Introduction to ISIS. ISP/IXP Workshops. 1999, Cisco Systems, Inc.

Transcription

1 Introduction to ISIS ISP/IXP Workshops 1999, Cisco Systems, Inc. 1

2 Acknowledgements Thanks to Stefano Previdi Any questions 2

3 Basic ISIS 3

4 ISIS - Intermediate System to Intermediate System Link state protocol for IP and CLNS, RFC1195, ISO Each router sends Hello PDU IIHs and build adjacencies with neighbours Each router creates an LSP and flood it to neighbours Each router receives all LSPs from neighbours 4

5 Introduction to ISIS Each router maintains database (L1, L2, or both) Each router runs the Dijkstra (SPF) algorithm on the database to calculate topology Each router runs Partial Route Calculation (PRC) to calculate IP routing information Each router has the full visibility of the network (or the area) 5

6 ISIS Levels ISIS has a 2 layer hierarchy Level-2 (the backbone) Level-1 (the areas) A router can be Level-1 (L1) router Level-2 (L2) router Level-1-2 (L1L2) router 6

7 Level-1 router ISIS Levels Has neighbours only on the same area Has a level-1 LSDB with all routing information for the area Level-2 router May have neighbours in the same or other areas Has a Level-2 LSDB with all routing information about inter-area Level-1-2 router May have neighbours on any area. Has two separate LSDBs: level-1 LSDB & level-2 LSDB 7

8 Backbone ISIS does not have a backbone area The backbone is the contiguous collection of Level-2 capable routers 8

9 L1, L2, and L1L2 Routers L1-only L1L2 Area-3 Area-2 L1L2 L1-only L1L2 L2-only Area-4 Area-1 L1L2 L1L2 L1-only L1-only 9

10 ISIS Areas ISIS area borders are on links, not routers Each router is identified with Network Entity Title (NET) NET: An NSAP where n-selector is 0 10

11 NSAP and Addressing NSAP: Network Service Access Point Total length between 8 and 20 bytes Area Address: variable length field (up to 13 bytes) System ID: defines an ES or IS in an area. NSEL: N-selector. identifies a network service user (transport entity or the IS network entity itself) NET: the address of the network entity itself 11

12 An Addressing Example 39.0f Area f f Area f Area f Area f f

13 Adjacencies Hello PDU IIHs are exchanged between routers to form adjacencies ISIS adjacency through IIH Area addresses are exchanged in IIH PDUs 13

14 Link State PDU (LSP) Each router creates an LSP and flood it to neighbors A level-1 router will create level-1 LSP(s) A level-2 router will create level-2 LSP(s) A level-1-2 router will create level-1 LSP(s) and level-2 LSP(s) 14

15 LSP header LSPs have Fixed header TLV coded contents The LSP header contains LSP-id Sequence number Remaining Lifetime Checksum Type of LSP (level-1, level-2) Attached bit Overload bit 15

16 LSP Contents The LSP contents are coded as TLV (Type, Length, Value) Area addresses IS neighbors Authentication Info 16

17 LSDB content Each router maintains a separate LSDB for level-1 and level-2 LSPs LSP headers and contents SRM bits: set per interface when router has to flood this LSP SSN bits: set per interface when router has to send a PSNP for this LSP 17

18 Flooding of LSPs New LSPs are flooded to all neighbors It is necessary that all routers get all LSPs Each LSP has a sequence number 2 kinds of flooding Flooding on a p2p link Flooding on LAN 18

19 Flooding on a p2p link Once the adjacency is established both routers send CSNP packet Missing LSPs are sent by both routers if not present in the received CSNP Missing LSPs may be requested through PSNP 19

20 Flooding on a P2P Link Receives LSP id=x seqnr=22 It s new. Put it in the LSDB Now let s flood it Set SRM bit Send over p2p Received ack Clear SRM bit LSP -> id=x seqnr=22 <- PSNP id=x seqnr=22 Received it. My own copy has seqnr=21. So it is new, Install it in LSDB. Acknowledge it. Maybe flood further. 20

