ISIS is simple to operate normally while everything is working, most common deployments are flat network based on L2, however when there is a problem and we need to start troubleshooting then people start to get lost.
So I would like to provide some tools on how to read ISIS database.
- notice to the “*” sign, that mean LSP was generated on the router you did the show command, you can see that host name from the show command match also host name on the LSPID,
- LSPID identified by hostname.xx-yy, xx is normally 00 unless that LSP is pseudo node LSP generated by DIS , yy is representing the number of fragments for that LSP 00 – FF (max 255 fragments, plenty), most cases all the important information will be in 00 unless there are many fragments.
- LSP Holdtime is the amount of time an LSP will stay in database without any refresh.
- ATT/P/OL - 0/0/0, ATT bit or attached bit is used on L1/L2 connected to L1 node, if set to 1 L1 node will generate default route to the best L1/L2 node (best metric)
- ATT/P/OL - 0/0/0, OL bit or overload bit, in the past it was used when router was over loaded to set all links on the router as unusable, preventing others from transiting, that node, today it is used mostly to wait for bgp convergence.
P_london_someisp.net#show isis databaseIS-IS Level-2 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLP_london_somei.00-00* 0x00000005 0x65C5 1177 0/0/0P_dublin_somei.00-00 0x00000004 0x8346 1176 0/0/0P_cyprus_somei.00-00 0x00000005 0x5634 1183 0/0/0P_LA_someisp.n.00-00 0x00000005 0xDE33 1175 0/0/0PE_newyork_som.00-00 0x00000003 0x2EF0 1179 0/0/0PE_telaviv_som.00-00 0x00000004 0x2877 1181 0/0/0PE_Jerusalem_s.00-00 0x00000002 0x2994 1172 0/0/0PE_Jerusalem_s.02-00 0x00000001 0x2ED2 1171 0/0/0
From the database each router build a topology using SPF (dijkstra algorithm), if I would like to understand how to get from one router to another I can look into the ISIS database detail and understand that:
For the example, I would like to see only with the database, how to get from my selfto PE_telaviv without looking into the topology or routing table (just for fun):Notice that according to my LSP I can see the neighbors I am connected to, and the net addressfor each link along with metric.P_london_someisp.net#$atabase level-2 det P_london_someisp.net.00-00IS-IS Level-2 LSP P_london_somei.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLP_london_somei.00-00* 0x00000006 0x63C6 409 0/0/0Area Address: 49.0001NLPID: 0xCCHostname: P_london_someisp.netIP Address: 1.1.1.1Metric: 10 IS-Extended P_dublin_somei.00Metric: 10 IS-Extended P_cyprus_somei.00Metric: 10 IS-Extended P_LA_someisp.n.00Metric: 10 IS-Extended PE_newyork_som.00Metric: 10 IP 1.1.1.1/32Metric: 10 IP 10.100.1.4/30Metric: 10 IP 10.100.1.16/30Metric: 10 IP 10.100.1.20/30Metric: 10 IP 10.100.1.24/30P_london_someisp.net#
now if I take the first neighbor from my LSP P_dublin and look into his LSP:notice that one of his neighbors is PE_telaviv.So 10 to reach dublin + 10 to reach telaviv = 20 total cost from london to telavivI have taken the next neighbor cyprus and it looks like we have also 20 metric path usingcyprus, so we will have load sharing between them.notice that P_LA and PE_newyork does not have direct link to PE_telaviv, that mean we do notneed to explorer more in that direction as any route using them will have higher cost.
