IS-IS protocol

1© 2003 Cisco Systems, Inc. All rights reserved.Session NumberPresentation_ID

IS-IS Introduction

222© 2003 Cisco Systems, Inc. All rights reserved.Presentation_IDRST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.

Agenda

• IS-IS Overview• CLNS Addressing

• IS-IS Levels• IS-IS PDUs• LSP Header

• Flooding• Configuration

• New Features• Deployment Scenarios


IS-IS Overview


Terminology

• AFI: Authority and Format Identifier (the first octet of all OSI NSAP addresses—identifies format of the rest of the address)

• CLNP: Connection-Less Network Protocol (ISO 8473—the OSI connectionless network layer protocol—very similar to IP)

• ES: End System (the OSI term for a host)• IS: Intermediate System (the OSI term for a router)• ES-IS: End System to Intermediate System routing exchange

protocol (ISO 9542—OSI protocol between routers and end systems)

• IS-IS: Intermediate System to Intermediate System routing exchange protocol (the ISO protocol for routing within a single routing domain)

• IS-IS Hello: A Hello packet (defined by the IS-IS protocol)• LSP: Link State Packet (a type of packet used by the IS-IS protocol)• TLV: Type Length Value


IS-IS Overview

• IS-IS was originally designed for use as a dynamic routing protocol for the ISO Connectionless Network Protocol (CLNP); (ISO10589 or RFC 1142)

• Adapted for routing IP in addition to CLNP (RFC1195) as integrated or dual IS-IS

• IS-IS is a Link State Protocol similar to the Open Shortest Path First (OSPF)


IS-IS Overview (Cont.)

• IS-IS is an Interior Gateway Protocol (IGP) used for routing within an Autonomous System (AS) also referred to as a routing domain

• BGP is normally used dynamic routing between IP domains

• ISO-IGRP is a Cisco proprietary routing protocol that can be used between CLNP domains



• 3 network protocols play together to deliver the ISO defined Connectionless Network ServiceØ CLNP

Ø IS-IS

Ø ES-IS—End System to Intermediate System Protocol

• All 3 protocols independently ride over layer 2



• CLNP is the ISO equivalent of IP for datagram delivery services (ISO 8473, RFC 994)

• IS-IS carries routing information; integrated IS-IS works within the ISO CNLS framework if even used for routing IP (ISO 8473, RFC 1142)

• ES-IS is a dynamic protocol for discovering layer 2 adjacencies (ISO9542, RFC 995); hosts and routers discover each other via ES-IS


CLNS Addressing


Area ID SEL

CLNS Addressing

• CLNS addressing consists of 3 parts:ØArea—variable

ØIDØSEL(ector)


• ISO/IEC 10589 distinguishes only 3 fields in the NSAP address format

• Area address: Variable length field composed of high order octets of the NSAP excluding the SystemID and SEL fields

• SystemID: Defines an ES or IS in an area; Cisco implements a fixed length of 6 octets for the SystemID

• NSEL: Selector, also designated as N-selector; it is the last byte of the NSAP and identifies a network service user (transport entity or the IS network entity itself)

IDP

AFI IDI

DSP

High Order DSP System ID NSELVariable Length Area Address 6 Bytes 1 Byte

NSAPs and Addressing


NSAPs and Addressing (Cont.)

• NSAP: Network Service Access Point • An NSAP has an address that consists of 3 parts

Ø Variable length area-address

Ø 6 Byte system ID

Ø Byte n-selector (indicating transport layer)

Ø Total length between 8 and 20 bytes


NETs versus NSAPs

• NET: Network Entity Title• Is the address of the network entity itself

• A NET is an NSAP where n-selector is 0 (common practice)

• A NET implies the routing layer of the IS itself (no transport layer)

• ISs (routers) do not have any transport layer (selector=0)

• Multiple NETs are like secondary IP addresses; only use them when merging or splitting areas


OSI Addressing—NET and System Identifier Rules• NET must begin with an octet

Ø47.xxxx....;Ø0111.xxxx... Not 111.xxxx...

