Discussion on Overlapped IPGs in 802.1Qbf wei.yuehua@zte.com.cn ao.ting@zte.com.cn 2010-01

IPG overlap description In 802.1Qbf draft 0.1, Several paragraphs discuss overlap topology: (26.11.3.1) IPGs are said to overlap if a Segment associated with one IPG and a Segment associated with another IPG share a sequence of PNPs. NOTE2—It is recommended that the values of HoldOffTime (26.11.5.3.9) associated with overlapping IPGs be provisioned such the failure of a common component results in IPS action associated with one of the IPGs that completes (i.e., the value of MAdefectIndication associated with that IPG returns to “false”) before a failure of the other IPG is signaled (i.e., the value of MAdefectIndication associated with that IPG is set to “true”).

3 possible topologies IPG1 and IPG2 protect the same group of TESIs. The green line indicates the overlapped part of working segment of the two IPGs. Working segment of IPG2 is totally overlapped with working segment of IPG1 in figure(1) and figure(2), We say IPG2 is nested with IPG1. In figure(3), two IPGs are partially overlapped (1) (2) (3)

Expanded view of nested topology Configure HoldOffTime(IPG1)=50ms, HoldOffTime(IPG2)=0. When a failure of link between Pb3 and Pc3 occurs, IPG2 Monitoring CC will detect the failure then IPG2 switch all protected TESIs from B-C to B-E-C including IPG1 monitoring TESI. After IPG2 switches, IPG1 monitoring CCM traverses A-B-E-C-D. So IPG1 WOULDN’T trigger switching after waiting an interval of HoldOffTime(IPG1)

Expanded view of partially overlapped topology Configure HoldOffTime(IPG1)=50ms, HoldOffTime(IPG2)=0. When a failure of link between Pb2 and Pc2 occurs, IPG2 Monitoring CC will detect the failure then IPG2 switch all protected TESIs from B-C-D to B-F-D NOT including IPG1 monitoring TESI. After IPG2 switches, IPG1 monitoring CCM STILL traverses A-B-C. So IPG1 WOULD trigger switching after waiting an interval of HoldOffTime(IPG1) Both IPGs trigger switching due to failure of overlapped link, So configure different holdOffTime values don’t bring any benefits to this topology

Conclusions & Proposals holdoff-timer can apply to nested topology. It can’t apply to partially overlapped topology. Proposal: Replace “overlapped IPGs” with “nested IPGs” Suggest to use the following definition for “Nested IPGs” Both SEPs of a working segment in one IPG share the same PNP with SIPs/SEPs of a working segment in another IPG Add specification on how the nested IPGs work.

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