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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
4RST-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.
IS-IS Overview
555RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
666RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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)
777RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
888RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
IS-IS Overview (Cont.)
• 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
999RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
IS-IS Overview (Cont.)
• 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
10RST-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.
CLNS Addressing
111111RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Area ID SEL
CLNS Addressing
• CLNS addressing consists of 3 parts:
Area—variable
ID
SEL(ector)
121212RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
• 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
131313RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
141414RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
151515RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
161616RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
171717RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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?
18RST-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.
IS-IS Levels
191919RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
202020RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
212121RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Areas and Backbone Routers (Cont.)
• 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
222222RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
L1L2
L1L2
L1L2L1
L1
L1
Area 49.001
Area 49.003Area 49.0002
Areas and Backbone Routers (Cont.)
232323RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
L1L2
L1L2
L1L2
L1L2
L1L2
L1 Only
L2 Only
L1 Only
Area 2
Area 1
Area 3
Area 4L1 Only
L1 Only
Areas and Backbone Routers (Cont.)
• Backbone must be L2 contiguous
242424RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
Areas and Backbone Routers (Cont.)
“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.
252525RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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)
26RST-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.
IS-IS PDUs
272727RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
282828RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
292929RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
303030RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
313131RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
323232RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
333333RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
343434RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
353535RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
363636RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
373737RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
383838RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
393939RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
LSP for Router AIS: 10 P
LSP for Router AIS: 10 P
LSP for Router AIS: 10 P
Pseudonode in the LSPDB
404040RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
414141RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
424242RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
43RST-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.
LSP Header
444444RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
LSP Header
• The LSP header contains
LSP-id
Sequence number
Remaining lifetime
Checksum
Type of LSP (Level 1, Level 2)
Attached bit
Overload bit
454545RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
464646RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
474747RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
484848RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
49RST-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.
LSP flooding
49
505050RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
515151RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
525252RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
535353RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
545454RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Basic flooding rules
• If same age:
Acknowledge the LSP with a PSNP
• If older:
Acknowledge the LSP with a PSNP
Send our version of the same LSP
Wait for PSNP
555555RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
565656RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
575757RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
585858RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
595959RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
606060RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
61RST-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.
TLVs
626262RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
636363RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
646464RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
656565RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
666666RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
676767RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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)
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Configuration
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!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
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!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
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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
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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
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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
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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
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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
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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
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Areas and levels
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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)
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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
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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
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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
828282RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
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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
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Design Considerations
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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
868686RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Design guidelinesOverload-bit
• 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
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Design guidelinesOverload-bit
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)
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Design guidelinesOverload-bit
• 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
898989RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Design guidelinesOverload-bit
• 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
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Design guidelinesOverload-bit
• 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
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Design guidelinesOverload-bit
• 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
929292RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
Set over Load Bit (Cont.)
router isis
set-overload-bit
set-overload-bit on-startup <sec>
set-overload-bit on-startup wait- for-bgp
939393RST-2083010_05_2001_c1 © 2001, Cisco Systems, Inc. All rights reserved.
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
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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
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IP routing specifics
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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
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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
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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
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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
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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
2. Level-2 LSP withIP prefix: 10.14.0.0/16
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Route Leaking
L1L2
L1
L1L2
4. Level-2 LSP withIP prefix: 10.1.0.0/16
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
3. Level-2 LSP withIP prefix: 10.1.0.0/16
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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
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Route Leaking
• Solution for several issues:
• optimal inter-area routing
• BGP shortest path to AS exit point
• MPLS-VPN
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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
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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
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Route Leaking
• Recommendation: use wide Metric TLV (TLV 135)
• Configured with:
Router isis metric-style wide
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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>
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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
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