IS-ISIntermediate System-to-Intermediate System Rick GrazianiCabrillo Collegegraziani@cabrillo.eduFall 2009

SourcesSources used in creating this IS-IS presentation:Cisco Online curriculum, CCNP 1 version 3.0IS-IS Network Design Solutions, Cisco PressVery Good ReferenceCCNP Self-Study (BSCI), Cisco PressRouting TCP/IP Volume I by Jeff Doyle

IS-IS versus OSPFIS-IS is exactly the same as OSPF only completely different.

IS-IS versus OSPFOSPF discussed in CCNA and CCNP (Single Area and Multi-Area)The following slides show terminology differences between IS-IS and OSPF.Similar to the differences between Spanish and Italian.Suggestion: Put the following on the board.

IS-IS versus OSPF - Terminology

IS-ISOSPFCommentsES (End System)HostIS (Intermediate System)RouterCircuitLinkSNPA (Subnetwork Point of Attachment)Datalink AddressPDU (Protocol Data Unit)PacketDIS (Designated Intermediate System)DR (Designated Router)N/ABDRIIH (IS-to-IS Hello Packet)Hello packet

IS-IS versus OSPF - Terminology

IS-ISOSPFCommentsLSP (Link-State Packet)LSA (Link -State Advertisement)LSAs are actually comparable to TLVs used in LSPs.CSNP (Complete Sequence Number PDU or Packet) DBD (Data Base Description Packet) PSNP (Partial Sequence Number PDU or Packet) LSAck or LSR (Link State Request) Routing Domain AS The term routing domain is also used with OSPF.Level 1 Area Area (non-backbone) Level 2 AreaBackbone area (Area 0)IS-IS uses a backbone path connected by contiguous L2 routers. There is no backbone area in IS-IS

IS-IS versus OSPF ISs (Routers)

IS-ISOSPFCommentsLevel 1 IS (router)Internal Non-backbone RouterInternal, non-backbone router in a Totally Stubby AreaLevel 2 IS (router)Internal Backbone Router or ASBRAny Level 2 router can distribute externals into the domain. No special name. (Cisco IOS allows Level 1 routers to distribute externals.)Level 1-2 IS (router)ABRSystem IDRouter IDThe System ID is the key for SPF calculations. Sometimes the NET address is thought of as the Router ID.AFI = 49RFC 1918 AddressesAFI is part of the NSAP.

IS-IS versus OSPF - Timers

InterfaceIS-ISOSPFPoint-to-PointHello 10 secHoldtime 30 secHello 10 secDead 40 secBroadcastHello 10 secHoldtime 30 secHello 10 secDead 40 secNBMAN/AHello 30 secDead 120 sec

OtherIS-ISOSPFLS Aging1,200 sec or 20 min(counts down)3,600 sec or 60 min(counts up)LS RefreshEvery 15 minEvery 30 minNBMAN/AHello 30 secDead 120 secSPF Delay/Holdtime5.5 sec / 10 sec5 sec / 10 sec

History of IS-IS and OSPFJust like the MacIntosh versus Windows debate

History of IS-IS and OSPF http://www.nanog.org1985Originally called DECnet Phase V1987 IS-IS (from DEC) selected by ANSI as OSI intradomain protocol (CLNP only) 1988 NSFnet deployed, IGP based on early IS-IS draft OSPF work begins, loosely based on IS-IS mechanisms IP extensions to IS-IS defined1989 OSPF v.1 RFC published IS-IS becomes ISO proposed standard Public bickering ensues--OSPF and IS-IS are blessed as equals by IETF, with OSPF somewhat more equal Private cooperation improves both protocols 1990 Dual-mode IS-IS RFC published

History of IS-IS and OSPF http://www.nanog.org1991 OSPF v.2 RFC published Cisco ships OSI-only IS-IS 1992 Cisco ships dual IS-IS Lots of OSPF deployed, but very little IS-IS 1993 Novell publishes NLSP (IPX IS-IS knockoff)1994 Large ISPs need an IGP; IS-IS is recommended due to recent rewrite and OSPF field experience (and to lesser extent, NSF CLNP mandate) 1995 ISPs begin deployment of IS-IS, Cisco implementation firms up, protocol starts to become popular in niche

History of IS-IS and OSPF http://www.nanog.org1996-1998 IS-IS niche popularity continues to grow (some ISPs switch to it from OSPF) IS-IS becomes barrier to entry for router vendors targeting large ISPs Juniper and other vendors ship IS-IS capable routers 1999-present Extensions continue for both protocols

Who uses IS-IS?IS-IS is popular amongst telcos and large ISPs (at least in U.S.).A Tier 1 carrier is a telco or ISP that is at the top of the telecommunications peering and settlements food chain. Tier 1 operators typically have operations in more than one country Tier 1 operators own and operate their own physical networks, and either own or part-own their international submarine cable links.

