Introduction to ISISftp.ipsyn.net/.../isp-routing/3-Introduction_to_ISIS-1up.pdf · 2001. 5. 1. ·...

1© 1999, Cisco Systems, Inc.

Introduction to ISISIntroduction to ISISIntroduction to ISIS

ISP/IXP WorkshopsISP/IXP WorkshopsISP/IXP Workshops

2ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

AcknowledgementsAcknowledgementsAcknowledgements

• Thanks to Stefano Previdi

• Any questions

<[email protected]>


Basic ISISBasic ISISBasic ISIS



ISIS - Intermediate System toISIS - Intermediate System toIntermediate SystemIntermediate System

• Link state protocol for IP and CLNS,RFC1195, ISO 10589

• Each router sends Hello PDU IIHs andbuild adjacencies with neighbours

• Each router creates an LSP and floodit to neighbours

• Each router receives all LSPs fromneighbours


Introduction to ISISIntroduction to ISIS

• Each router maintains database (L1, L2, orboth)

• Each router runs the Dijkstra (SPF)algorithm on the database to calculatetopology

• Each router runs Partial Route Calculation(PRC) to calculate IP routing information

• Each router has the full visibility of thenetwork (or the area)


ISIS LevelsISIS 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



• Level-1 router

Has neighbours only on the same area

Has a level-1 LSDB with all routing information for thearea

• Level-2 routerMay have neighbours in the same or other areas

Has a Level-2 LSDB with all routing information aboutinter-area

• Level-1-2 router

May have neighbours on any area.

Has two separate LSDBs: level-1 LSDB & level-2 LSDB


BackboneBackbone

• ISIS does not have a backbone

area

• The backbone is the contiguous

collection of Level-2 capable

routers


L1L2

L1L2

L1L2

L1L2

L1L2

L1-only

L2-only

L1-only

L1, L2, and L1L2 Routers

Area-2

Area-1

Area-3

Area-4

L1-only

L1-only


ISIS AreasISIS Areas

• ISIS area borders are on links, notrouters

• Each router is identified withNetwork Entity Title (NET)

• NET: An NSAP where n-selector is 0


NSAP and AddressingNSAP 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


An Addressing ExampleAn Addressing Example

39.0f01.0002.3333.3333.3333.00

39.0f01.0001.2222.2222.2222.00

39.0f01.0001.1111.1111.1111.00

39.0f01.0004.7777.7777.7777.00

39.0f01.0003.6666.6666.6666.00

39.0f01.0004.8888.8888.8888.00

39.0f01.0002.4444.4444.4444.00

Area 1

Area 3

Area 4

Area 2


• Hello PDU IIHs are exchangedbetween routers to formadjacencies

• Area addresses are exchanged inIIH PDUs

AdjacenciesAdjacencies

ISIS adjacency through IIH


Link State PDU (LSP)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)


LSP headerLSP 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


LSP ContentsLSP Contents

• The LSP contents are coded asTLV (Type, Length, Value)

Area addresses

IS neighbors

Authentication Info


LSDB contentLSDB content

• Each router maintains a separate LSDB forlevel-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


Flooding of LSPsFlooding of LSPs

• New LSPs are flooded to all neighbors

• It is necessary that all routers get allLSPs

• Each LSP has a sequence number

• 2 kinds of flooding

Flooding on a p2p link

Flooding on LAN


Flooding on a p2p linkFlooding on a p2p link

• Once the adjacency isestablished both routers sendCSNP packet

• Missing LSPs are sent by bothrouters if not present in thereceived CSNP

• Missing LSPs may be requestedthrough PSNP


Flooding on a P2P LinkFlooding on a P2P Link

Receives LSPid=x seqnr=22

It’s new.Put it in the LSDBNow let’s flood itSet SRM bitSend over p2p

Received ackClear SRM bit

Received it.My own copyhas seqnr=21.So it is new,Install it in LSDB.Acknowledge it.Maybe flood further.

