[zebra 22868] Re: Clarification on OSPF RFC2328

Robin,
          Didn't find time on the week end, but here is the explanation:

Ra will advertise an lsa with
LinkID==Neighbors RouterID
LinkData==MIB-II ifindex value
Plus
An lsa of link type stub with
LinkID==IP Address
LinkData==Subnet mask

Likewise for Rb.

Answer to Q1:
Whatever is the cost of the link depending on "ip ospf cost" or
"auto-cost-reference-bandwidth" or the default 10**8/BW for cisco.

Juniper and 3Com use different kind of metrics - so where
interoperability is concerned - you would manually set the metric to
result in a meaningful value. For ex: assume 3com routers use simple
hop count and cisco 10**8/BW. Then you would configure cisco with a
manual cost that results in a practically useful hop count value,
after assessment of your routing domain and its impact on route
preference.

Answer to Q2: I don't have the time t olok up the rfc right now - but
let me tell you what I know. Every P2P link generates an lsa with
linkType P2P as well as an lsa with linkType stub.  So for 100 P2P
links, 100 stub lsa's will be generated. Because for each P2P link, a
network address and mask has to be advertised.

To reduce the number of router lsa's if database size and scalability
is an issue - you can make the interface types point-to-multipoint
with suitable Layer2 technology like FR and a matching L3 addressing
scheme.

Save me trouble and tell me quickly what option1 and option2 are in
the rfc - Then I'll tell you more... (:

HTH

Rgrds


On 5/12/06, Robin Mathew <[email protected]> wrote:
> Hi,
> I need a small clarification on Router LSA discussed in RFC2328(Section
> 12.4.1.1 page130).
> Configuration:
> --------------------
>    Ia                 Ib
> Ra----------------Rb
>
> 1. Ra and Rb are OSPF routers connected over point to point links.
> 2. Ia and Ib are the interfaces ip addresses of each of the routers. The
> interfaces are unnumbered and are assigned ip addresses with a netmask
> of 32.
>
> Question:
> -------------
>  From my understanding of RFC2328, router LSAs of Ra should contain-
> a. A link of type point-to-point with Link ID set to Rb and LinkData set
> to Ia.
> b. A link of type stub network with LinkID set to Ib and LinkData set to
> 255.255.255.255.
>
>    This is assuming that we are following the option 1 for stub networks.
>
> Question 1:
> Shouldnt we advertise the interface Ia in the router LSA of Ra with a
> cost of 0? I am unable to find any indication about that from the RFC.
>
>  From my understanding, it looks like its required for the correct
> calculation of cost if there are 3 nodes. Or am I missing something??
>
> Question2:
> The RFC2328 talks about two options for router LSA stub networks. Is
> there any restrictions on their use? If we have a mix of routers
> advertising router LSA containing stub networks with option 1 and option
> 2 in the network, how will they interoperate?
>
> Any help regarding this will be greatly appriciated..
> Regards,
> Robin
>
>
>
>
>
>
>
>
>
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-- 
Nonchalantly yours
GobbledeGeek
[Everything but Gobbledegook.. !!]