• NET must end with a single octet set to 00, identifying network entity (for example, router) itselfØ...xxxx.00

• System ID normally six octets (on Cisco six!) and has to be the same length everywhere

• Examples: 47.0001.0000.0c12.3456.0001.1921.6811.1003.00

1047.0001.1234.5678.9101.00


CLNS Addressing: NSAP Examples

• Example 1:Ø47.0001.aaaa.bbbb.cccc.00Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00

• Example 2:Ø39.0f01.0002.0000.0c00.1111.00Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00

• Example 3:Ø49.0002.0000.0000.0007.00Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00


Identifying Systems in IS-IS

• The area address uniquely identifies the routing area and the System ID identifies each node ØAll routers within an area must use the same area address

ØAn ES may be adjacent to a level-1 router only if they both share a common area address

ØArea address is used in level-2 routing


Identifying Systems in IS-IS—System ID

• System ID may be the MAC address (CLNS) or IP address of an interface (IP world)ØSystem ID used in level-1 routing and has to be unique within an area (and of same length)

ØSystem ID has to be unique within level-2 routers that form routing domain

ØGeneral recommendation: domain-wide unique System ID


39.0f01.0003.6666.6666.6666.00

39.0f01.0001.1111.1111.1111.00

39.0f01.0004.7777.7777.7777.00

CLNS Addressing: NSAP Examples (Cont.)

39.0f01.0002.3333.3333.3333.00

39.0f01.0002.4444.4444.4444.00

39.0f01.0001.2222.2222.2222.00


The LOOPBACK IP Address: 192.168.3.25The AREA the Router Under Is: 49.0001IP Address Conversion Process to System ID:

192.168.3.25192.168.3.25

192.168.003.025192.168.003.025

1921.6800.30251921.6800.3025

49.0001.1921.6800.302549.0001.1921.6800.3025

CLNS Addressing: How Do Most ISP Define System IDs?


IS-IS Levels


Areas and Backbone Routers

• IS-IS has a 2 layer hierarchyØ The backbone (Level 2)

Ø The areas (Level 1)

• An IS can beØ Level 1 router (intra-area routing)

Ø Level 2 router (inter-area routing)

Ø Level 1-2 router (intra and inter-area routing)

• For each level (1 and 2) a DIS will be elected on LANs


Areas and Backbone Routers (Cont.)

• Level 1 routerØ Has neighbors only on the same area

Ø Has the Level 1 LSDB with all routing information for the area

Ø Use the closest Level 2 router to exit the area

Ø This may result in sub-optimal routing

• Level 2 routerØ May have neighbors in other areas

Ø Has a Level 2 LSDB with all information about inter-area routing



• Level 1–2 routerØMay have neighbors on any area

ØHas two LSDBs:

Level 1 for the intra-area routing

Level 2 for the inter-area routing

ØIf 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


L1L2

L1L2

L1L2L1

L1

L1

Area 49.001

Area 49.003Area 49.0002



L1L2

L1L2

L1L2

L1L2

L1L2

L1 Only

L2 Only

L1 Only

Area 2

Area 1

Area 3

Area 4L1 Only

L1 Only


• Backbone must be L2 contiguous


Area 1 Router A

Area 3Router F

Area 2Router D

Area 2Router E

Area 2Router B Area 2

Router CArea 4

Router G

Remember, the Backbone Must Be Contiguous:IS-IS Router Cannot Determine If They Need to Be L1 or L1L2,So All Routers Try to Be a L1L2 IS by Default


“I’m in area 2 and ALL my neighbors are in thesame area. I must be a L1-only router ?”

!! NO !!Router C must have a full L2 LSDB to route between areas 1, 3, and 4. Remember, the backbone must be contiguous.