Example of Tier 1 carriers (not necessarily IS-IS users): AT&T Global Crossing Level 3 NTT/Verio Qwest Sprint Verizon Business (UUNET)

Introduction to IS-ISProtocolRoutersAreas

OSI: Two Network Services, Two Network ProtocolsCMNS (Connection Mode Network Service) Requires establishment of a path between transport layer entites.CONP (Connection-Oriented Network Protocol) OSI network layer protocol that carries upper layer data over connection-oriented links.

CLNS (Connectionless Network Service) Performs datagram support, does not require circuit to be establishedCLNP (Connectionless Network Protocol) OSI network layer protocol that carries upper layer data over connectionless links. (Similar to IP)

IS-IS Protocol OptionsIS-IS (ISO 10589)Dynamic link state routing protocol used in an ISO CLNS environment.

Integrated IS-IS (RFC 1195)IS-IS for mixed ISO CLNS and IP environments.Either:Purely ISOPurely IP (CCNP 1)Both

OSI Routing Protocols: ES-IS and IS-ISES-ISAnalogous to Address Resolution Protocol (ARP) in IPNot technically a routing protocolSometimes referred to as Level 0 routing.ESs (hosts) discover nearest IS (router) by listening to IS Hello (ISH) packetsISs (routers) know which hosts are on their subnetwork by listening to ES Hello (ESH) packets.Not applicable for IP networks ISHESH

OSI Routing Protocols: ES-IS and IS-ISIS-IS OSI distinguishes between Level 1, Level 2, and Level 3 routing.Level 1 RoutingIf DA is an ES on another subnetwork in the same area, the IS knows the correct route and forwards packet appropriately. Level 2 Routing If DA is an ES on another area, the Level 1 IS sends the packet to the nearest Level 2 IS. (coming)Level 3 Routing is between separate domains. Pure CLNS environment IDRP or ISO-IGRP can be used, in IP, BGP is used. (Not applicable to CCNP)Boundary areas in IS-IS exists on a link between routers and not on a router itself as in OSPF.These routers should be entirely in Area 1 and Area 2.

IS-IS AreasIS-IS Routers:Level 1 IS (L1 IS, router)Analogous to OSPF Internal non-backbone router (Totally Stubby)Responsible for only routing to ESs inside an area.Level 2 IS (L2 IS, router)Analogous to OSPF Internal Backbone routerResponsible for routing between areasLevel 1 and Level 2 IS (L1-L2 IS, router)Analogous to OSPF ABR routerParticipate in both L1 intra-area routing and L2 inter-area routing.

Level 1 RouterLevel 1 IS (L1 IS, router)Analogous to OSPF Internal non-backbone router (Totally Stubby)Responsible for only routing to ESs inside an area.Level 1 routers maintain the Level 1 database for the area and exit points to neighboring areas.

Level 2 RouterLevel 2 IS (L2 IS, router)Analogous to OSPF Internal Backbone routerResponsible for routing between areasAlso referred to as area routers.Interconnects the Level 1 areasStore separate database of only inter-area topology

Level 1 Level 2RouterLevel 1 and Level 2 IS (L1-L2 IS, router)Analogous to OSPF ABR routerParticipate in both L1 intra-area routing and L2 inter-area routing.Maintain both Level 1 and Level 2 LSDBSupport Level 1 function communicating with other Level 1 routers in their areaInform other Level 1 routers that they are the exit point (default route) from the area.Support Level 2 function communicating with the rest of the backbone path.

IS-IS BackboneIS-IS does not share the concept of a backbone area 0 with OSPF.IS-IS backbone a set of distinct areas interconnected by a chain of Level 2 routers, weaving their way through and between the Level 1 Areas.The IS-IS backbone (path) consists of a contiguous set of Level 1-2 and Level 2 routers.Where is the backbone (path)?