LSP ->id=x seqnr=22

<- PSNPid=x seqnr=22


Flooding on a LANFlooding 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 tasksConducting the flooding over the LANCreating and updating a special LSP describing the

LAN topology (Pseudonode LSP)

• Pseudonode represents LAN (created by theDIS)


Flooding on a LAN Flooding on a LAN

LAN

Physical view Logical view

the DIS

the DIS

a Pseudonode

ES ES


Flooding on a LANFlooding on a LAN

• DIS conducts the flooding over theLAN

• DIS multicasts CSNP every 10seconds

• All routers in the LAN check theCSNP against their own LSDB (andmay ask specific re-transmissionswith PSNPs)


<- CSNPid=y seqnr=...id=x seqnr=22id=z ...

<- LSP

Flooding on a LANFlooding on a LANReceived new LSPid=x seqnr=22Install in LSDBSet SRM bitFlood the LSPClear SRM bit !

Periodic CSNPevery 10 sec's

Oh, he has an oldLSP, better resendhim latestPSNP ->

id=xseqnr=21

a LANthe DISAn ordinary

router

Dropped LSP !(bad bad) :-

Gosh, I’ve gotup-to-date y and zBut my x is old !Request latest x

Got it. Install andrun SPF, but don’tsend PSNP <- LSP

id=x seqnr=22


Complete Sequence NumberComplete Sequence NumberPDUPDU

• Describes all LSPs in your LSDB (inrange)

• If LSDB is large, multiple CSNPs aresent

• Used at 2 occasionsPeriodic multicast by DIS (every 10

seconds) to synchronise LSDB overLAN subnets

On p2p links when link comes up


Partial Sequence Number Partial Sequence Number PDUsPDUs

• PSNPs Exchanged on p2p links (ACKs)

• Two functionsAcknowledge receipt of an LSPRequest transmission of latest LSP

• PSNPs describe LSPs by its headerLSP identifierSequence numberRemaining lifetimeLSP checksum


ConfigurationConfiguration

• L1, L2, L1-L2By default cisco routers will be L1L2 routersRouters can be manually configured to

behave asLevel-1 only, Level-2 only, Level-1-2

Configuration can be done perinterface or at the router level

Area-2 Area-3 Area-1

Rtr-A Rtr-B Rtr-C



Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Router-B

Interface Loopback0ip address 192.168.1.1 255.255.255.255!Interface Pos2/0/0ip address 192.168.222.1 255.255.255.0ip router isisisis circuit-type level-2!FastEthernet4/0/0ip address 192.168.120.10 255.255.255.0ip router isisisis circuit-type level-1!router isispassive-interface Loopback0net 49.0001.1921.6800.1001.00

Router-A

Interface Loopback0ip address 192.168.1.5 255.255.255.255!interface FastEthernet0/0ip address 192.168.120.5 255.255.255.0ip router isis!router isisis-type level-1passive-interface Loopback0net 49.0001.1921.6800.1005.00



Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Router-C

Interface Loopback0ip address 192.168.2.2 255.255.255.255!Interface Pos1/0/0ip address 192.168.222.2 255.255.255.0ip router isisisis circuit-type level-2!interface Fddi3/0ip address 192.168.111.2 255.255.255.0ip router isisisis circuit-type level-1!router isispassive-interface Loopback0net 49.0002.1921.6800.2002.00

Router-D

Interface Loopback0ip address 192.168.2.4 255.255.255.255!interface Fddi6/0ip address 192.168.111.4 255.255.255.0ip router isis!router isisis-type level-1passive-interface Loopback0net 49.0002.1921.6800.2004.00


Show CLNSShow CLNS

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> sh clnsGlobal CLNS Information: 2 Interfaces Enabled for CLNS NET: 49.0001.1921.6800.1001.00 Configuration Timer: 60, Default Holding Timer: 300, PacketLifetime 64 ERPDU's requested on locally generated packets Intermediate system operation enabled (forwarding allowed) IS-IS level-1-2 Router: Routing for Area: 49.0001