Example #1: Area Configuration—PhysicalView

Area-1 Area-2

R3 R2

R1 R4

L1L2 routers

L1 routers

• R2 and R3 belong to their respective level-1 areas and provide a physical connection between them


Example #1: Area Configuration—LogicalView

L1

R3 R2

R1 R4

• R2 and R3 are still L1 routers, but, in addition, they provide an entry point to the level-2 backbone interconnecting both level-1 areas

L2

L2

L1 L1


Example #2: L2 and L1/L2 Routers Forming L2 Backbone

L1L2

L1L2

Backbone links

L1L2

L1L2

L1-only

L1-only

Area-2

Area-1

Area-3

Area-4L1-only

L1-only

L1L2

IS-IS domain

This router must behave as level 2 as well in order to guarantee backbone continuity.This router must behave as level 2 as well in order to guarantee backbone continuity.

L2-only


SPF (Dijkstra) and Partial Route Calculation

• SPF (Dijkstra) is run when topology has to be calculated (SPF tree)

• PRC (Partial Route Calculation) is executed when IP routing information has to be calculated

• If an IS receives an LSP where only IP information has changed, it will run PRC only (less CPU)


IS-IS PDUs


IS-IS PDUs

• IS-IS packets are encapsulated directly in a data-link frame

• There is no CLNS or IP headerØ Hello PDUs (IIH, ISH, ESH)

Ø LSP

Non-pseudonode LSP

Pseudonode LSPs

Ø CSNPØ PSNP


Datalink Header(OSI Family

0xFEFE)

Datalink Header(OSI Family

0xFEFE)IS-IS Fixed Header (First Byte Is 0x83) IS-IS TLVsIS-IS TLVsIS-IS

Datalink Header (OSI Family 0xFEFE)


ESIS Fixed Header (First Byte is 0x81)ESIS Fixed Header (First Byte is 0x81) ESIS TLVsESIS TLVsESIS



CLNS Header (with NSAPs) (First Byte Is 0x80) User DataUser DataCLNS

Encapsulation


Mac Layer Addresses

• On LANs IS-IS PDUs are forwarded to the following well known MAC layer broadcast addresses

AllL1ISs 01-80-C2-00-00-14AllL2ISs 01-80-C2-00-00-15

AllIntermediateSystems 09-00-2B-00-00-05

AllEndSystems 09-00-2B-00-00-04


Hello PDUs

• IIHs are between routers (IS-IS)• Exchanged by ISs to form adjacencies

Ø Point-to-point IIH

Ø Level 1 LAN IIH

Ø Level 2 LAN IIH

• Multipoint and P2P IIHs are padded to full MTU SizeØUseful to detect MTU inconsistencies


Hello PDUs (Cont.)

• Circuit-type: Ø1—Level 1 only

Ø2—Level 2 only (no IS-ES hello)

Ø3—Level 1–2

• Source ID: Transmitting router’s network layer address

• Holding time: Time at which neighbors can legally declare this route dead if they haven’t gotten a hello from it

• Packet length: The length of the entire IS-IS hello message

• Local circuit ID: Identifier to the interface and unique relative to the transmitting router’s other interfaces

Point-to-Point IS-IS Hello


Hello PDUs (Cont.)

• Priority: The transmitting routers’ priority for becoming designated router on the LAN, with higher #s having a higher priority

• LAN ID: The name of the LAN as assigned by the DIS; it consists of DIS-ID + extra octet to differentiate this LAN from others with the same DIS

LAN IS-IS Hello


ES Sends ESH

IS-IS Adjacency through IIH

IS Send ISH for ES

Hello PDUs (Cont.)

• ISs send IIH to establish IS-IS adjacencies

• ISs listen to ESH to discover ESs

• ISs send ISH for ESs

• Es sends ESH and listen to ISH

• ESs select IS as default router by listening to ISH


Node and Pseudonode LSP

• 2 kinds of Link State PDUsØ Non-Pseudonodes represent routers

Ø Pseudonodes represents LANs (created by the DIS)

• A Level 1 router will create a Level 1 LSP• A Level 2 router will create a Level 2 LSP• A Level 1–2 router will create

ØA Level 1 LSP and a Level 2 LSP


Non-Pseudonode LSP Generation

• Each IS will create and flood a new Non-Pseudonode LSPØWhen a new neighbor comes up or goes away

ØWhen new IP prefixes are inserted or removed

ØWhen the metric of a link did change

ØWhen refresh interval timer expires


Pseudonode LSP Generation

• The DIS will create and flood a new Pseudonode LSPØWhen a new neighbor comes up or goes away

ØWhen refresh interval timer expires

• Pseudonode LSP is created by the DISØ One for each level (Level 1 and/or Level 2)

Ø One for each LAN

• Reduces adjacencies and flooding over LAN subnets


Pseudonode LSP Generation (Cont.)

