OSPF areas

trackit

Hi!

im neck deep in the routing world, preparing for BSCI

I have one question though. I know that every OSPF area has to have a connection to backbone area (in other words has to have a router with direct or logical connection to area 0). What is not so clear to me is how is this requirement exactly enforced? Are InterArea routes allowed to be exchanged only between area 0 and some area x (ie if one of the areas is not area 0 then IA routes are not exchanged)? or is there some other logic behind it?

thanks

kryolla

this is from the RFC 2328 section 16.2

"The inter-area routes are calculated by examining summary-LSAs. If the router has active attachments to multiple areas, only backbone summary-LSAs are examined"

This doesnt take into account the transit capabilty parameter which overides this behavior

ilcram19-2

also keep in mind that a router that does not have an interface on area 0 wont be able to advirtize a route unless it uses a virtual link to a router with an interface in area 0

iproute

ilcram19-2 wrote: »

also keep in mind that a router that does not have an interface on area 0 wont be able to advirtize a route unless it uses a virtual link to a router with an interface in area 0

Not entirely true. You can have a single area OSPF domain that uses a non-0 area number. Just don't plan on adding any additional areas w/o first creating an area 0. A bad design idea that I've actually seen practiced

ilcram19-2

iproute wrote: »

A bad design idea that I've actually seen practiced

im trying not to confuse the guy and give him usable information, a routers will be able to advertize to area 0 even if it doesnt have a physical interface in area 0, using a virtual link will allow you to do that.

iproute

ilcram19-2 wrote: »

im trying not to confuse the guy and give him usable information, a routers will be able to advertize to area 0 even if it doesnt have a physical interface in area 0, using a virtual link will allow you to do that.

I'm trying to show him that if he encounters a single-area (non-0) OSPF domain he shouldn't be confused.