hi xuxiaohu, On Nov 7, 2013, at 11:51 PM, Xuxiaohu wrote: > 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 a crossing > level boudaries. Furthermore, it requires the L1/L2 routers much be > SR-capable.
and why is it a problem squeezing a few thousand transport loopbacks across an ABR / L1L2 router ? > 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. most SPs running hierarchical IS-IS have turned on domain wide prefix leaking. in fact doing aggregation in those environments is prohibitive in determining the true cost to a particular BGP next hop. - so we do not really loose much. > 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. i second that opinion - the only extensible container for IP prefixes are the extended IP reach TLVs. > 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. keep in mind that we do not advertise a label per prefix but rather a global *index*. it is my understanding that the semantics of draft-xu-rtgwg-global-label-adv-00 are quite different in the sense that absolute label values are being advertised here. /hannes _______________________________________________ OSPF mailing list [email protected] https://www.ietf.org/mailman/listinfo/ospf