• Broadcast link represented as virtual node, referred to as Pseudonode (PSN)

• PSN role played by the Designated Router (DIS)

• DIS election is preemptive, based on interface priority with highest MAC address being tie breaker

• IS-IS has only one DIS; DIS helps routers on broadcast link to synchronize their IS-IS databases

PSN

DIS DIS


LSP for Router CIS: 10 A

10 B10 D

ES: 10 E

LSP for Router CIS: 10 A

10 B10 D

ES: 10 E

LSP for Router BIS: 10 A

10 C10 D

ES: 10 E

LSP for Router BIS: 10 A

10 C10 D

ES: 10 E

LSP for Router DIS: 10 A

10 B10 C

ES: 10 E

LSP for Router DIS: 10 A

10 B10 C

ES: 10 E

LSP for Router AIS: 10 B

10 C10 D

ES: 10 E

LSP for Router AIS: 10 B

10 C10 D

ES: 10 E

EndSystem E

LSPDB without Pseudonode


LSP for Router AIS: 10 PLSP for Router AIS: 10 P

EndSystem E

LSP for thePseudonode PIS: 0 A

0 B0 C0 D

ES: 0 E

LSP for thePseudonode PIS: 0 A

0 B0 C0 D

ES: 0 E

LSP for Router AIS: 10 PLSP for Router AIS: 10 P

LSP for Router AIS: 10 P




Pseudonode in the LSPDB


Neighbors and Adjacencies

• IIH (IS-IS Hello) between routersØTwo types of HELLOS on LAN—L1 and L2

ØOnly one on p2p (with the type of desired adjacency described—L1, L2 or both)

ØHELLOS sent every 10 seconds, holdtime 30 seconds(default)

• Separate adjacencies are built for L1 and L2 routersØL1/L2 routers keep two tables

• Routers form adjacencies with all other routers and send LSPs to all routers on the LAN (unlike OSPF routers)


LAN Adjacencies

L1

L1/L2 L1/L2 L1

L1 adjacency L2 adjanceny

• Adjacencies are established based on the area address announced in the incoming IIHs and the type of the router

L1/L2 L1/L2

L1/L2

Area-1

Area-1

Area-1 Area-1

Area-2


WAN Adjacencies

L1 L1

L1/L2

Area-1 Area-1

L1/L2 L1 L1/L2

Area-1 Area-1

L1

L2 L2

Area-1 Area-1

Area-1 Area-2

L1

L1L2

L1

#

L1/L2 L2 L1/L2

Area-1 Area-1L2

L1/L2 L2 L1/L2

Area-1 Area-2L2

L2

L1/L2 L1/L2Area-1 Area-1


CSNP/PSNP

• For both Level 1 and Level 2 databases, we have CSNPs and PSNPsØ Level 1 CSNP

Ø Level 2 CSNP

Ø Level 1 PSNP

Ø Level 2 PSNP


Complete Sequence Number PDU

• Describes all LSPs in your LSDB (in range)Ø Contains an address range

Ø LSPid, seqnr, checksum, remaining lifetime

• Used at 2 occasionsØ Periodic multicast by DIS (every 10 seconds)

Ø On p2p links when link comes up

• Created and flooded by the DISØ Every 10 seconds

Ø On each LAN the IS is the DIS

• If LSDB is large, multiple CSNPs are sent


Partial Sequence Number PDU

• PSNPs have 2 functionsØ Exchanged by ISs on p2p links (ACKs)

Ø Acknowledge receipt of an LSP

Ø Request transmission of latest LSP

• PSNPs describe LSPs by its headerØ LSP identifier

Ø Sequence numberØ Remaining lifetime

Ø LSP checksum


LSP Header


LSP Header

• The LSP header contains Ø LSP-id

Ø Sequence number

Ø Remaining lifetime

Ø Checksum

Ø Type of LSP (Level 1, Level 2)

Ø Attached bit

Ø Overload bit


LSP Header (Cont.)

