

\ |

R3 (ABR for area 2)

If there is a virtual link between R1(area 0) and R2(area 1 ABR) and another virtual link between R1 and R3(area 2 ABR) then the circuit between R2 and R3 will be used for traffic between area 1 and area 2.

Hope this helps,

Jim Coffey