Is is

2RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.© 2001, Cisco Systems, Inc. All rights reserved.

The IS-IS Routing Protocol

333RST-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

• TLVs

• Configuration

• Design Considerations

• 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 Service

CLNP

IS-IS

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:

Area—variable

ID

SEL(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 NSEL

Variable 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

Variable length area-address

6 Byte system ID

Byte n-selector (indicating transport layer)

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


CLNS Addressing: NSAP Examples

• Example 1:

47.0001.aaaa.bbbb.cccc.00Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00

• Example 2:

39.0f01.0002.0000.0c00.1111.00Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00

• Example 3:

49.0002.0000.0000.0007.00Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00


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: 192.168.3.25

The AREA the Router Under Is: 49.0001

IP 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 Did Most ISP’s Define System IDs?


IS-IS Levels


Areas and Backbone Routers

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

The areas (Level 1)

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

Level 2 router (inter-area routing)

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 router Has neighbors only on the same area

Has the Level 1 LSDB with all routing information for the area Use the closest Level 2 router to exit the area

This may result in sub-optimal routing

• Level 2 router May have neighbors in other areas

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



• Level 1–2 router

May have neighbors on any area

Has two LSDBs:

Level 1 for the intra-area routing

Level 2 for the inter-area routing

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


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 2Router C

Area 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.


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 header Hello PDUs (IIH, ISH, ESH)

LSP

Non-pseudonode LSP

Pseudonode LSPs

CSNP

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)Datalink Header

(OSI Family 0xFEFE)ESIS Fixed Header (First Byte is 0x81)ESIS Fixed Header (First Byte is 0x81) ESIS TLVsESIS TLVsESIS

Datalink Header (OSI Family 0xFEFE)

Datalink Header (OSI Family 0xFEFE)

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-C2-00-00-14

AllL2ISs 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 Point-to-point IIH

Level 1 LAN IIH

Level 2 LAN IIH

• Multipoint and P2P IIHs are padded to full MTU Size

Useful to detect MTU inconsistencies


Hello PDUs (Cont.)

• Circuit-type: 1—Level 1 only

2—Level 2 only (no IS-ES hello)

3—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 PDUs Non-Pseudonodes represent routers

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 createA Level 1 LSP and a Level 2 LSP


Non-Pseudonode LSP Generation

• Each IS 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 did change

When refresh interval timer expires


Pseudonode LSP Generation

• The DIS will create and flood a new Pseudonode LSP

When a new neighbor comes up or goes away

When refresh interval timer expires

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

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 B 10 DES: 10 E

LSP for Router CIS: 10 A 10 B 10 DES: 10 E

LSP for Router BIS: 10 A 10 C 10 DES: 10 E

LSP for Router BIS: 10 A 10 C 10 DES: 10 E

LSP for Router DIS: 10 A 10 B 10 CES: 10 E

LSP for Router DIS: 10 A 10 B 10 CES: 10 E

LSP for Router AIS: 10 B 10 C 10 DES: 10 E

LSP for Router AIS: 10 B 10 C 10 DES: 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 B 0 C 0 DES: 0 E

LSP for thePseudonode PIS: 0 A 0 B 0 C 0 DES: 0 E

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

LSP for Router AIS: 10 P




Pseudonode in the LSPDB


CSNP/PSNP

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

Level 1 CSNP

Level 2 CSNP

Level 1 PSNP

Level 2 PSNP


Complete Sequence Number PDU

• Describes all LSPs in your LSDB (in range) Contains an address range LSPid, seqnr, checksum, remaining lifetime

• Used at 2 occasions Periodic multicast by DIS (every 10 seconds) On p2p links when link comes up

• Created and flooded by the DIS Every 10 seconds On each LAN the IS is the DIS

• If LSDB is large, multiple CSNPs are sent


Partial Sequence Number PDU

• PSNPs have 2 functions Exchanged by ISs on p2p links (ACKs) Acknowledge receipt of an LSP Request transmission of latest LSP

• PSNPs describe LSPs by its header LSP identifier Sequence number Remaining lifetime LSP checksum


LSP Header


LSP Header

• The LSP header contains

LSP-id

Sequence number

Remaining lifetime

Checksum

Type of LSP (Level 1, Level 2)

Attached bit

Overload bit


LSP Header (Cont.)

• LSP identifier consists of 3 partsSource ID

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

Pseudonode ID

Zero for router LSP, non-zero for Pseudonode LSP

LSP number

Fragmentation number00c0.0040.1234.01-00

System IDSystem IDPN-IDPN-ID

Frag-Nr Frag-Nr


LSP Header (Cont.)