21 Flooding on a LAN There s a Designated Router (DIS) DIS election is based on priority Breaking-tie is the highest MAC address DIS has two tasks Conducting the flooding over the LAN Creating and updating a special LSP describing the LAN topology (Pseudonode LSP) Pseudonode represents LAN (created by the DIS) 21

22 Flooding on a LAN Physical view the DIS Logical view the DIS a Pseudonode LAN ES ES 22

23 Flooding on a LAN DIS conducts the flooding over the LAN DIS multicasts CSNP every 10 seconds All routers in the LAN check the CSNP against their own LSDB (and may ask specific re-transmissions with PSNPs) 23

24 Flooding on a LAN An ordinary router Dropped LSP! (bad bad) :- Gosh, I ve got up-to-date y and z But my x is old! Request latest x a LAN the DIS <- LSP <- CSNP id=y seqnr=... id=x seqnr=22 id=z... PSNP -> Got it. Install and run SPF, but don t id=x send PSNP seqnr=21 <- LSP id=x seqnr=22 Received new LSP id=x seqnr=22 Install in LSDB Set SRM bit Flood the LSP Clear SRM bit! Periodic CSNP every 10 sec's Oh, he has an old LSP, better resend him latest 24

25 Complete Sequence Number PDU Describes all LSPs in your LSDB (in range) If LSDB is large, multiple CSNPs are sent Used at 2 occasions Periodic multicast by DIS (every 10 seconds) to synchronise LSDB over LAN subnets On p2p links when link comes up 25

26 Partial Sequence Number PDUs PSNPs Exchanged on p2p links (ACKs) Two functions Acknowledge receipt of an LSP Request transmission of latest LSP PSNPs describe LSPs by its header LSP identifier Sequence number Remaining lifetime LSP checksum 26

27 Configuration Area-2 Area-1 Area-3 Rtr-C Rtr-A Rtr-B L1, L2, L1-L2 By default cisco routers will be L1L2 routers Routers can be manually configured to behave as Level-1 only, Level-2 only, Level-1-2 Configuration can be done per interface or at the router level 27

28 Configuration L1L2 routers Rtr-B Rtr-C Router-B Area Area Interface Loopback0 ip address ! Interface Pos2/0/0 ip address ip router isis isis circuit-type level-2! FastEthernet4/0/0 ip address ip router isis isis circuit-type level-1! router isis passive-interface Loopback0 net Rtr-A Rtr-D L1routers Router-A Interface Loopback0 ip address ! interface FastEthernet0/0 ip address ip router isis! router isis is-type level-1 passive-interface Loopback0 net

29 Configuration L1L2 routers Rtr-B Rtr-C Router-C Area Area Interface Loopback0 ip address ! Interface Pos1/0/0 ip address ip router isis isis circuit-type level-2! interface Fddi3/0 ip address ip router isis isis circuit-type level-1! router isis passive-interface Loopback0 net Rtr-A Rtr-D L1routers Router-D Interface Loopback0 ip address ! interface Fddi6/0 ip address ip router isis! router isis is-type level-1 passive-interface Loopback0 net

30 Show CLNS L1L2 routers Rtr-B Rtr-C Area Area Rtr-B> sh clns Global CLNS Information: 2 Interfaces Enabled for CLNS NET: Configuration Timer: 60, Default Holding Timer: 300, Packet Lifetime 64 ERPDU's requested on locally generated packets Intermediate system operation enabled (forwarding allowed) IS-IS level-1-2 Router: Routing for Area: Rtr-A Rtr-D L1routers 30

31 Show clns neighbors L1L2 routers Rtr-B Rtr-C Area Area Rtr-A Rtr-D L1routers Rtr-B> show clns neighbors System Id SNPA Interface State Holdtime Type Protocol *PPP* PO2/0/0 Up 29 L2 IS-IS e c00 Fa4/0/0 Up 9 L1 IS-IS 31

