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Re: [OSPF] ospf backbone area
Acee Lindem <acee at lindem.com> wrote on 06/10/2009 20:19:25:
> From:
>
> Acee Lindem <acee at lindem.com>
>
> To:
>
> Joakim Tjernlund <joakim.tjernlund at transmode.se>
>
> Cc:
>
> ospf at ietf.org, ospf-bounces at ietf.org
>
> Date:
>
> 06/10/2009 20:20
>
> Subject:
>
> Re: [OSPF] ospf backbone area
>
>
> On Oct 6, 2009, at 1:25 PM, Joakim Tjernlund wrote:
>
> <pruned>
>
> >>
> >> 2) why should routers connected by a point to point link having
> >> interface addresses assigned,
> >> should advertise a stub link a stub connection to the other
> >> router's interface address.
> >>
> >> This dates back to the fact that P2P links were represented as a host
> >> route to the destination network on BSD based systems. Those of you
> >> who are familiar with GateD will remember this. Note that there is
> >> also an option to represent/advertise P2P links as a subnets. Most if
> >> not all routers vendors advertise the subnet.
> >>
> >> Do you have a choice here? I read the RFC that if the P2P link has
> >> a host route (mask = 255.255.255.255) you have to advertise a stub
> >> link to the other router's interface address.
> >>
> >> From RFC 2328:
> >>
> >> 12.4.1.1. Describing point-to-point interfaces
> >>
> >> 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]
> >
> > but mask 255.255.255.255 does not qualify as a subnet? So
> > that leaves only Option 1
>
> Yes - if you use 255.255.255.255, you will need to support option 1.
> We already have 2 options, we don't need a third.
Maybe I read too much into this statement:
"This dates back to the fact that P2P links were represented as a host
route to the destination network on BSD based systems. Those of you
who are familiar with GateD will remember this. Note that there is
also an option to represent/advertise P2P links as a subnets. Most if
not all routers vendors advertise the subnet."
I got the impression that you meant that you could choose to advertise P2P/32
with Option 2 instead of Option 1. I guess I read too much into it :)