• 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 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 LSDB

Indicates that the router has an incomplete LS database, and hence cannot be trusted to compute any correct routes

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

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

LSP overload bit


LSP flooding

49


Why do we need flooding

• All routers generate an LSP

• All LSPs need to be flooded to all routers in the network

if LSPDB is not synchronised, routing loops or blackholes might occur

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


What triggers a new LSP ?

• When something changes …

Adjacency came up or went down

Interface up/down (connected IP prefix !)

Redistributed IP routes change

Inter-area IP routes change

An interface is assigned a new metric

Most other configuration changes

Periodic refresh


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


Basic flooding rules

• When receiving an LSP, compare with old version of LSP in LSPDB

• If newer:

install it in the LSPDB

Acknowledge the LSP with a PSNP

Flood to all other neighbors

Check if need to run SPF


Basic flooding rules

• If same age:


• If older:


Send our version of the same LSP

Wait for PSNP


Sequence number

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

• When router boots, it sets 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 LSPs

• Periodic refresh needed to keep stable LSPs valid

• IS-IS counts down from 1200 sec to 0

we allows to start at 65535 sec (18.7h)

• When lifetime expires, the LSP is purged from the network

Header with lifetime = 0 is flooded


Flooding on a P2P Link

RouterARouterB

Received ack

Received it. Local copy has seqNr = 21.So the received one is newer. Install it in LSDB. Acknowledge it. Maybe flood further.id=x seqnr=22

PSNP

Now flood it:Send over p2p.

id=x seqnr=22LSP

Receives LSPid=x seqNr=22 It’s new. Put it in the LSPDB

id=x seqnr=22LSP


The Designated Router

• DIS is like the DR in OSPF

• DIS is only on LANs, not on p2p

• DIS has two tasks

create/update pseudonode LSP

conduct flooding over the LAN

• DIS sends periodic CSNPs

LSPid, SeqNr, Checksum, Lifetime of all LSPs present in the LSPDB


The Designated IS

• No Backup DIS in ISIS

not necessary, no LSPDB resync

• DIS is elected by priority and MAC

actually is “self-elected”

• LAN circuitID shows who is DIS

use show clns interface


Flooding on a LAN

Periodic CSNPevery 10 secs

id=y seqnr=...id=x seqNr=22id=z ...

CSNP

LAN

DISRtr-A

Got it. Install andrun SPF

Local copies of LSP-y and LSP-z are up-to-date but local copy of LSP-x is older.Request latest LSP-x via PSNP

id=x seqNr=21PSNP

!!! Problem !!!Dropped LSP

LSP

Received new LSPid=x seqNr=22Install in LSPDB.Flood the LSP.

id=x seqNr=22LSP

Neighbor has an old LSP, better resend him latest

id=x seqNr=22LSP


TLVs


TLV NameTLV Name TypeType

Level 1 TLVs

OriginOrigin

Intermediate System NeighborsIntermediate System Neighbors

End System NeighborsEnd System Neighbors

Authentication InformationAuthentication Information

IP Internal Reachability InformationIP Internal Reachability Information

Protocols SupportedProtocols Supported

IP Interface AddressIP Interface Address

22

33

1010

128128

129129

132132

RFC 1195RFC 1195

RFC 1195RFC 1195

RFC 1195RFC 1195

ISO 10589ISO 10589

ISO 10589ISO 10589

ISO 10589ISO 10589

Area AddressArea Address 11 ISO 10589ISO 10589


TLV NameTLV Name TypeType OriginOrigin

Area AddressArea Address

Intermediate System NeighborsIntermediate System Neighbors

Partition Designated Level 2 ISPartition Designated Level 2 IS

Authentication InformationAuthentication Information

IP Internal Reachability InformationIP Internal Reachability Information

Protocols SupportedProtocols Supported

11

22

44

1010

128128

129129

ISO 10589ISO 10589

RFC 1195RFC 1195

RFC 1195RFC 1195

ISO 10589ISO 10589

ISO 10589ISO 10589

ISO 10589ISO 10589

55Prefix NeighborsPrefix Neighbors ISO 10589ISO 10589

Level 2 TLVs

IP Interface AddressIP Interface Address 132132 RFC 1195RFC 1195

IP External Reachability InformationIP External Reachability Information 130130 RFC 1195RFC 1195

Inter-Domain Routing Protocol Information Inter-Domain Routing Protocol Information 131131 RFC 1195RFC 1195


TLV NameTLV Name TypeType CommentsComments

New TLVs

Extended IS Reachability InformationExtended IS Reachability Information 22 22 Used in Place of TLV 2 forTraffic Engineering (TE)