Show Show clns clns neighborsneighbors

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> show clns neighborsSystem Id SNPA Interface State Holdtime Type Protocol1921.6800.2002 *PPP* PO2/0/0 Up 29 L2 IS-IS1921.6800.1005 00e0.1492.2c00 Fa4/0/0 Up 9 L1 IS-IS


Show Show clns clns interfaceinterface

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> sh clns int POS2/0/0POS2/0/0 is up, line protocol is up Checksums enabled, MTU 4470, Encapsulation PPP ERPDUs enabled, min. interval 10 msec. RDPDUs enabled, min. interval 100 msec., Addr Mask enabled Congestion Experienced bit set at 4 packets CLNS fast switching disabled CLNS SSE switching disabled DEC compatibility mode OFF for this interface Next ESH/ISH in 47 seconds Routing Protocol: IS-IS Circuit Type: level-1-2 Interface number 0x0, local circuit ID 0x100 Level-1 Metric: 10, Priority: 64, Circuit ID: 1921.6800.2002.00 Number of active level-1 adjacencies: 0 Level-2 Metric: 10, Priority: 64, Circuit ID: 1921.6800.1001.00 Number of active level-2 adjacencies: 1 Next IS-IS Hello in 2 seconds


ShowShow clns clns interface interface

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> sh clns int FastEthernet4/0/0FastEthernet4/0/0 is up, line protocol is up Checksums enabled, MTU 1497, Encapsulation SAP ERPDUs enabled, min. interval 10 msec. RDPDUs enabled, min. interval 100 msec., Addr Mask enabled Congestion Experienced bit set at 4 packets CLNS fast switching enabled CLNS SSE switching disabled DEC compatibility mode OFF for this interface Next ESH/ISH in 1 seconds Routing Protocol: IS-IS Circuit Type: level-1 Interface number 0x1, local circuit ID 0x1 Level-1 Metric: 10, Priority: 64, Circuit ID: 1921.6800.1005.01 Number of active level-1 adjacencies: 1 Next IS-IS LAN Level-1 Hello in 3 seconds


Show Show clns clns protocolprotocol

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> show clns protocolIS-IS Router: <Null Tag> System Id: 1921.6800.1001.00 IS-Type: level-1-2 Manual area address(es): 49.0001 Routing for area address(es): 49.0001 Interfaces supported by IS-IS: FastEthernet4/0/0 - IP POS2/0/0 - IP Redistributing: static Distance: 110Rtr-B>


Show Show clns clns traffictraffic

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> show clns trafficCLNS & ESIS Output: 14, Input: 436CLNS Local: 0, Forward: 0CLNS 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: 0CLNS Options: Packets 0, total 0, bad 0, GQOS 0, cngstn exprncd 0CLNS Segments: Segmented: 0, Failed: 0CLNS Broadcasts: sent: 0, rcvd: 0Echos: Rcvd 0 requests, 0 replies Sent 0 requests, 0 repliesESIS(sent/rcvd): ESHs: 0/0, ISHs: 14/15, RDs: 0/0, QCF: 0/0ISO-IGRP: Querys (sent/rcvd): 0/0 Updates (sent/rcvd): 0/0ISO-IGRP: Router Hellos: (sent/rcvd): 0/0ISO-IGRP Syntax Errors: 0IS-IS: Level-1 Hellos (sent/rcvd): 84/244IS-IS: Level-2 Hellos (sent/rcvd): 0/0IS-IS: PTP Hellos (sent/rcvd): 84/82


