Hi, Gyan: I am not sure what you meant by "non-deterministic" behavior. If you referred to dampening the IGP metric export, there is this knob "bgp dynamic-med-interval".
If one wants symmetric routing with scale at the prefix-level, "exporting IGP metric as MED" is the best option. Thanks. -- Enke On Mon, Oct 18, 2021 at 1:50 PM Gyan Mishra <[email protected]> wrote: > > Hi Enke > > Understood. Sending IGP metric into BGP can have unwanted consequences as > MED is higher up in BGP path selection and so can cause much non > deterministic behavior. It is better to not match internal but rather do > set metric to set higher MED on all prefix to make the path preferred. Or > you can de-prefer the path by setting metric/med to 0. In my experience > there are better ways such as prepending and local preference to maintain > symmetry then exporting IGP metric to BGP. > > Kind Regards > > Gyan > On Mon, Oct 18, 2021 at 1:50 PM Enke Chen <[email protected]> > wrote: > >> Hi, Gyan: >> >> > IGP metric exported to BGP as MED does not come into play here as we >> are not redistribution ISIS into BGP. >> > We are doing next hop self on egress PE and it’s the PE loopback next >> hop attribute that is flooded. >> >> "Exporting IGP metric to BGP" (also discussed in Sect. 1.1, RFC 5302) is >> different from "redistributing IGP into BGP". In Cisco IOS the feature is >> configured as "set metric-type internal" in an outbound route-map. >> >> Thanks. -- Enke >> >> On Sat, Oct 16, 2021 at 9:35 AM Gyan Mishra <[email protected]> >> wrote: >> >>> >>> Hi Enke >>> >>> See section 5 of PUA draft use case. >>> >>> >>> https://datatracker.ietf.org/doc/html/draft-wang-lsr-prefix-unreachable-annoucement-07#section-5 >>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__datatracker.ietf.org_doc_html_draft-2Dwang-2Dlsr-2Dprefix-2Dunreachable-2Dannoucement-2D07-23section-2D5&d=DwMFaQ&c=V9IgWpI5PvzTw83UyHGVSoW3Uc1MFWe5J8PTfkrzVSo&r=OPLTTSu-451-QhDoSINhI2xYdwiMmfF5A2l8luvN11E&m=EP67fgSmvoUQ60_OUUqc0BJ3zy3bmRVf8DrsucnyeGk&s=juwHypuac_GTZeZhN_M7ms5blX0_6WSIt7YGOCKzJe4&e=> >>> >>> Also see RFC 5283 LDP Inter-area extension and use of LPM matching >>> summarization and issue with black hole routing with RFC 5283 summarization >>> gap that exists when the BGP next hop attribute LPM prefixes go away do fo >>> PE node failover. >>> >>> Also RFC 5302 ramifications of domain wide flooding of loopbacks >>> scalability issue for large areas and solution needed to make RFC 5283 LPM >>> summarization possible. >>> >>> Operators don’t want to flood the loopbacks domain wide for scalability >>> reasons. >>> >>> This issue with LPM matching as well for IP or SRv6 based networks when >>> component prefix goes away from node failure when summarization is utilized. >>> >>> The issue with multiple multiple paths can be resolved with ECMP path >>> between areas with multiple level1-2 routes redundant paths so that you >>> don’t run into symmetry issues. Also their are cases where symmetry is not >>> necessary. Also you can use hardcoded metrics to set preferred path. IGP >>> metric exported to BGP as MED does not come into play here as we are not >>> redistribution ISIS into BGP. We are doing next hop self on egress PE and >>> it’s the PE loopback next hop attribute that is flooded. >>> >>> Kind Regards >>> >>> Gyan >>> >>> On Fri, Oct 15, 2021 at 1:21 PM Enke Chen <[email protected]> >>> wrote: >>> >>>> Hi, Folks: >>>> >>>> As I recall, the loopbacks and their metrics need to be propagated >>>> individually to the whole network (including leaking from Level 2 to Level >>>> 1) for the following reasons: >>>> >>>> 1) Path selection: in case there are multiple Level 2 / Level 1 >>>> routers in an area. >>>> 2) Export IGP metric : when the IGP metrics are exported to BGP as MED >>>> for symmetric routing. >>>> >>>> Not sure if there is still a problem to be solved. >>>> >>>> Regards, >>>> >>>> -- Enke >>>> >>>> ---------- Forwarded message ---------- >>>> From: Tony Li <[email protected]> >>>> To: "Les Ginsberg (ginsberg)" <[email protected]> >>>> Cc: Gyan Mishra <[email protected]>, lsr <[email protected]> >>>> Bcc: >>>> Date: Wed, 13 Oct 2021 22:50:33 -0700 >>>> Subject: Re: [Lsr] "Prefix Unreachable Announcement" and "IS-IS and >>>> OSPF Extension for Event Notification" >>>> >>>> Les, >>>> >>>> If you’re advertising loopbacks that are outside of the summarized >>>> space, then you end up with reachability and liveness. Yes, there’s a cost >>>> in scalability… it ain’t free. >>>> >>>> Tony >>>> _______________________________________________ >>>> Lsr mailing list >>>> [email protected] >>>> https://www.ietf.org/mailman/listinfo/lsr >>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__www.ietf.org_mailman_listinfo_lsr&d=DwMFaQ&c=V9IgWpI5PvzTw83UyHGVSoW3Uc1MFWe5J8PTfkrzVSo&r=OPLTTSu-451-QhDoSINhI2xYdwiMmfF5A2l8luvN11E&m=EP67fgSmvoUQ60_OUUqc0BJ3zy3bmRVf8DrsucnyeGk&s=g9Thil3w---hguNz-FkPmxjLHnurnmrUFzDHaSm1jaI&e=> >>>> >>> -- >>> >>> >>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.verizon.com_&d=DwMFaQ&c=V9IgWpI5PvzTw83UyHGVSoW3Uc1MFWe5J8PTfkrzVSo&r=OPLTTSu-451-QhDoSINhI2xYdwiMmfF5A2l8luvN11E&m=EP67fgSmvoUQ60_OUUqc0BJ3zy3bmRVf8DrsucnyeGk&s=IiPSPRNx8poa0PgzIswRH-Tmd8aPbgntkDetpL9kPdc&e=> >>> >>> *Gyan Mishra* >>> >>> *Network Solutions A**rchitect * >>> >>> *Email [email protected] <[email protected]>* >>> >>> >>> >>> *M 301 502-1347* >>> >>> -- > > > <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.verizon.com_&d=DwMFaQ&c=V9IgWpI5PvzTw83UyHGVSoW3Uc1MFWe5J8PTfkrzVSo&r=OPLTTSu-451-QhDoSINhI2xYdwiMmfF5A2l8luvN11E&m=gtx8sc-_Jh00dX84ZuQEFj2HYti4M3KJRg7fxox4MpQ&s=omEgWRPZkDBZvAYi_C_wSKbG5YiBS-7XK703pp6uiM8&e=> > > *Gyan Mishra* > > *Network Solutions A**rchitect * > > *Email [email protected] <[email protected]>* > > > > *M 301 502-1347* > >
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