32 Show clns interface Rtr-B> sh clns int POS2/0/0 POS2/0/0 is up, line protocol is up Checksums enabled, MTU 4470, Encapsulation PPP ERPDUs enabled, min. interval 10 msec. L1L2 routers RDPDUs enabled, min. interval 100 msec., Addr Mask enabled Congestion Experienced bit set at 4 packets CLNS fast switching disabled CLNS SSE switching disabled Rtr-B Rtr-C DEC compatibility mode OFF for this interface Next ESH/ISH in 47 seconds Routing Protocol: IS-IS Area Area Circuit Type: level-1-2 Rtr-A Interface number 0x0, local circuit ID 0x100 L1routers Rtr-D Level-1 Metric: 10, Priority: 64, Circuit ID: Number of active level-1 adjacencies: 0 Level-2 Metric: 10, Priority: 64, Circuit ID: Number of active level-2 adjacencies: 1 Next IS-IS Hello in 2 seconds 32

33 Show clns interface Rtr-B> sh clns int FastEthernet4/0/0 FastEthernet4/0/0 is up, line protocol is up Checksums enabled, MTU 1497, Encapsulation SAP ERPDUs enabled, min. interval 10 msec. L1L2 routers RDPDUs enabled, min. interval 100 msec., Addr Mask enabled Congestion Experienced bit set at 4 packets CLNS fast switching enabled CLNS SSE switching disabled Rtr-B Rtr-C DEC compatibility mode OFF for this interface Next ESH/ISH in 1 seconds Routing Protocol: IS-IS Area Area Circuit Type: level-1 Rtr-A Rtr-D Interface number 0x1, local circuit ID 0x1 L1routers Level-1 Metric: 10, Priority: 64, Circuit ID: Number of active level-1 adjacencies: 1 Next IS-IS LAN Level-1 Hello in 3 seconds 33

34 Show clns protocol Rtr-B> show clns protocol IS-IS Router: <Null Tag> System Id: IS-Type: level-1-2 Manual area address(es): Routing for area address(es): L1L2 routers Interfaces supported by IS-IS: FastEthernet4/0/0 - IP POS2/0/0 - IP Redistributing: Rtr-B Rtr-C static Distance: 110 Area Area Rtr-B> Rtr-A Rtr-D L1routers 34

35 Show clns traffic Rtr-B> show clns traffic CLNS & ESIS Output: 14, Input: 436 CLNS Local: 0, Forward: 0 CLNS Discards: Hdr Syntax: 0, Checksum: 0, Lifetime: 0, Output cngstn: 0 No Route: 0, Dst Unreachable 0, Encaps. Failed: 0 NLP Unknown: 0, Not an IS: 0 CLNS Options: Packets 0, total 0, bad 0, GQOS 0, cngstn exprncd 0 CLNS Segments: Segmented: 0, Failed: 0 CLNS Broadcasts: sent: 0, rcvd: 0 Echos: Rcvd 0 requests, 0 replies Sent 0 requests, 0 replies ESIS(sent/rcvd): ESHs: 0/0, ISHs: 14/15, RDs: 0/0, QCF: 0/0 L1L2 routers ISO-IGRP: Querys (sent/rcvd): 0/0 Updates (sent/rcvd): 0/0 ISO-IGRP: Router Hellos: (sent/rcvd): 0/0 ISO-IGRP Syntax Errors: 0 IS-IS: Level-1 Hellos (sent/rcvd): 84/244 Rtr-B Rtr-C IS-IS: Level-2 Hellos (sent/rcvd): 0/0 IS-IS: PTP Hellos (sent/rcvd): 84/82 Area Area Rtr-A Rtr-D L1routers 35

36 Show clns traffic IS-IS: Level-1 LSPs sourced (new/refresh): 7/1 IS-IS: Level-2 LSPs sourced (new/refresh): 5/0 IS-IS: Level-1 LSPs flooded (sent/rcvd): 5/5 IS-IS: Level-2 LSPs flooded (sent/rcvd): 1/6 IS-IS: LSP Retransmissions: 0 IS-IS: Level-1 CSNPs (sent/rcvd): 0/82 IS-IS: Level-2 CSNPs (sent/rcvd): 1/1 IS-IS: Level-1 PSNPs (sent/rcvd): 1/0 IS-IS: Level-2 PSNPs (sent/rcvd): 3/1 IS-IS: Level-1 DR Elections: 2 IS-IS: Level-2 DR Elections: 1 IS-IS: Level-1 SPF Calculations: 3 IS-IS: Level-2 SPF Calculations: 3 IS-IS: Level-1 Partial Route Calculations: 0 IS-IS: Level-2 Partial Route Calculations: 0 IS-IS: LSP checksum errors received: 0 IS-IS: Update process queue depth: 0/200 IS-IS: Update process packets dropped: 0 Rtr-B L1L2 routers Rtr-C Area Area Rtr-A Rtr-D L1routers 36