ShowShow clns clns traffic traffic

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

IS-IS: Level-1 LSPs sourced (new/refresh): 7/1IS-IS: Level-2 LSPs sourced (new/refresh): 5/0IS-IS: Level-1 LSPs flooded (sent/rcvd): 5/5IS-IS: Level-2 LSPs flooded (sent/rcvd): 1/6IS-IS: LSP Retransmissions: 0IS-IS: Level-1 CSNPs (sent/rcvd): 0/82IS-IS: Level-2 CSNPs (sent/rcvd): 1/1IS-IS: Level-1 PSNPs (sent/rcvd): 1/0IS-IS: Level-2 PSNPs (sent/rcvd): 3/1IS-IS: Level-1 DR Elections: 2IS-IS: Level-2 DR Elections: 1IS-IS: Level-1 SPF Calculations: 3IS-IS: Level-2 SPF Calculations: 3IS-IS: Level-1 Partial Route Calculations: 0IS-IS: Level-2 Partial Route Calculations: 0IS-IS: LSP checksum errors received: 0IS-IS: Update process queue depth: 0/200IS-IS: Update process packets dropped: 0


Show Show isis isis databasedatabase

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routersRtr-B> show isis databaseIS-IS Level-1 Link State DatabaseLSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00* 0x00000019 0x2783 1153 1/0/0 (11)1921.6800.1005.00-00 0x0000000C 0x2179 905 0/0/0 (5)1921.6800.1005.01-00 0x00000009 0x40EC 831 0/0/0 (4)

IS-IS Level-2 Link State DatabaseLSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00* 0x00000010 0xFC45 1153 0/0/0 (9)1921.6800.1001.01-00* 0x00000001 0x4CB7 1137 0/0/0 (10)1921.6800.2002.00-00 0x00000018 0x86A6 1141 0/0/0 (3)1921.6800.2002.02-00 0x00000004 0x8558 881 0/0/0 (2)


ShowShow isis isis database database

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> sh isis database 1921.6800.1001.00-00 detail

IS-IS Level-1 LSP 1921.6800.1001.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00* 0x00000006 0x4D70 991 1/0/0 (1) Area Address: 49.0001 NLPID: 0xCC IP Address: 192.168.120.10 Metric: 10 IP 192.168.222.0 255.255.255.0 Metric: 10 IP 192.168.120.0 255.255.255.0 Metric: 0 IP 192.168.1.1 255.255.255.255 Metric: 10 IS 1921.6800.1005.01 Metric: 0 ES 1921.6800.1001


ShowShow isis isis database database

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

IS-IS Level-2 LSP 1921.6800.1001.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00* 0x00000004 0x1539 980 0/0/0 (2) Area Address: 49.0001 NLPID: 0xCC IP Address: 192.168.222.1 Metric: 10 IS 1921.6800.1001.01 Metric: 10 IS 1921.6800.2002.00 Metric: 10 IP 192.168.120.0 255.255.255.0 Metric: 0 IP 192.168.1.1 255.255.255.255 Metric: 10 IP 192.168.1.5 255.255.255.255 Metric: 10 IP 192.168.222.0 255.255.255.0


Show Show isis spfisis spf-log-log

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routersRtr-B> show isis spf-log Level 1 SPF log When Duration Nodes Count Last trigger LSP Triggers00:06:30 0 1 6 1921.6800.1001.00-00 NEWAREA RTCLEARED NEWADJ NEWLSP

TLVCONTENT00:06:20 0 3 3 1921.6800.1005.01-00 NEWADJ TLVCONTENT00:06:05 0 3 2 1921.6800.1001.00-00 ATTACHFLAG LSPHEADER

Level 2 SPF log When Duration Nodes Count Last trigger LSP Triggers00:06:31 0 1 2 1921.6800.1001.00-00 RTCLEARED NEWLSP00:06:21 0 1 3 1921.6800.1001.00-00 NEWADJ TLVCODE TLVCONTENT00:06:11 0 2 1 1921.6800.2002.00-00 LSPHEADER