OSI AddressingNSAP (Area, System ID, NSEL)SNAP

OSI AddressingIS-IS uses OSI network layer addressing In IP only networks used to identify the router (IS)A variety of NSAP formats exist, which we will not go into.Represented in hexadecimal (up to 40 hex digits)Cisco format: Area System ID NSEL (always 00 on ISs) 49.0001.2222.2222.2222.00NSAP

NSAPsFormat of the Cisco NSAP address consists of three parts. Area addressSystem IDNSAP selector byte

Area address is a variable length field The system ID is the ES or IS identifier in an area, similar to the OSPF router ID. The system ID has a fixed length of six bytes as engineered in the Cisco IOS. The NSAP selector byte is a service identifier. Analogous to that of a port or socket in TCP/IP.

Do I have to know this, I am only routing IP?Even in IP only networks, IS-IS uses OSI addresses:Identify the router (IS)Build the topology table Build the SPF treeLSPsHello and other PDUs

NSAPs Cisco FormatAreaAddresses starting with 49 (AFI=49) are considered private IP address, analogous to RFC 1918.Routed by IS-ISShould not be advertised to other CLNS networks (outside this IS-IS domain)Additional 2 bytes (0001) added for the area IDAll routers in the same are must have the same area addressArea System ID NSEL 49.0001.2222.2222.2222.00

NSAPs Cisco FormatSystem IDCisco fixes the System ID at 6 bytes.Customary to use one of the following:MAC address from the routerIP address of loopback interface192.168.111.3 -> 192.168.111.003 -> 1921.6811.1003Each device (IS and ES) must have a unique System ID within the area. (Recommended to make them unique within the domain.)Area System ID NSEL 49.0001.2222.2222.2222.00

NSAPs Cisco FormatNSEL (NSAP Selector)NSEL is a service identifier. Loosely equivalent to that of a port or socket in TCP/IP.Not used in routing decisions.NET (Network Entity Title) When NSEL = 00, it identifies the device itself, the network level address.The NSAP with a NSEL = 00 is known as a Network Entity Title (NET)A NET is an NSAP with the NSEL set to (00)Area System ID NSEL 49.0001.2222.2222.2222.00

NSAP (NETs)Example 1: NSAP 47.0001.aaaa.bbbb.cccc.00Area ID is: 47.0001System ID is: aaaa.bbbb.ccccNSAP selector byte is: 00Example 2: NSAP 39.0f01.0002.0000.0c00.1111.00Area ID is: 39.0f01.0002System ID is: 0000.0c00.1111NSAP selector byte is: 00Other ExamplesArea System ID NSEL 49.0001.2222.2222.2222.00

Configuring IS-IS (so far)ip router isis: IS-IS must be enabled on the interfaceNote: IS-IS routing cannot be enabled on an interface until an IP address has been configured on the interface.IOS: Cisco IOS 12.2(12) with Enterprise Plus (16 MB Flash/48 MB RAM) or Enter Plus IPSec56 (16 MB Flash/64 MB RAM) SanJose1interface FastEthernet0/0 ip address 172.16.0.1 255.255.255.0 ip router isis isis priority 100router isis net 49.0001.1111.1111.1111.00

SanJose2interface FastEthernet0/0 ip address 172.16.0.2 255.255.255.0 ip router isisrouter isis net 49.0001.2222.2222.2222.00 Area . System ID . NSELSanJose3interface FastEthernet0/0 ip address 172.16.0.3 255.255.255.0 ip router isisrouter isis net 49.0001.3333.3333.3333.00Area 49.0001

Configuring IS-IS (so far)To display both ES and IS neighbors.SNPA (Subnetwork Point of Attachment) address is the interface circuit ID.Ethernet: MAC address of the remote router.Serial, would show encapsulation, I.e. HDLCType: Cisco routers default to L1-L2 type routers.We will see how to change this in a moment.Area 49.0001

IS-IS AdjacenciesAre we half way yet?