Used in Place of TLV 2 forTraffic Engineering (TE)

Router-IdRouter-Id 134134 TE Extension to IS-ISTE Extension to IS-IS

Extended IP Reachability InformationExtended IP Reachability Information135135

TE Extension to IS-IS, Used in Place of TLV 128

or 130

TE Extension to IS-IS, Used in Place of TLV 128

or 130

Dynamic Hostname InformationDynamic Hostname Information 137137For Dynamic Distribution

of Hostname to NET Mapping via LSP Flooding

For Dynamic Distribution of Hostname to NET Mapping

via LSP Flooding

Point-to-Point Adjacency StatePoint-to-Point Adjacency State 240240Reliable Point-to-PointAdjacency Formation

Reliable Point-to-PointAdjacency Formation


Old IS-IS Metrics

• ISO 10589 specifies 4 types of metricDefault—supported by all routers

Delay—measures transit delay

Expense—measures the monetary cost of link utilization

Error—measures error probability

• Default metric type must be supported by all implementations

• Other types specified for QoS routing are not available most implementation


00 I/EI/E Default Metric (6 bits)Default Metric (6 bits) 11

Byte(s)Byte(s)

Old IS-IS Metrics (Cont.)

• Maximum LINK_METRIC per interface is 63

• Maximum PATH_METRIC is 1023

• There is no automatic interpretation based on interface bandwidth

• Cisco uses default of 10 on all interfaces regardless of bandwidth


New IS-IS Metrics (Wide Metrics)

• With the draft-ietf-isis-traffic-02.txt

Max Link_METRIC is 16777215 (2^24 – 1)

Max PATH_METRIC is 4261412864 (2^32 – 2^25)


Configuration


!interface Loopback0 ip address 172.16.1.1 255.255.255.255!interface Ethernet0 ip address 172.16.12.1 255.255.255.0 ip router isis !router isis passive-interface Loopback0 net 49.0001.1720.1600.1001.00!

How to Configure?

R1 Configuration

R1R1

R2R2

R3R3

s0

s0

e0

e0


!interface Loopback0 ip address 172.16.2.2 255.255.255.255!interface Ethernet0 ip address 172.16.12.2 255.255.255.0 ip router isis !interface Serial0 ip address 172.16.23.1 255.255.255.252 ip router isis!router isis passive-interface Loopback0 net 49.0001.1720.1600.2002.00!

How to Configure? (Cont.)

R1R1

R2R2

R3R3

s0

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 0

Ethernet0 is up, line protocol is up

Checksums enabled, MTU 1497, Encapsulation SAP

Routing Protocol: IS-IS

Circuit Type: level-1-2

Interface number 0x0, local circuit ID 0x1

Level-1 Metric: 10, Priority: 64, Circuit ID: R2.01

Number of active level-1 adjacencies: 1

Level-2 Metric: 10, Priority: 64, Circuit ID: R2.01

Number of active level-2 adjacencies: 1

Next IS-IS LAN Level-1 Hello in 5 seconds

Next IS-IS LAN Level-2 Hello in 1 seconds


Looking into the Database

R2#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 database

IS-IS Level-1 Link State Database:

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

R1.00-00 0x0000008B 0x6843 55 0/0/0

R2.00-00 * 0x00000083 0x276E 77 0/0/0

R2.01-00 * 0x00000004 0x34E1 57 0/0/0

R3.00-00 0x00000086 0xF30E 84 0/0/0

IS-IS Level-2 Link State Database:

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

R1.00-00 0x00000092 0x34B2 41 0/0/0

R2.00-00 * 0x0000008A 0x7A59 115 0/0/0

R2.01-00 * 0x00000004 0xC3DA 50 0/0/0

R3.00-00 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: 49.0001 NLPID: 0xCC Hostname: R2 IP Address: 172.16.2.2 Metric: 10 IP 172.16.12.0 255.255.255.0 Metric: 0 IP 172.16.2.2 255.255.255.255 Metric: 10 IP 172.16.23.0 255.255.255.252 Metric: 10 IS R2.01 Metric: 10 IS R3.00IS-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: 49.0001 NLPID: 0xCC Hostname: R2 IP Address: 172.16.2.2 Metric: 10 IS R2.01 Metric: 10 IS R3.00 Metric: 10 IP 172.16.23.0 255.255.255.252 Metric: 10 IP 172.16.1.1 255.255.255.255 Metric: 10 IP 172.16.3.3 255.255.255.255 Metric: 0 IP 172.16.2.2 255.255.255.255 Metric: 10 IP 172.16.12.0 255.255.255.0