Network Design IssuesNetwork Design Issues

• As in all IP network designs, the key issueis the addressing lay-out

• ISIS support large number of routers in asingle area

• When using areas, use summary-addresses

• 400 routers in the backbone has been done


Network Design IssuesNetwork Design Issues

• Possible link costDefault on all interface is 10Manually 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 partitioningDesign so that backbone can not be partitioned


Scaling IssuesScaling Issues

• Areas vs. single areaUse areas where

sub-optimal routing is not an issue

areas with one single exit point

Start with L2-only everywhere is a goodchoice

Future implementation of level-1 areas will beeasier

Backbone continuity is ensured from start


Advanced ISISAdvanced ISISAdvanced ISIS




•Level-1 router

Use the closest Level-2 router to exitthe area

•Level-1-2 router

If the router has adjacencies to otherareas, it will inform the level-1 routers(intra-area) it is a potential exit pointfor the area


Non-Pseudonode LSPNon-Pseudonode LSP

• Non-Pseudonode LSP represents router

• Each router will create and flood a newNon-Pseudonode LSPWhen 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


Pseudonode LSPPseudonode LSP

• Pseudonode LSP represents LAN

• The DIS will create and flood anew Pseudonode LSPWhen a new neighbour comes up or

goes away

When refresh interval timer expires

• Reduces adjacencies and floodingover LAN subnets


LSP IdentifierLSP Identifier

• LSP identifier consists of 3 partsSource ID

System-ID of router (non-PN) or DIS (Pseudonode)

Pseudonode IDZero for router LSP, non-zero for Pseudonode LSP

LSP numberFragmentation number

• Example: 00c0.0040.1234.02-00

SystemID

PN-ID

Frag-Nr


LSP HeaderLSP Header

• LSP sequence numberUsed to determine the newest LSP

version

• LSP remaining lifetimeUsed to purge old LSPs

• LSP checksum

• LSP typelevel-1 or level2



• LSP Attached bitSet in the level-1 LSP by a L1-L2 router

if it has connectivity to another areaIndicate to the area routers (level-1)

that it is a potential exit point of thearea

Level-1 routers select the closest (bestmetric) level-2 router with the ATT-bitset



• LSP Overload bit

Set by the router when it has an overloadproblem on its LSDB

The O-bit Indicates that the router has anincomplete LSP database, and hence cannot betrusted to compute any correct routes

LSP with O-bit is used in the LSDB but topologybehind it is not calculated

Therefore other routers do not compute routeswhich would require the PDU to pass throughthe overloaded router


Flooding in depthFlooding in depth

• LSPs hold a Remaining LifetimeSet to 20 minutes by the originating routerAfter 20 min. each router must remove the LSPPrevents old LSPs to stay in the LSDB forever

• Periodic refresh of LSPsrouter resends LSP at least every 15 mins

• Can be set up to 18.7 HoursCut back on “background LSP noise”



• Network wide LSP purgesWhen a router detects a corrupted or an

expired LSP, it sets RL to 0, and floods theLSP header

All routers will now remove the LSPsimultaneous

New DIS purges old Pseudonode LSP

• LSPs hold a checksum

To detect LSDB corruption in router’s memory



• Sequence number overflow is unlikelyto happen4 Bytes seqnr is enough for 120 years

• Restarting of a routerUse sequence number = 1Neighbour will flood old LSP with old

sequence numberRouter can now set sequence number higher

then before reboot, so everyone will usethe latest LSP


LSP refreshLSP refresh

• LSP are refreshed when refresh timerexpiresRefresh-timer trigger new LSP generation and

