Re: [Lsr] Link Data value for Multi-area links

Alexander Okonnikov <> Mon, 30 November 2020 10:26 UTC

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From: Alexander Okonnikov <>
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Date: Mon, 30 Nov 2020 13:26:49 +0300
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To: Peter Psenak <>
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Subject: Re: [Lsr] Link Data value for Multi-area links
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Hi Peter,

> 30 нояб. 2020 г., в 12:56, Peter Psenak <> написал(а):
> Hi Alex,
> On 27/11/2020 13:49, Alexander Okonnikov wrote:
>> Hi Peter,
>> Which kind of ambiguity is meant? In case of numbered point-to-point each link has its own unique IP address, so there is no ambiguity.
>> Per my understanding this problem has appeared due to follow reasons:
>> 1) In old versions of the draft (up to -05) it was proposed that multi-area links are treated as unnumbered. ifIndex to be encoded in Link Data field, irrespectively whether interface has its own IP address (numbered) or borrow it (unnumbered);
>> 2) From -06 to -08 multi-area links are still treated as unnumbered, but if interface is numbered, then IP address of the neighbor (rather than local one) to be encoded into Link Data, in order to make the link look like unnumbered;
>> 3) In version -09 of the draft and in RFC 5185 itself there is no more mentions that multi-area link to be treated as unnumbered. Rather, another approach is used - if router's interface is numbered, then link is also numbered; if router's interface is unnumbered, then link is unnumbered. The rule that specifies omitting corresponding type 3 link is added. Mention of 'unnumbered' link is also removed from section 3 in RFC 5185. >
>> Hence, in version -09 with removing unnumbered nature of multi-area links Link Data for numbered links had to be changed from Neighbor's IP address to own IP address, as it is specified in RFC 2328. From perspective of other routers this link can be treated as numbered or unnumbered, depending on configuration of neighbor's corresponding interface.
> you are free to advertise the link as unnumbered. RFC5185 is not mandating to send IP address really.

The same valid for numbered ones. I.e. I'm free to advertise the link as numbered. This is straightforward when the link is numbered indeed. And if we would prefer to have deal with unnumbered interfaces, we would not need RFC 5185 (section 1.2).

>> One question - how neighboring router will perform next-hop calculation (in case it needs to do so)? If neighbor is configured with numbered interface, it will treat Link Data as IP next hop, which will be its own IP interface address.
>> Another question - how router will be able to match Link TLV (RFC 3630) to corresponding Link in Router LSA? For example, we want to calculate RSVP-TE LSP based on IGP metric (RFC 3785) and thus router needs to match IGP link to TE link.
> I don't believe you are going to do any traffic engineering over a multi-area adjacency. MADJ is there to address the OSPF route preference rules that may lead to sub-optimal routing. MADJ link is not advertised for TE purposes.

Why not? We need multi-area configuration and at the same time we need ability to build intra-area RSVP-TE LSPs within each of areas. And what about calculating IP next hop? Which compatibility is meant in section 3?

> thanks,
> Peter

Thank you.

>> Thank you.
>>> 27 нояб. 2020 г., в 14:50, Peter Psenak <> написал(а):
>>> Alexander,
>>> On 26/11/2020 17:58, Alexander Okonnikov wrote:
>>>> Hi WG,
>>>> RFC 5185 says that Neighbor's IP address to be encoded into Link Data field. Per RFC 2328 router's own IP address to be encoded into Link Data. What is the reason to advertise neighbor's IP address for multi-area links and not local IP address? It seems like bug. Could someone comment on this?
>>> Advertising a neighbor address/ifindex helps to eliminate ambiguity in case of parallel point-to-point adjacencies. It's not perfect, but that's how it was specified. So it's not a bug.
>>> thanks,
>>> Peter
>>>> Thanks in advance.
>>>> _______________________________________________
>>>> Lsr mailing list