37 Show isis database L1L2 routers Rtr-B Rtr-C Area Area Rtr-A Rtr-D Rtr-B> show isis database L1routers IS-IS Level-1 Link State Database LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x x /0/0 (11) x C 0x /0/0 (5) x x40EC 831 0/0/0 (4) IS-IS Level-2 Link State Database LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x xFC /0/0 (9) * 0x x4CB /0/0 (10) x x86A /0/0 (3) x x /0/0 (2) 37

38 Show isis database Rtr-B> sh isis database detail IS-IS Level-1 LSP LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x x4D /0/0 (1) Area Address: NLPID: 0xCC IP Address: Metric: 10 IP Metric: 10 IP Metric: 0 IP Metric: 10 IS Metric: 0 ES Rtr-B L1L2 routers Rtr-C Area Area Rtr-A Rtr-D L1routers 38

39 Show isis database IS-IS Level-2 LSP LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x x /0/0 (2) Area Address: NLPID: 0xCC IP Address: Metric: 10 IS Metric: 10 IS Metric: 10 IP Metric: 0 IP Metric: 10 IP Metric: 10 IP Rtr-B L1L2 routers Rtr-C Area Area Rtr-A Rtr-D L1routers 39

40 Show isis spf-log L1L2 routers Rtr-B Rtr-C Area Area Rtr-B> show isis spf-log Level 1 SPF log When Duration Nodes Count Rtr-A Rtr-D L1routers Last trigger LSP Triggers 00:06: NEWAREA RTCLEARED NEWADJ NEWLSP TLVCONTENT 00:06: NEWADJ TLVCONTENT 00:06: ATTACHFLAG LSPHEADER Level 2 SPF log When Duration Nodes Count Last trigger LSP Triggers 00:06: RTCLEARED NEWLSP 00:06: NEWADJ TLVCODE TLVCONTENT 00:06: LSPHEADER 40

41 Network Design Issues As in all IP network designs, the key issue is the addressing lay-out ISIS support large number of routers in a single area When using areas, use summaryaddresses 400 routers in the backbone has been done 41

42 Network Design Issues Possible link cost Default on all interface is 10 Manually configured according to routing strategy Summary address cost Equal to the best more specific cost Plus cost to reach neighbor of best specific Backbone has to be contiguous Ensure continuity by redundancy Area partitioning Design so that backbone can not be partitioned 42

43 Scaling Issues Areas vs. single area Use areas where sub-optimal routing is not an issue areas with one single exit point Start with L2-only everywhere is a good choice Future implementation of level-1 areas will be easier Backbone continuity is ensured from start 43

44 Advanced ISIS 44

45 ISIS Levels Level-1 router Use the closest Level-2 router to exit the area Level-1-2 router If the router has adjacencies to other areas, it will inform the level-1 routers (intra-area) it is a potential exit point for the area 45

46 Non-Pseudonode LSP Non-Pseudonode LSP represents router Each router will create and flood a new Non-Pseudonode LSP When a new neighbor comes up or goes away When new IP prefixes are inserted or removed When the metric of a link is changed When refresh interval timer expires 46

47 Pseudonode LSP Pseudonode LSP represents LAN The DIS will create and flood a new Pseudonode LSP When a new neighbour comes up or goes away When refresh interval timer expires Reduces adjacencies and flooding over LAN subnets 47

48 LSP Identifier LSP identifier consists of 3 parts Source ID System-ID of router (non-pn) or DIS (Pseudonode) Pseudonode ID Zero for router LSP, non-zero for Pseudonode LSP LSP number Fragmentation number Example: 00c SystemID PN-ID Frag-Nr 48