flooding

LSP lifetime (LSP header) gives remaining time

LSP lifetime is used by routers anddecremented

When lifetime is 0, LSP purged from LSDB

Decrement allows different lifetimes per router


Configurable TimersConfigurable Timers

• Timers are configurablecsnp-interval Set CSNP interval in seconds

hello-interval Set Hello interval in seconds

hello-multiplier Set multiplier for Hello holding time

lsp-interval Set LSP transmission interval

retransmit-interval Set per-LSP retransmission interval

retransmit-throttle-interval Set interface LSP retransmission interval

lsp-gen-interval Minimum interval between regenerating same LSP

lsp-refresh-interval Set LSP refresh interval

max-lsp-lifetime Set maximum LSP lifetime

prc-interval Minimum interval between partial route calculations

spf-interval Minimum interval between SPF calculations


New FeaturesNew Features

• Metrics are 3 bytes (2^24) wide with 12.0S

• Specific routes leaking from L2 to L1 with12.0S

• Mesh-groups

Mesh-groups allow LSP flooding on selected sub-interface (thus neighbours)

Avoid flooding over all interfaces

Use is recommended on NBMA clouds


Mesh-groupMesh-group

• 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

Router-B

Router-A


Timer IssuesTimer Issues

• Timers can be adapted to large topologiesLSP, Hello, CSNP, retransmission intervals

Refresh intervalTrade-off between reduced flooding and purge of corrupted

LSPs

SPF and PRC interval

LSP maximum lifetimeMaximum value is recommended

Will significantly reduce LSP re-flooding

Old copies of LSPs may be present in LSDB (but will not beused).


Timer IssuesTimer Issues

• In some topological cases really fastconvergence is needed

HSRP and ISISWith HSRP the routers have to converge

rapidly in order to switch from/to active andpassive states

Hello-interval and lsp-gen-interval can bereduced

routing traffic will increase (possible routeflapping)


IP SummarisationIP Summarisation

• A L1L2 router will insert L1 IPprefixes into its L2 LSP

• Summarisation reduces L2 LSDBsize

• Configurable on Level-1 andLevel-2 routers

• More specific should be present


IP SummarisationIP Summarisation

• Summary addresses for level-1summary address is created forredistributed prefixes

• Summary addresses for level-2summary address is created for

redistributed prefixes

L1 routes


IP specificIP specific

• Default routingBoth level-1 and level-2 ISs can

generate a default routeA 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 L2non-PN LSP


Default routeDefault route

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-A> show ip routeGateway of last resort is 192.168.120.10 to network 0.0.0.0

C 192.168.120.0/24 is directly connected, FastEthernet0/0 192.168.1.0/32 is subnetted, 2 subnetsi L1 192.168.1.1 [115/10] via 192.168.120.10, FastEthernet0/0C 192.168.1.5 is directly connected, Loopback0i L1 192.168.222.0/24 [115/20] via 192.168.120.10, FastEthernet0/0i*L1 0.0.0.0/0 [115/10] via 192.168.120.10, FastEthernet0/0


Summary routeSummary route

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routersrouter isis summary-address 55.0.0.0 255.0.0.0 level-1-2 summary-address 192.168.1.0 255.255.255.0 level-1-2 redistribute static ip metric 44 level-1-2 passive-interface Loopback0 net 49.0001.1921.6800.1001.00!ip route 55.1.0.0 255.255.0.0 Null0ip route 55.2.0.0 255.255.0.0 Null0



Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B> sh isis da 1921.6800.1001.00-00 det

IS-IS Level-1 LSP 1921.6800.1001.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00* 0x00000062 0x770C 1028 1/0/0 (1) Area Address: 49.0001 NLPID: 0xCC IP Address: 192.168.120.10 Metric: 10 IP 192.168.222.0 255.255.255.0 Metric: 10 IP 192.168.120.0 255.255.255.0 Metric: 0 IP 192.168.1.1 255.255.255.255 Metric: 10 IS 1921.6800.1005.01 Metric: 0 ES 1921.6800.1001 Metric: 44 IP-External 171.68.0.0 255.255.0.0 Metric: 44 IP-External 172.17.0.0 255.255.0.0 Metric: 44 IP-External 144.254.0.0 255.255.0.0 Metric: 44 IP-External 55.0.0.0 255.0.0.0



Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

IS-IS Level-2 LSP 1921.6800.1001.00-00

LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL

1921.6800.1001.00-00* 0x0000005F 0xB264 1027 0/0/0 (2)

Area Address: 49.0001

NLPID: 0xCC

IP Address: 192.168.222.1

Metric: 10 IS 1921.6800.1001.01

Metric: 10 IS 1921.6800.2002.00

Metric: 44 IP-External 171.68.0.0 255.255.0.0

Metric: 10 IP 192.168.120.0 255.255.255.0



Metric: 10 IP 192.168.222.0 255.255.255.0


Metric: 0 IP 192.168.1.0 255.255.255.0


Attached bitAttached bit

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-A> show isis database

IS-IS Level-1 Link State DatabaseLSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL1921.6800.1001.00-00 0x00000006 0x4D70 666 1/0/0 (4)1921.6800.1001.01-00 0x00000001 0x1136 0 (648) 0/0/0 (3)1921.6800.1005.00-00* 0x00000002 0x356F 647 0/0/0 (1)1921.6800.1005.01-00* 0x00000001 0x50E4 647 0/0/0 (2)


Debug Debug isis adjisis adj-packets-packets

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Rtr-B# debug isis adj-packets

ISIS-Adj: Rec L1 IIH from 00e0.1492.2c00 (FastEthernet4/0/0), cir type 1, cir id 1921.6800.1005.01 ISIS-Adj: Sending L1 IIH on FastEthernet4/0/0 ISIS-Adj: Rec L1 IIH from 00e0.1492.2c00 (FastEthernet4/0/0), cir type 1, cir id 1921.6800.1005.01 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


Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Debug Debug isis spfisis spf-events-events

Rtr-B# debug isis spf-eventsISIS-SPF: Compute L1 SPTISIS-SPF: Move 1921.6800.1001.00-00 to PATHS, metric 0ISIS-SPF: thru 2147483647/2147483647/2147483647, delay 0/0/0, mtu 2147483647/2147483647/2147483647, hops 0/0/0, ticks 0/0/0ISIS-SPF: Add 1921.6800.1005.01-00 to TENT, metric 10ISIS-SPF: Next hop localISIS-SPF: Move 1921.6800.1005.01-00 to PATHS, metric 10ISIS-SPF: thru 2147483647/2147483647/2147483647, delay 0/0/0, mtu 2147483647/2147483647/2147483647, hops 0/0/0, ticks 0/0/0ISIS-SPF: considering adj to 1921.6800.1005 (FastEthernet4/0/0) metric 10ISIS-SPF: (accepted)ISIS-SPF: Add 1921.6800.1005.00-00 to TENT, metric 10ISIS-SPF: Next hop 1921.6800.1005 (FastEthernet4/0/0)ISIS-SPF: Move 1921.6800.1005.00-00 to PATHS, metric 10ISIS-SPF: Add 192.168.120.0/255.255.255.0 to IP route table, metric 20ISIS-SPF: Next hop 1921.6800.1005/192.168.120.5 (FastEthernet4/0/0) (rejected)ISIS-SPF: Add 192.168.1.5/255.255.255.255 to IP route table, metric 10ISIS-SPF: Next hop 1921.6800.1005/192.168.120.5 (FastEthernet4/0/0) (accepted)ISIS-SPF: Add 144.254.0.0/255.255.0.0 to IP route table, metric 60ISIS-SPF: Next hop 1921.6800.1005/192.168.120.5 (FastEthernet4/0/0) (rejected)


Debug Debug isis snpisis snp-packets-packets

Rtr-B# debug isis snp-packets

ISIS-SNP: Rec L1 CSNP from 1921.6800.1005 (FastEthernet4/0/0)ISIS-SNP: CSNP range 0000.0000.0000.00-00 to FFFF.FFFF.FFFF.FF-FFISIS-SNP: Same entry 1921.6800.1001.00-00, seq 93ISIS-SNP: Same entry 1921.6800.1005.00-00, seq 78ISIS-SNP: Same entry 1921.6800.1005.01-00, seq 76

Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers


Area 49.0001 Area 49.0002

Rtr-C Rtr-B

Rtr-A Rtr-D

L1L2 routers

L1routers

Debug Debug isis spfisis spf-update-update

Rtr-B(config)#int fa4/0/0Rtr-B(config-if)#isis metric 13Rtr-B(config-if)#^ZRtr-B#ISIS-SPF-TRIG: L1, new metricISIS-Update: Building L1 LSPISIS-Update: TLV contents different, code 80ISIS-Update: TLV contents different, code 2ISIS-SPF-TRIG: L1, 1921.6800.1001.00-00 TLV contents changed, code 2ISIS-Update: Full SPF requiredISIS-Update: Sending L1 LSP 1921.6800.1001.00-00, seq 96, ht 1199 on FastEthernet4/0/0ISIS-SNP: Rec L1 CSNP from 1921.6800.1005 (FastEthernet4/0/0)ISIS-Stats: Compute L1 SPTISIS-Stats: Complete L1 SPT, Compute time 0.000, 3 nodes, 2 links on SPT, 0 suspendsISIS-Update: Building L2 LSPISIS-Update: TLV contents different, code 80ISIS-Update: TLV contents different, code 80ISIS-Update: Leaf routes changedISIS-Update: Sending L2 LSP 1921.6800.1001.00-00, seq 96, ht 1199 on POS2/0/0ISIS-Update: Building L2 LSPISIS-Update: Rate limiting L2 LSP 1921.6800.1001.00-00, seq 97ISIS-Update: TLV contents different, code 80ISIS-Update: TLV contents different, code 80ISIS-Update: Leaf routes changedISIS-Update: Sending L2 LSP 1921.6800.1001.00-00, seq 97, ht 1196 on POS2/0/0


2 routers with the same System-ID2 routers with the same System-ID

• This is an illegal configuration

• “Possible duplicate System-IDs” warning2 Routers with same System-ID will see each

others LSP as their own, but not correct. Sothey will generate a new LSP with highersequence number.

This is an infinite ping-pong gameWhen an IS has to regenerate its LSP frequently,

it will generate the above warning


<- LSPid=x.00-00 seqnr=1

Two ISs Have Same SystemID xTwo ISs Have Same SystemID x

this IS has generatedan LSP withLSPid=x.00-00and seqnr=22

we ignore the LSPwith seqnr=1, aswe think it is old

we try to overrideseqnr=23

now this IS bootswith SystemID=x

during the floodingthey will notice themismatch.

when we receiveseqnr=22, we tryto override it bycreating a new LSPwith seqnr=23



LSP ->id=x.00-00 seqnr=24

LSP ->id=x.00-00 seqnr=22

etc, etc, etc .....


LSP Corruption StormsLSP Corruption Storms

• Suppose a link that corrupt packets,but regenerates the data-link layerCRC

Example is frame-relay to ATM switches

• ISIS gets LSP with incorrectchecksum

• Receiving router will do network-wide purge


LSP Corruption StormsLSP Corruption Storms

• Originating router will regenerateLSP

• This can create purge/re-floodstorms

• Configure “ignore-LSP-errors”

• New spec will allow the LSP discardwithout network-wide purge


LSP ->id=x.00-00seqnr=22RL=1200

LSP Checksum ThrashingLSP Checksum Thrashing

<- LSPid=x.00-00seqnr=22RL=0

LSP -> id=x.00-00 seqnr=23 RL=1200


this IS got the LSP witha checksum error.So it purges it.

<- LSPid=x.00-00seqnr=22RL=0


someone purgedour LSP. let’sregenerate it

And on and on and ...

78

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Introduction to ISISftp.ipsyn.net/.../isp-routing/3-Introduction_to_ISIS-1up.pdf · 2001. 5. 1. ·...

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