Looking into the Routing-Table

R1#show ip route isis

i L1 172.16.2.2/32 [115/10] via 172.16.12.2, Ethernet0

i L1 172.16.3.3/32 [115/20] via 172.16.12.2, Ethernet0

R2#show ip route isis

i L1 172.16.1.1/32 [115/10] via 172.16.12.1, Ethernet0

i L1 172.16.3.3/32 [115/10] via 172.16.23.2, Serial0


Show IS-IS SPF-Log

R1#show isis spf-log

Level 1 SPF log

When Duration Nodes Count First trigger LSP Triggers

04:07:42 12 5 1 PERIODIC

03:52:41 12 5 1 PERIODIC

03:37:40 12 5 1 PERIODIC

00:37:31 12 5 1 PERIODIC

00:22:31 21 5 1 PERIODIC

00:07:30 19 5 1 PERIODIC


Show IS-IS LSP Log

R1#show isis lsp-log

Level 1 LSP log

When Count Interface Triggers

5d05h 1 Serial1 DELADJ

5d05h 1 ATTACHFLAG

5d04h 2 Ethernet0 NEWADJ DIS

5d04h 3 Ethernet0 CONFIG DELADJ DELADJ

5d04h 1 Serial1 NEWADJ

00:23:10 1 Loopback0 CONFIG


Areas and levels

77


Hierarchy

• IS-IS has 2 layers of hierarchy

the backbone is called level-2

areas are called level-1

• Same algorithms apply for L1 and L2

• A router can take part in L1 and L2

inter-area routing (or inter-level routing)


Level-1 Routers

• Neighbors only in the same area

• L1 has information about own area

• L1-only routers look at the attached-bit in L1 LSPs to find the closest L1L2 router

• L1-only routers install a default route to the closest L1L2 router in the area


Level-2 routers

• May have neighbors in other areas

• L2 has information about L2 topology

• L2 has info on what L1 destinations are reachable and how to reach them via the L2 topology

• L2 routers often also do L1 routing

so called L1L2 routers


Adjacency levels

L1-Adjacency L2-Adjacency

L2-AdjacencyL2-Adjacency

L1L2Adjacency

L1L2Adjacency

Router with adjacencies within the same area.

However, needs to have a L2 database as well since it is a transit node

Therefore L1L2 adjacency is required


Level-1, Level-2 & Level-1-2 Routers

• Backbone MUST BE L2 contiguous

L1-only

L1-only

L1-only

L1-only

L1-L2

L1-L2

L2-only

L1-only

L1-L2

L1-L2

L1-only

This router has to behave as level-2 as well in order to guarantee backbone continuity


Level-1, Level-2 & Level-1-2 Routers

• Backbone MUST BE L2 contiguous

L1-only

L1-only

L1-only

L1-L2

L1-L2

L2-only

L1-L2

L1-L2

L1-only

This router has to behave as level-2 as well in order to guarantee backbone continuity

L1-L2


Design Considerations


Design guidelinesOverload-bit

• 10589 defines for each LSP a special bit called the LSPDB Overload Bit

• While having problems, a router could set the OL bit, and other routers would route around it

• Connected IP prefixes still reachable



• With IS-IS you can manually set the overload bit in the router’s LSP

• This router will therefore never be used for transit during the path calculation, but it is still reachable



R1 R2

R5R3R5-LSP Overload-bit

Neighbors: R1, R4

R4

When R1 computes SPT, he will find that R5 LSP has Overload-bit set. Therefore R5 cannot be used as transit node and shortest path to R4 is: R1->R2->R3->R4

• Why/When use Overload-Bit ?

When the router is not ready to forward traffic for ALL destinationsTypically when ISIS is up but BGP not yet

When the router has other functions (Network Management)



• BGP will typically converge much slower than the IGP (a few minutes)

• During this time, other routers in the AS will use this new router for transit

• But if the new router does not have all BGP routes yet, it will drop traffic

• New router should first converge BGP before carrying traffic



• IS-IS can set the OL bit after each reboot, and allow BGP to converge before it advertises itself as transit by unsetting the OL bit

• Network admin needs to specify how long IS-IS should wait for BGP to converge

typically 2 to 5 minutes



• BGP can tell IS-IS to unset the Overload-bit immediately

• Default BGP update delay is 2 min

• When BGP never informs ISIS, the Overload-bit will be cleared after 10 minutes



• Overload-bit on-startup recommended in MPLS networks

• During boot-up a router may have all IGP routes but not all labels

• During this time it’s better not to use the router as a transit point

router isisset-overload-bit on-startup 120


Set over Load Bit (Cont.)

router isis

set-overload-bit

set-overload-bit on-startup <sec>

set-overload-bit on-startup wait- for-bgp


Set over Load Bit (Cont.)