49 LSP Header LSP sequence number Used to determine the newest LSP version LSP remaining lifetime Used to purge old LSPs LSP checksum LSP type level-1 or level2 49

50 LSP Header LSP Attached bit Set in the level-1 LSP by a L1-L2 router if it has connectivity to another area Indicate to the area routers (level-1) that it is a potential exit point of the area Level-1 routers select the closest (best metric) level-2 router with the ATT-bit set 50

51 LSP Header LSP Overload bit Set by the router when it has an overload problem on its LSDB The O-bit Indicates that the router has an incomplete LSP database, and hence cannot be trusted to compute any correct routes LSP with O-bit is used in the LSDB but topology behind it is not calculated Therefore other routers do not compute routes which would require the PDU to pass through the overloaded router 51

52 Flooding in depth LSPs hold a Remaining Lifetime Set to 20 minutes by the originating router After 20 min. each router must remove the LSP Prevents old LSPs to stay in the LSDB forever Periodic refresh of LSPs router resends LSP at least every 15 mins Can be set up to 18.7 Hours Cut back on background LSP noise 52

53 Flooding in depth Network wide LSP purges When a router detects a corrupted or an expired LSP, it sets RL to 0, and floods the LSP header All routers will now remove the LSP simultaneous New DIS purges old Pseudonode LSP LSPs hold a checksum To detect LSDB corruption in router s memory 53

54 Flooding in depth Sequence number overflow is unlikely to happen 4 Bytes seqnr is enough for 120 years Restarting of a router Use sequence number = 1 Neighbour will flood old LSP with old sequence number Router can now set sequence number higher then before reboot, so everyone will use the latest LSP 54

55 LSP refresh LSP are refreshed when refresh timer expires Refresh-timer trigger new LSP generation and flooding LSP lifetime (LSP header) gives remaining time LSP lifetime is used by routers and decremented When lifetime is 0, LSP purged from LSDB Decrement allows different lifetimes per router 55

56 Configurable Timers Timers are configurable csnp-interval hello-interval hello-multiplier lsp-interval retransmit-interval Set CSNP interval in seconds Set Hello interval in seconds Set multiplier for Hello holding time Set LSP transmission interval Set per-lsp retransmission interval retransmit-throttle-interval Set interface LSP retransmission interval lsp-gen-interval lsp-refresh-interval max-lsp-lifetime prc-interval spf-interval Minimum interval between regenerating same LSP Set LSP refresh interval Set maximum LSP lifetime Minimum interval between partial route calculations Minimum interval between SPF calculations 56

57 New Features Metrics are 3 bytes (2^24) wide with 12.0S Specific routes leaking from L2 to L1 with 12.0S Mesh-groups Mesh-groups allow LSP flooding on selected subinterface (thus neighbours) Avoid flooding over all interfaces Use is recommended on NBMA clouds 57

58 Mesh-group Router-B Router-A Router-B will receive 4 instances of Router-A LSP Mesh-groups will allow to flood only on selected links Mesh-groups are configured on interfaces 58

59 Timer Issues Timers can be adapted to large topologies LSP, Hello, CSNP, retransmission intervals Refresh interval Trade-off between reduced flooding and purge of corrupted LSPs SPF and PRC interval LSP maximum lifetime Maximum value is recommended Will significantly reduce LSP re-flooding Old copies of LSPs may be present in LSDB (but will not be used). 59

60 Timer Issues In some topological cases really fast convergence is needed HSRP and ISIS With HSRP the routers have to converge rapidly in order to switch from/to active and passive states Hello-interval and lsp-gen-interval can be reduced routing traffic will increase (possible route flapping) 60

61 IP Summarisation A L1L2 router will insert L1 IP prefixes into its L2 LSP Summarisation reduces L2 LSDB size Configurable on Level-1 and Level-2 routers More specific should be present 61

62 IP Summarisation Summary addresses for level-1 summary address is created for redistributed prefixes Summary addresses for level-2 summary address is created for redistributed prefixes L1 routes 62

