Hi Peter, ________________________________________ 发件人: Peter Psenak [[email protected]] 发送时间: 2013年11月8日 7:45 收件人: Xuxiaohu 抄送: [email protected]; [email protected] 主题: Re: [OSPF] Inconsistency between OSPF extention and IS-IS extension for segment routing
Xiaohu, please see inline: On 11/7/13 14:51 , Xuxiaohu wrote: > 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. if you aggregate on area/L1L2 boundary, SIDs/labels for individual prefixes that are covered by the aggregate are useless in the area to which you aggregate - there will be no FIB entries for these individual prefixes in such area. So if you aggregate, there is no need to propagate SIDs/labels for aggregated prefixes. [Xiaohu] "In the multi-area/level scenario where route summary between areas/levels is required, the IP longest-match algorithm SHOULD be used by SR-capable routers when processing label bindings advertised by the mapping server" For more details, please read the Introduction section of http://tools.ietf.org/html/draft-xu-rtgwg-global-label-adv-00 Best regards, Xiaohu " thanks, Peter > > 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 > _______________________________________________ > OSPF mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/ospf > _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