• LSP identifier consists of 3 partsØSource ID

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

ØPseudonode ID

Zero for router LSP, non-zero for Pseudonode LSP

ØLSP number

Fragmentation number

00c0.0040.1234.01-00

System IDSystem IDPN-IDPN-ID

Frag-NrFrag-Nr


LSP Header (Cont.)

• LSP sequence numberØUsed to determine the newest LSP version

• LSP remaining lifetimeØUsed to purge old LSPs

• LSP checksum• LSP type

ØLevel 1 or Level 2


LSP Header (Cont.)

• 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

LSP Attached Bit


LSP Header (Cont.)

• Set by the IS when it has an overload problem on its LSDBØIndicates that the router has an incomplete LS database, and hence cannot be trusted to compute any correct routesØIs used in the LSDB, but topology behind it is not calculated

ØTherefore other routers do not compute routes which would require the PDU to pass through the overloaded router

ØException—ES neighbors—since these paths are guaranteed to be non-looping

LSP overload bit


Flooding


Reasons for Flooding

• All routers generate an LSP• All LSPs need to be duplicated and sent to all

routers in the networkØIf LSPDB is not synchronized, routing loops might occur

• IS-IS two components are the SPF computation and reliable flooding


What Triggers a New LSP?

• When something changes…Ø Adjacency came up or went down

Ø Interface up/down (connected IP prefix!)

Ø Redistributed IP routes change

Ø Inter-area IP routes change

Ø An interface is assigned a new metric

Ø Most other config changes


What to Do with a New LSP?

• Create new LSP, install in your own LSPDB and mark it for flooding

• Send the new LSP to all neighbors• Neighbors flood the LSP further• Only flood new LSPs, ack old ones

ØBecause we have state in our LSPDB, we can prevent infinite looping of LSPs


Flooding on a P2P Link

• Once the adjacency is established both IS send CSNP packet

• Missing LSPs are sent by both ISs if not present in the received CSNP

• Missing LSPs may be requested through PSNP


RTA RTB RTCInterface 1Interface 1

Interface 2Interface 2 Interface 3Interface 3

Interface 4Interface 4

PSNPRTA.00-00SEQ#100




LSPRTA.00-00SEQ#100

LSPRTA.00-00SEQ#100

LSPRTA.00-00SEQ#100

LSPRTA.00-00SEQ#100

Flooding on a P2P Link (Cont.)


Link-State Database Synchronization—P2P

R1 R3 LSP 33

PSNP

III. ACK: Thank you for

LSP 33

III. ACK: Thank you for

LSP 33 I. Link went downI. Link went down

II. New LSP describing the

current situation

II. New LSP describing the

current situation

s0R2


Flooding on a LAN (Broadcast Links)

• On LANs only, there’s a designated router (DIS)• DIS has two tasks

ØCreating and updating the Pseudonode LSP

ØConducting the flooding over the LAN

• A DIS is elected for each LAN• DIS election is based on priority

• Breaking-tie is the highest SNPA (MAC address)• DIS election is deterministic


RTA (DIS) RTBInterface 1

Interface 2

PSNPRTB.00-00SEQ#200

LSPRTA.00-00SEQ#100

RTCInterface 3

CSNP

LSPRTC.00-00

SEQ#1RTA

RTB

RTBRTA

Flooding on a LAN (Broadcast Links) (Cont.)