Hello MessagesIS-IS uses Hello PDUs to establish adjacencies with other routers (ISs) and ESs.IS-IS has three types of Hello PDUs:ESH, sent by ES to an ISISH, sent by IS to an ESIIH, used between two ISs (CCNP)

Neighbors and AdjacenciesIS-IS discover neighbors and forms adjacencies using IS-IS Hello PDUs.Transmitted every 10 secondsCan be changed using the interface command, is hello-intervalHold time defaults to 3 times the Hello time (30 seconds), before declaring a neighbor dead.Changed using the interface command is hello-multiplierDefault is 3

LAN Representation and AdjacenciesSimilar to the DR in OSPFDIS (Designated IS) is elected to generate the LSP (Link State Packet, ie. LSA) representing the virtual router connecting all attached routers to a star-shaped topologyLAN uses a virtual node called pseudonode.Represents the LANSent by the DISDIS

LAN Representation and AdjacenciesElection of DIS:Router with highest priority (Cisco default is 64)Router with highest MAC addressNo BDRNo way to make a router ineligible from being DIS (no OSPF priority 0)New router (IS) can cause a new election, unlike OSPFPeriodically broadcasts CSNPs (OSPF DBD) every 10 secondsDIS

Configuring IS-IS (so far)isis priority: Sets DIS priority on a LAN interface, default 64SanJose1interface FastEthernet0/0 ip address 172.16.0.1 255.255.255.0 ip router isis isis priority 100router isis net 49.0001.1111.1111.1111.00

SanJose2interface FastEthernet0/0 ip address 172.16.0.2 255.255.255.0 ip router isisrouter isis net 49.0001.2222.2222.2222.00 Area . System ID . NSELSanJose3interface FastEthernet0/0 ip address 172.16.0.3 255.255.255.0 ip router isisrouter isis net 49.0001.3333.3333.3333.00Area 49.0001DIS

AdjacenciesL1 routers form L1 adjacencies with L1 and L1-L2 routers in their area.L2 routers form L2 adjacencies with L2 and L1-L2 routers in their area or another area.L1 router does not form an adjacency with an L2 routerLAN Adjacencies

AdjacenciesWhat are the adjacencies? L1? L2? L1L2? None?

The adjacencies also determine what type of routes the IS (router) will have in its routing table. L1 Intra-area routes (routes only within that area)L2 Inter-area routes (routes from other areas)Or both

AdjacenciesFor an adjacency to be formed and maintained, routers must agree on:If they are both Level 1, them must be in the same areaThe System ID must be unique to each router.The Hello timers do not have to match.

Metric and Calculate the Shortest PathFour types of metrics:Default Cisco only supports this metric.DelayExpenseErrorEach metric expressed as an integer between 0 and 63.

MetricCisco assigns a default metric of 10 to every interface regardless of interface type.Total cost of any route = sum of metrics of the outgoing interfaces.By default, IS-IS metric becomes a simple measure of hop count.Interface command: isis metric changes the default value.

Multi-Area IS-IS Example

Levelsis-type {level-1 | level-1-2 | level-2-only}L1L2L1L2SanJose1interface FastEthernet0/1 ip address 192.168.10.1 255.255.255.252 ip router isisinterface FastEthernet0/0 ip address 172.16.0.1 255.255.255.0 ip router isisinterface Serial0/0 ip address 10.0.0.2 255.255.255.252 ip router isisrouter isis net 49.0001.1111.1111.1111.00 is-type level-1-2

SanJose2interface FastEthernet0/0 ip address 172.16.0.2 255.255.255.0 ip router isisinterface FastEthernet0/1 ip address 192.168.20.1 255.255.255.0 ip router isisrouter isis net 49.0001.2222.2222.2222.00 is-type level-1

Phoenixinterface Serial0/0 ip address 10.0.0.1 255.255.255.252 ip router isisinterface FastEthernet0/1 ip address 192.168.30.1 255.255.255.0 ip router isis router isis net 49.0002.3333.3333.3333.00 is-type level-2-onlyNote: ip router isis command must be added not only to transit interfaces but also to interfaces connected to stub networks whose IP addresses should be advertised by IS-IS.

show clns neighSanJose1#show clns neighborsSystem Id Interface SNPA State Holdtime Type ProtocolSanJose2 Fa0/0 0003.6be9.d480 Up 26 L1 IS-ISPhoenix Se0/0 *HDLC* Up 28 L2 IS-IS