63 IP specific Default routing Both level-1 and level-2 ISs can generate a default route A level-1 IS will always prefer a level-1 default route BEFORE using the ATT (closest level-2 capable IS) Creates an IP External prefix in L1 or L2 non-pn LSP 63

64 Default route L1L2 routers Rtr-B Rtr-C Area Area Rtr-A> show ip route Gateway of last resort is to network C /24 is directly connected, FastEthernet0/ /32 is subnetted, 2 subnets i L [115/10] via , FastEthernet0/0 C is directly connected, Loopback0 i L /24 [115/20] via , FastEthernet0/0 i*l /0 [115/10] via , FastEthernet0/0 Rtr-A Rtr-D L1routers 64

65 Summary route L1L2 routers Rtr-B Rtr-C Area Area router isis summary-address level-1-2 summary-address level-1-2 redistribute static ip metric 44 level-1-2 passive-interface Loopback0 net ! ip route Null0 ip route Null0 Rtr-A Rtr-D L1routers 65

66 Summary route Rtr-B> sh isis da det IS-IS Level-1 LSP LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x x770C /0/0 (1) Area Address: NLPID: 0xCC IP Address: Metric: 10 IP Metric: 10 IP Metric: 0 IP Metric: 10 IS Metric: 0 ES Metric: 44 IP-External Metric: 44 IP-External Metric: 44 IP-External Metric: 44 IP-External Rtr-B L1L2 routers Rtr-C Area Area Rtr-A Rtr-D L1routers 66

67 Summary route IS-IS Level-2 LSP LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL * 0x F 0xB /0/0 (2) Area Address: NLPID: 0xCC IP Address: Metric: 10 IS L1L2 routers Metric: 10 IS Metric: 44 IP-External Metric: 10 IP Rtr-B Rtr-C Metric: 44 IP-External Metric: 44 IP-External Metric: 10 IP Area Area Metric: 44 IP-External Rtr-A Rtr-D L1routers Metric: 0 IP

68 Attached bit L1L2 routers Rtr-B Rtr-C Area Area Rtr-A Rtr-D L1routers Rtr-A> show isis database IS-IS Level-1 Link State Database LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL x x4D /0/0 (4) x x (648) 0/0/0 (3) * 0x x356F 647 0/0/0 (1) * 0x x50E /0/0 (2) 68

69 Debug isis adj-packets L1L2 routers Rtr-B Rtr-C Area Area Rtr-A Rtr-D L1routers Rtr-B# debug isis adj-packets ISIS-Adj: Rec L1 IIH from 00e c00 (FastEthernet4/0/0), cir type 1, cir id ISIS-Adj: Sending L1 IIH on FastEthernet4/0/0 ISIS-Adj: Rec L1 IIH from 00e c00 (FastEthernet4/0/0), cir type 1, cir id ISIS-Adj: Sending serial IIH on POS2/0/0 ISIS-Adj: Rec serial IIH from *PPP* on POS2/0/0, cir type 3, cir id 00 69

70 Debug isis spf-events L1L2 routers Rtr-B Rtr-C Area Area Rtr-B# debug isis spf-events ISIS-SPF: Compute L1 SPT Rtr-A Rtr-D ISIS-SPF: Move to PATHS, metric 0 L1routers ISIS-SPF: thru / / , delay 0/0/0, mtu / / , hops 0/0/0, ticks 0/0/0 ISIS-SPF: Add to TENT, metric 10 ISIS-SPF: Next hop local ISIS-SPF: Move to PATHS, metric 10 ISIS-SPF: thru / / , delay 0/0/0, mtu / / , hops 0/0/0, ticks 0/0/0 ISIS-SPF: considering adj to (FastEthernet4/0/0) metric 10 ISIS-SPF: (accepted) ISIS-SPF: Add to TENT, metric 10 ISIS-SPF: Next hop (FastEthernet4/0/0) ISIS-SPF: Move to PATHS, metric 10 ISIS-SPF: Add / to IP route table, metric 20 ISIS-SPF: Next hop / (FastEthernet4/0/0) (rejected) ISIS-SPF: Add / to IP route table, metric 10 ISIS-SPF: Next hop / (FastEthernet4/0/0) (accepted) ISIS-SPF: Add / to IP route table, metric 60 ISIS-SPF: Next hop / (FastEthernet4/0/0) (rejected) 70