Link-State Database Synchronization—LAN

R1

CSNP sent periodically (every 10 s) by DIS

R2/DIS

PSNP

II. Request:Sorry. I

missed LSP 77

II. Request:Sorry. I

missed LSP 77 CSNP PSNP

I. CSNP:LSP76LSP77LSP88

I. CSNP:LSP76LSP77LSP88


Sequence Number

• Each LSP (and LSP fragment) has its own sequence number

• When router boots, set seqnr to one• When there is a change, the seqnr is incremented, a

new version of the LSP is generated with the new seqnr

• Higher seqnr means newer LSP


Remaining Lifetime

• Used to age out old LSPsØWhen the originator is not anymore

• We need periodic refresh (with higher seqnr) to keep stable LSPs valid

• IS-IS counts down from 1200 sec to 0ØWe allow to start at 65535 sec (18.7h)


How Is Remaining Lifetime Used?

• If the remaining lifetime expires, the first router that notices, purges the LSP:ØRemove LSP body, only keep header; set age to zero; flood via normal way in the network; zero lifetime LSP are newer than non-zero lifetime LSPs

ØAfter a while all routers remove the purged LSP from their LSPDB


LSP Checksum

• Used to detect LSP corruption during floodingØDepending on Layer 2 CRC is not enough, corruption happens in routers and switches

ØCompute checksum of received LSP, check against checksum inside LSP

ØIf corrupt, drop LSP; sender retransmits


Compare LSP on Checksum

• If two LSPs have same LSPid, same seqnr, and remaining lifetime, keep the LSP with highest checksumØGuarantees consistent LSPDBs all across the networkØCan happen after a router reboots, or is reconnected to the network


Configuration


!interface Loopback0ip address 172.16.1.1 255.255.255.255!interface Ethernet0ip address 172.16.12.1 255.255.255.0ip router isis

!router isispassive-interface Loopback0net 49.0001.1720.1600.1001.00

!

How to Configure?

R1 Configuration

R1R1

R2R2

R3R3s0

s0

e0

e0


!interface Loopback0ip address 172.16.2.2 255.255.255.255!interface Ethernet0ip address 172.16.12.2 255.255.255.0ip router isis

!interface Serial0ip address 172.16.23.1 255.255.255.252ip router isis

!router isispassive-interface Loopback0net 49.0001.1720.1600.2002.00

!

How to Configure? (Cont.)

R1R1

R2R2

R3R3s0

s0

e0

e0

R2 Configuration


Looking at the Show Commands

R1#show clns neighborSystem Id Interface SNPA State Holdtime Type ProtocolR2 Et0 0000.0c47.b947 Up 24 L1L2 IS-IS

R1#show clns interface ethernet 0Ethernet0 is up, line protocol is up

Checksums enabled, MTU 1497, Encapsulation SAPRouting Protocol: IS-ISCircuit Type: level-1-2Interface number 0x0, local circuit ID 0x1Level-1 Metric: 10, Priority: 64, Circuit ID: R2.01Number of active level-1 adjacencies: 1Level-2 Metric: 10, Priority: 64, Circuit ID: R2.01Number of active level-2 adjacencies: 1Next IS-IS LAN Level-1 Hello in 5 secondsNext IS-IS LAN Level-2 Hello in 1 seconds


Looking into the DatabaseR2#show clns neighborSystem Id Interface SNPA State Holdtime Type ProtocolR1 Et0 0000.0c09.9fea Up 24 L1L2 IS-ISR3 Se0 *HDLC* Up 28 L1L2 IS-IS

R2#show isis databaseIS-IS Level-1 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLR1.00-00 0x0000008B 0x6843 55 0/0/0R2.00-00 * 0x00000083 0x276E 77 0/0/0R2.01-00 * 0x00000004 0x34E1 57 0/0/0R3.00-00 0x00000086 0xF30E 84 0/0/0IS-IS Level-2 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLR1.00-00 0x00000092 0x34B2 41 0/0/0R2.00-00 * 0x0000008A 0x7A59 115 0/0/0R2.01-00 * 0x00000004 0xC3DA 50 0/0/0R3.00-00 0x0000008F 0x0766 112 0/0/0


Looking into the Database Detail

R2#show isis database R2.00-00 detailIS-IS Level-1 LSP R2.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLR2.00-00 * 0x00000093 0x077E 71 0/0/0