SanJose2#show clns neighborsSystem Id Interface SNPA State Holdtime Type ProtocolSanJose1 Fa0/0 0002.b9ee.5ee0 Up 4 L1 IS-IS

Phoenix#show clns neighborsSystem Id Interface SNPA State Holdtime Type ProtocolSanJose1 Se0/0 *HDLC* Up 27 L2 IS-ISL1L2L1L2System ID Cisco IOS uses the hostname.SNPA is the MAC of the remote router or HDLC on the serial link.The Type is the is-type of the remote router.

show isis dataSanJose1#show isis database

IS-IS Level-1 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLSanJose1.00-00 * 0x00000005 0xDE15 916 1/0/0SanJose1.01-00 * 0x00000003 0xBBFE 1165 0/0/0SanJose2.00-00 0x00000006 0xBDFB 470 0/0/0IS-IS Level-2 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLSanJose1.00-00 * 0x0000001B 0xAB48 914 0/0/0SanJose1.01-00 * 0x0000001B 0x5526 480 0/0/0Phoenix.00-00 0x0000001E 0xA3D3 912 0/0/0Phoenix.01-00 0x00000002 0x54A6 826 0/0/0L1L2L1L2SanJose1 has a L1 LSDB and a L2 LSDB.

show isis dataSanJose2#show isis databaseIS-IS Level-1 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLSanJose1.00-00 0x00000005 0xDE15 907 1/0/0SanJose1.01-00 0x00000003 0xBBFE 1155 0/0/0SanJose2.00-00 * 0x00000006 0xBDFB 464 0/0/0

Phoenix#show isis databaseIS-IS Level-2 Link State Database:LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OLSanJose1.00-00 0x0000001B 0xAB48 893 0/0/0SanJose1.01-00 0x0000001B 0x5526 459 0/0/0SanJose2.00-00 0x00000024 0x2FD3 0 (932) 0/0/0Phoenix.00-00 * 0x0000001E 0xA3D3 895 0/0/0Phoenix.01-00 * 0x00000002 0x54A6 808 0/0/0L1L2L1L2SanJose2 only has a L1 LSDB.Phoenix only has a L2 LSDB.

show ip routeSanJose1#show ip route

i L2 192.168.30.0/24 [115/20] via 10.0.0.1, Serial0/0C 192.168.10.0/24 is directly connected, FastEthernet0/1 172.16.0.0/24 is subnetted, 1 subnetsC 172.16.0.0 is directly connected, FastEthernet0/0i L1 192.168.20.0/24 [115/20] via 172.16.0.2, FastEthernet0/0 10.0.0.0/30 is subnetted, 1 subnetsC 10.0.0.0 is directly connected, Serial0/0L1L2L1L2Because SanJose1 is an L1L2 router, it contains both:L1 routes for Area 49.0001 and L2 routes for the other area 49.0002.

show ip routeSanJose2#show ip route

i L1 192.168.10.0/24 [115/20] via 172.16.0.1, FastEthernet0/0 172.16.0.0/24 is subnetted, 1 subnetsC 172.16.0.0 is directly connected, FastEthernet0/0C 192.168.20.0/24 is directly connected, FastEthernet0/1 10.0.0.0/30 is subnetted, 1 subnetsi L1 10.0.0.0 [115/20] via 172.16.0.1, FastEthernet0/0i*L1 0.0.0.0/0 [115/10] via 172.16.0.1, FastEthernet0/0L1L2L1L2Because SanJose2 is only an L1 router, it only contains:L1 routes for Area 49.0001 andThe 0.0.0.0/0 default route sent by SanJose1.L1-only routers, always learn a default route from a neighboring L1-L2router

show ip routePhoenix#show ip route

Gateway of last resort is not set

C 192.168.30.0/24 is directly connected, FastEthernet0/1i L2 192.168.10.0/24 [115/20] via 10.0.0.2, Serial0/0 172.16.0.0/24 is subnetted, 1 subnetsi L2 172.16.0.0 [115/20] via 10.0.0.2, Serial0/0i L2 192.168.20.0/24 [115/30] via 10.0.0.2, Serial0/0 10.0.0.0/30 is subnetted, 1 subnetsC 10.0.0.0 is directly connected, Serial0/0L1L2L1L2Because Phoenix is an L2 router, it contains:L2 routes which are L1 routes from Area 49.0001.No L1 routes from Area 49.0002.