71 Debug isis snp-packets L1L2 routers Rtr-B Rtr-C Area Area Rtr-A Rtr-D L1routers Rtr-B# debug isis snp-packets ISIS-SNP: Rec L1 CSNP from (FastEthernet4/0/0) ISIS-SNP: CSNP range to FFFF.FFFF.FFFF.FF-FF ISIS-SNP: Same entry , seq 93 ISIS-SNP: Same entry , seq 78 ISIS-SNP: Same entry , seq 76 71

72 Debug isis spf-update L1L2 routers Rtr-B Rtr-B(config)#int fa4/0/0 Rtr-B(config-if)#isis metric 13 Rtr-B(config-if)#^Z Rtr-B# ISIS-SPF-TRIG: L1, new metric ISIS-Update: Building L1 LSP ISIS-Update: TLV contents different, code 80 ISIS-Update: TLV contents different, code 2 ISIS-SPF-TRIG: L1, TLV contents changed, code 2 ISIS-Update: Full SPF required ISIS-Update: Sending L1 LSP , seq 96, ht 1199 on FastEthernet4/0/0 ISIS-SNP: Rec L1 CSNP from (FastEthernet4/0/0) ISIS-Stats: Compute L1 SPT ISIS-Stats: Complete L1 SPT, Compute time 0.000, 3 nodes, 2 links on SPT, 0 suspends ISIS-Update: Building L2 LSP ISIS-Update: TLV contents different, code 80 ISIS-Update: TLV contents different, code 80 ISIS-Update: Leaf routes changed ISIS-Update: Sending L2 LSP , seq 96, ht 1199 on POS2/0/0 ISIS-Update: Building L2 LSP ISIS-Update: Rate limiting L2 LSP , seq 97 ISIS-Update: TLV contents different, code 80 ISIS-Update: TLV contents different, code 80 ISIS-Update: Leaf routes changed ISIS-Update: Sending L2 LSP , seq 97, ht 1196 on POS2/0/0 Rtr-C Area Area Rtr-A Rtr-D L1routers 72

73 2 routers with the same System-ID This is an illegal configuration Possible duplicate System-IDs warning 2 Routers with same System-ID will see each others LSP as their own, but not correct. So they will generate a new LSP with higher sequence number. This is an infinite ping-pong game When an IS has to regenerate its LSP frequently, it will generate the above warning 73

74 Two ISs Have Same SystemID x this IS has generated an LSP with LSPid=x and seqnr=22 we ignore the LSP with seqnr=1, as we think it is old we try to override seqnr=23 etc, etc, etc... LSP -> id=x seqnr=22 LSP -> id=x seqnr=24 <- LSP id=x seqnr=1 <- LSP id=x seqnr=23 <- LSP id=x seqnr=25 now this IS boots with SystemID=x during the flooding they will notice the mismatch. when we receive seqnr=22, we try to override it by creating a new LSP with seqnr=23 74

75 LSP Corruption Storms Suppose a link that corrupt packets, but regenerates the data-link layer CRC Example is frame-relay to ATM switches ISIS gets LSP with incorrect checksum Receiving router will do networkwide purge 75

76 LSP Corruption Storms Originating router will regenerate LSP This can create purge/re-flood storms Configure ignore-lsp-errors New spec will allow the LSP discard without network-wide purge 76

77 LSP Checksum Thrashing LSP -> id=x seqnr=22 RL=1200 someone purged our LSP. let s regenerate it LSP -> id=x seqnr=23 RL=1200 <- LSP id=x seqnr=22 RL=0 LSP -> id=x seqnr=22 RL=1193 LSP -> id=x seqnr=23 RL=1194 this IS got the LSP with a checksum error. So it purges it. <- LSP id=x seqnr=22 RL=0 And on and on and... 77

78 78