[OSPF] Inconsistency between OSPF extention and IS-IS extension for segment routing

[Xuxiaohu <[e-mail]>]

Hi co-authors of the above two drafts,

OSPF extension draft proposes to use Extended Prefix Opaque LSA to carry SR-related attributes. Since the Extended Prefix Opaque LSA does not advertise reachability of the prefix, but only its attributes, the prefixes contained within those LSAs for building IP routing table (e.g., Router LSAs) can be aggregated when crossing area boundaries while the Extended Prefix Opaque LSAs containing prefix SIDs can be intactly propagated across area boudaries. The final effect is much similar to the mechanism defined in RFC5283.

In contract, IS-IS extension draft proposes to reuse those Extended IP Reachability TLVs which are used for building IP routing table to carry SR-related attributes. Although this choice has the benefit of propagating less LSAs, it loses the capability of aggregating routes when acrossing level boudaries. Furthermore, it requires the L1/L2 routers much be SR-capable.

Although these two drafts are proposing extensions to two different IGPs, IMHO, it would better to provide similar capabilities if possible, especially advoid destroying the existing capabilities of these two IGPs,  e.g., inter-area/level route aggregation capability.

To Peter Psenak,

I don't agree with your argrment that the reason that IS-IS extension draft made that choice is because there is no choice for IS-IS. In fact, you can use the signalling mechanism for Label Request which has been proposed in draft-xu-rtgwg-global-label-adv-00. That's to say, you can use separate Extended IP Reachability TLVs other than those for IP reachability advertisement to carry SR-related attibutes. Since the former TLVs are intened for advertising label bindings other than building IP routing table, the Metric field of these TLVs is set to a value larger than MAX_PATH_METRIC (i.e., 0xFE000000). It's a normal approach for IS-IS. Of course, if SR is just used within a single level, it's good to use the existing approach proposed in the IS-IS extension draft.

Best regards,
Xiaohu