• Enhanced configuration:

Router IS-IS

set-overload-bit [ on-startup [ <timeout> | wait-for-bgp] ]

• keyword “wait-for-bgp”

• When BGP doesn’t inform IS-IS it is ready and “wait-for-bgp” is configured, the over Load Bit will be cleared after 10 minutes


TimerTimer Default ValueDefault Value Cisco IOS CommandCisco IOS Command

MaxageMaxage

LSP Refresh IntervalLSP Refresh Interval

LSP Transmission IntervalLSP Transmission Interval

LSP Retransmit IntervalLSP Retransmit Interval

CSNP IntervalCSNP Interval

1200s1200s

900s900s

33ms33ms

5s5s

10s10s

IS-IS Max-lSP-IntervalIS-IS Max-lSP-Interval

IS-IS Refresh-IntervalIS-IS Refresh-Interval

IS-IS lSP-IntervalIS-IS lSP-Interval

IS-IS Retransmit-IntervalIS-IS Retransmit-Interval

IS-IS CSNP-IntervalIS-IS CSNP-Interval

Database Timers


IP routing specifics

95


Dynamic Host Name

• All ISPs configure STATIC mappings of system-IDs

• This process has dis-adv of maintaining huge (identical) databases on all the routers

• Adding a router to the network, means updating this static mappings on all the routers


Dynamic Host Name (Cont.)

• TLV 137

• RFC 2763

• Floods the host names dynamically

• Show isis topology shows the NSAPs getting dynamically mapped to the hostname


L1 advertised into L2

• All L1L2 routers advertise all the IP prefixes they learn via L1 into L2

• Only advertise routes you use

• Summarization possible

At L1->L2 or when redistributing


Route Leaking

• ISIS feature/capability described in draft-ietf-isis-domain-wide

• Allows L1L2 routers to insert in their L1 LSP IP prefixes learned from L2 database if also present in the routing table

• ISIS areas are not stubby anymore


Route Leaking

L1L2

L1

L1L2L1L2

L1L2L1

L1L2

L1

1. Level-1 LSP withIP prefix: 10.14.0.0/16

4. At this point prefix 10.14.0.0/16 will NOT be inserted in L2 LSP since it has the Down-bit set

3. Level-1 LSP with IP prefix: 10.14.0.0/16 Up/Down-bit set

3. At this point prefix 10.14.0.0/16 will be inserted in L1 LSP since route leaking is configured AND the prefix is present in the routing table as a L2 route



Route Leaking

L1L2

L1

L1L2


L1L2

L1L2L1

L1L2

L11. Level-1 LSP withIP prefix: 10.1.0.0/16

2. Level-2 LSP withIP prefix: 10.1.0.0/16 2. Level-2 LSP with

IP prefix: 10.1.0.0/16

5. At this point the prefix 10.1.0.0/16 will NOT be inserted in the L1 LSP since a L1 route is preferred in the routing table

5. At this point the prefix 10.1.0.0/16 will NOT be inserted in the L1 LSP since a L1 route is preferred in the routing table

3. Level-1 LSP withIP prefix: 10.1.0.0/16Up/Down-Bit set



Route Leaking

• For IP only

• Prefixes MUST be present in the routing table as ISIS level-2 routes

Otherwise no leaking occurs

Same criteria than L1 to L2

Inter-area routing is done through the routing table


Route Leaking

• Solution for several issues:

• optimal inter-area routing

• BGP shortest path to AS exit point

• MPLS-VPN


Route Leaking

• 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


Route Leaking

• UP/Down bit Extended IP Reachability TLV (135) contains Up/Down bit

Described in draft-ietf-isis-traffic

• UP/Down bit is set each time a prefix is leaked into a lower level

• Prefixes with Up/Down bit set are NEVER propagated to a upper level


Route Leaking

• Recommendation: use wide Metric TLV (TLV 135)

• Configured with:

Router isis metric-style wide


Route Leaking

• Route leaking is implemented in 12.1Cisco IOS 12.1 command

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


Summarization is possible …..

• From L1 areas into the L2 backbone,

• From L2 leaking down into L1 areas,

• When redistributing into L2 or L1

router isis summary address 192.1.0.0 255.255.0.0


Is is

Documents

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iso protocol

network protocol iso

end systems

routing ip iso

dynamic routing protocol

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routing layer