Area Address: 49.0001NLPID: 0xCCHostname: R2IP Address: 172.16.2.2Metric: 10 IP 172.16.12.0 255.255.255.0Metric: 0 IP 172.16.2.2 255.255.255.255Metric: 10 IP 172.16.23.0 255.255.255.252Metric: 10 IS R2.01Metric: 10 IS R3.00

IS-IS Level-2 LSP R2.00-00LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLR2.00-00 * 0x0000009A 0x5A69 103 0/0/0

Area Address: 49.0001NLPID: 0xCCHostname: R2IP Address: 172.16.2.2Metric: 10 IS R2.01Metric: 10 IS R3.00Metric: 10 IP 172.16.23.0 255.255.255.252Metric: 10 IP 172.16.1.1 255.255.255.255Metric: 10 IP 172.16.3.3 255.255.255.255Metric: 0 IP 172.16.2.2 255.255.255.255Metric: 10 IP 172.16.12.0 255.255.255.0


Looking into the Routing-Table

R1#show ip route isisi L1 172.16.2.2/32 [115/10] via 172.16.12.2, Ethernet0i L1 172.16.3.3/32 [115/20] via 172.16.12.2, Ethernet0

R2#show ip route isisi L1 172.16.1.1/32 [115/10] via 172.16.12.1, Ethernet0i L1 172.16.3.3/32 [115/10] via 172.16.23.2, Serial0


Show IS-IS SPF-Log

R1#show isis spf-logLevel 1 SPF log

When Duration Nodes Count First trigger LSP Triggers04:07:42 12 5 1 PERIODIC03:52:41 12 5 1 PERIODIC03:37:40 12 5 1 PERIODIC00:37:31 12 5 1 PERIODIC00:22:31 21 5 1 PERIODIC00:07:30 19 5 1 PERIODIC


Show IS-IS LSP Log

R1#show isis lsp-logLevel 1 LSP log

When Count Interface Triggers5d05h 1 Serial1 DELADJ5d05h 1 ATTACHFLAG5d04h 2 Ethernet0 NEWADJ DIS5d04h 3 Ethernet0 CONFIG DELADJ DELADJ5d04h 1 Serial1 NEWADJ00:23:10 1 Loopback0 CONFIG


New Features


L2->L1 Route Leaking

• RFC1195 defines all routers as STUB routers• No information is leaked from routers in L2 into

routers in L1• Hence all L1-routers are forced to route to the

closest L2-router

• This may result in sub-optimal routing• This is IP only feature (CLNS still uses STUB)



• This new feature allows redistribution of L2-IP routes into L1 areas

• Enables Level 1-only routers to pick the best path to exit the area

• Enables MPLS-VPN (PE reachability) between areas

• Redistribution is controlled via distribute-lists



• Prefixes MUST be present in the routing table as ISIS level-2 routesØOtherwise no leaking occurs

ØSame criteria than L1 to L2

ØInter-area routing is done through the routing table



• When leaking routes from L2 backbone into L1 areas a loop protection mechanism need to be used in order to prevent leaked routes to be re-injected into the backbone



• Recommendation: use wide Metric TLV (TLV 135)

• Configure with:Ø router isis

metric-style wide


L2->L1 Route Leaking (Cont.)

• Route leaking is implemented in both 12.0S and 12.1ØCisco IOS 12.0S command

Øadvertise ip L2-into-L1 <100-199>

ØCisco IOS 12.1 command

Ø redistribute isis ip level-2 into level-1 distribute-list <100-199>

• Both commands are supported• 12.0S command will be converted into 12.1 syntax


Fast Hellos

• Hold-time can be set to 1 secondØinterface POS0/0

isis hello-interval minimal

• By default hello-multiplier is 3ØHello packets sent every 333 msecs


Fast Hellos (Cont.)

• AdvantagesØReduced link failure detection time

• DisadvantagesØIncreased BW/buffer/CPU usage can cause missed hellos; potential increased adjacency flapping can cause instability


Fast Hellos (Cont.)