P_london_someisp.net#$atabase level-2 det P_dublin_someisp.net.00-00IS-IS Level-2 LSP P_dublin_somei.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLP_dublin_somei.00-00 0x00000006 0x7F48 1077 0/0/0Area Address: 49.0001NLPID: 0xCCHostname: P_dublin_someisp.netIP Address: 2.2.2.2Metric: 10 IS-Extended P_london_somei.00Metric: 10 IS-Extended P_cyprus_somei.00Metric: 10 IS-Extended P_LA_someisp.n.00Metric: 10 IS-Extended PE_telaviv_som.00Metric: 10 IP 2.2.2.2/32Metric: 10 IP 10.100.1.0/30Metric: 10 IP 10.100.1.12/30Metric: 10 IP 10.100.1.16/30Metric: 10 IP 10.100.1.36/30P_london_someisp.net#$atabase level-2 det P_cyprus_someisp.net.00-00IS-IS Level-2 LSP P_cyprus_somei.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLP_cyprus_somei.00-00 0x00000007 0x5236 854 0/0/0Area Address: 49.0001NLPID: 0xCCHostname: P_cyprus_someisp.netIP Address: 3.3.3.3Metric: 10 IS-Extended P_london_somei.00Metric: 10 IS-Extended P_dublin_somei.00Metric: 10 IS-Extended P_LA_someisp.n.00Metric: 10 IS-Extended PE_telaviv_som.00Metric: 10 IS-Extended PE_Jerusalem_s.02Metric: 10 IP 3.3.3.3/32Metric: 10 IP 10.100.1.0/30Metric: 10 IP 10.100.1.4/30Metric: 10 IP 10.100.1.8/30Metric: 10 IP 10.100.1.32/30Metric: 10 IP 20.0.0.0/24P_london_someisp.net#$atabase level-2 det P_LA_someisp.net.00-00IS-IS Level-2 LSP P_LA_someisp.n.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLP_LA_someisp.n.00-00 0x00000007 0xDA35 690 0/0/0Area Address: 49.0001NLPID: 0xCCHostname: P_LA_someisp.netIP Address: 4.4.4.4Metric: 10 IS-Extended P_london_somei.00Metric: 10 IS-Extended P_dublin_somei.00Metric: 10 IS-Extended P_cyprus_somei.00Metric: 10 IS-Extended PE_newyork_som.00Metric: 10 IP 4.4.4.4/32Metric: 10 IP 10.100.1.8/30Metric: 10 IP 10.100.1.12/30Metric: 10 IP 10.100.1.20/30Metric: 10 IP 10.100.1.28/30P_london_someisp.net#show isis database level-2 det PE_newyork_someisp.net.00$IS-IS Level-2 LSP PE_newyork_som.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLPE_newyork_som.00-00 0x00000005 0x2AF2 499 0/0/0Area Address: 49.0001NLPID: 0xCCHostname: PE_newyork_someisp.netIP Address: 5.5.5.5Metric: 10 IS P_london_somei.00Metric: 10 IS P_LA_someisp.n.00Metric: 10 IS-Extended P_london_somei.00Metric: 10 IS-Extended P_LA_someisp.n.00Metric: 10 IP 5.5.5.5/32Metric: 10 IP 10.100.1.24/30Metric: 10 IP 10.100.1.28/30Metric: 10 IP 5.5.5.5 255.255.255.255Metric: 10 IP 10.100.1.24 255.255.255.252Metric: 10 IP 10.100.1.28 255.255.255.252
To see the topology now and see if we are on the right track, notice that we are correct 20 metricone with dublin and the other trough cyprus with 20 metric doing load sharing.
P_london_someisp.net#show isis topologyIS-IS paths to level-2 routersSystem Id Metric Next-Hop Interface SNPAP_london_someisp.net --P_dublin_someisp.net 10 P_dublin_someisp.net Se1/2 *HDLC*P_cyprus_someisp.net 10 P_cyprus_someisp.net Se1/1 *HDLC*P_LA_someisp.net 10 P_LA_someisp.net Se1/0 *HDLC*PE_newyork_someisp.ne10 PE_newyork_someisp.neMu1 *PPP*PE_telaviv_someisp.ne20 P_dublin_someisp.net Se1/2 *HDLC*P_cyprus_someisp.net Se1/1 *HDLC*PE_Jerusalem_someisp.20 P_cyprus_someisp.net Se1/1 *HDLC*
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