• Configuration:ØInterface configuration mode:

Rtr-A(config)#int POS0/0

Rtr-A(config-if)#isis hello-interval minimal

Rtr-A(config-if)#isis hello-multiplier 4

• Advertised hold time will now be 1 second, hello-interval will be 250 ms


Multi-Area Support

• Allows multiple IS-IS processes to be configured on a single router

• Each process serves a different area• One of the processes will be L1L2 to advertise all

area addresses from all processes into L2

• Maximum number of configurable IS-IS processes on a single router is 29


Why use Multi-Area ?

• ADMs used in Telco SDH/Sonet networks use CLNS for network management

• IS-IS implementation used on ADMs may have scaling limitations

• Thus, may only build small L1 areas

• Each ISIS router belongs to one area


Deployment Scenarios


L1-Only POPs

POP 1L1-Only

POP 3L1-Only

COREL1-Only

POP 4L1-Only

POP 2L1-Only


L1-Only POPs (Cont.)

• IS-IS is a newer protocol at that time at least operationally with the ISPs

• In this design—all the routers will be running in one area and are all doing L1-only routing

• This design is flat with a single L1-only database running on all the routers

• If you have a change in the topology, the SPF computation will be done in all the routers as they are in the L1-only sub-domain



• Also the Tier 1 ISPs picked up L1-only to avoid sub-optimal routing problems

• The other factor is when the router runs as L1L2—then the router(s) will have 2 instances of SPFs

• Since most of the routers were AGS+/7XXX at that time, the ISPs had chosen L1-only single-area IS-IS with in their network


L2-Only POPs

POP 1Area 49.0001

POP 3Area 49.0001

POP 2Area 49.0001

COREL2-Only

POP 4Area 49.0001



• Most of the Tier 1 ISPs are running Level 2-only on all the routers

• The rough approximation of routers L2-only are about 800–1000

• The SPF-computation may take up to 2–3 sec.

• Most of the uplinks into the core are OC-12 to OC-48 POS links

• As the network grows, easy to bring the L1-only POPs

• All the routers in L2 will share all the LSPs


L1 in the POP and L2 in the Core

POP 1L1-Only

Area 49.0001

POP 3L1-Only

Area 49.0003

POP 2L1-Only

Area 49.0002

COREL2-Only

POP 4L1-Only

Area 49.0004

L1L2L1L2

L1L2L1L2

L1L2L1L2

L1L2L1L2

L1L2L1L2

L1L2L1L2L1L2L1L2

L1L2L1L2


L1 in the POP and L2 in the Core (Cont.)

• Within a given local pop—all the routers will be in a separate area

• The L1L2 routers at the edge of the POPs will be running ØL1-adj going into the POP ØL2-adj into the core with the rest of the L1L2 routers

• The SPF computations will be limited to the respective L1-areas only

• All the L1-routers in a given pop will receive the ATT bit set by the L1L2 router at the edge of this pop

• This will cause the sub-optimal routing in reaching out the prefixes outside the POP by the local routers


L1 in the POP and L2 in the Core (Cont.)

• It is recommended to configure the L1L2 routers at the edge of the pop with route-leaking capabilities

• This way we leak the longer prefixes of the remote pop into the pop

• Hence the L1 routers will be able to take the right exist router based on the metric of the leaked IP-prefix

• Whenever you configure for route-leaking—make sure you configure the routers with metric-style wide

Route-Leaking


Suggested Reading

• ISO 10589 (IS-IS Intra-Domain Routing Exchange Protocol) • RFC 1195 (OSI IS-IS for Routing in TCP/IP and Dual Environments)

• draft-ietf-isis-traffic-02.txt (TE Extensions for IS-IS)

• draft-ietf-isis-3way-04.txt (3-Way Handshake)

• RFC 2966 (Route-leaking)

• RFC 2763 (Dynamic Hostname Exchange)

• draft-hsmit-mpls-igp-spf-00.txt

IS-IS protocol

Documents

Transcript of IS-IS protocol