Some final notesIS-IS encapsulate PDUs directly into the data-link frame.Uses an on-demand circuit to reduce Hello and LSP flooding across switched WAN links like ISDN.If no metric is specified for routes redistributed into IS-IS the default metric is 0.IS-IS router may have adjacencies with other routers on multipoint links.i su in the routing table code means the route is an IS-IS summary route. IS-IS provides support for multiple network layer protocolsIS-IS forms adjacencies with all neighbors.By default, Cisco IS-IS routers are Level 1-2 routers.

Additional Information

Sub-optimal RoutingSub-optimal routing is not necessarily a disadvantage.

IS-IS vs OSPFIS-IS is more scalable than OSPF because:More routers can reside in an areaFewer link-state PDUs are used

AuthenticationIS-IS authenticationInterface configurationisis password password

Stuck in INITRouters may have difficulty forming an adjacency, stuck in INIT if:Mismatched authentication configurationsMismatched MTU configurations

The IS-IS Routing Process, Summarization and Additional NotesAdditional information helpful to understand and some of those little items they like to ask us about on certification exams.

TerminologyCLV (Code/Length/Value) and TLV (Type/Length/Value)Same thing, CLV is more of the OSI term.There are variable length fields in a PDU.Code or Type specifies the type of information.Length specifies the size of the Value field.Value is the information itself.Example CLV or TLV 128 defines the capability to carry IP routes in IS-IS packets, in essence TLV 128 is Integrated IS-IS.

IS-IS Routing ProcessIS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

IS-IS Routing Process: UpdateThe Update ProcessRouters can only forward data packets if they have an understanding of the network topology.

LSPs are generated and flooded throughout the network whenever:An adjacency comes up or down (example: a new router comes online).An interface on a router changes state or is assigned a new metric.An IP route changes (example: because of redistribution)

IS-IS Routing Process: UpdateSending and Receiving an LSP

Receiving an LSPIf the LSP is already present in the database (LSDB), the router (IS) acknowledges (PSNP) and ignores it.The router sends the duplicated LSP it its neighbors.Level 1 LSPs are flooded throughout the areaLevel 2 LSPs are sent across all L2 adjacencies.

IS-IS Routing Process: UpdateSending and Receiving an LSP:

Propagating (sending) LSPs on a Point-to-Point InterfaceWhen an adjacency is established both routers send a CSNP (OSPF DBD) summary of their LSDB.If the receiving router has any LSPs that were not present in the CSNP it received, it sends a copy of the missing LSP to the other router. CSNPCSNPYou are missing LSP 3LSP 3PSNP (Ack)

IS-IS Routing Process: UpdateSending and Receiving an LSP:Propagating (sending) LSPs on a Point-to-Point InterfaceLikewise, if the receiving router is missing any LSPs received in the CSNP, the receiving router sends a PSNP (OSPF LSR) requesting the full LSP to be sent.LSPs are acknowledges with a PSNP (OSPF LSAck)When the LSP is sent, the router sets a timer.If the acknolwedgement (PSNP) is not received within 5 seconds (Cisco default), the LSP is resent.CSNPCSNPI am missing LSP 3LSP 3PSNP (Ack)PSNP 3

IS-IS Routing Process: UpdateSending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceThe DIS (OSPF DR) takes on much of the responsibility for synchronizing the databases on behalf of the pseudonode.DIS has three tasks:Creating and maintaining adjacenciesCreating and updating the pseudonode LSPFlooding the LSPs over the LAN.

IS-IS Routing Process: UpdateSending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceOn receiving a CSNP the router compares it with its LSDBIf the receiving router has a newer version of the LSP then what was sent in the CSNP, or if the CSNP did not contain one of its LSPs, the router multicasts the LSP to all routers on the LAN.CSNP (DBD)LSP 88I have a newer version of LSP 88Receipt of LSP 88 is acknowledged by all routers with a PSNP.

IS-IS Routing Process: UpdateSending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceOn receiving a CSNP the router compares it with its LSDBIf the database is missing an LSP that was in the CSNP, it sends a PSNP requesting the full LSP.The DIS sends the LSP.CSNP (DBD)PSNP (LSR)LSP 77 (LSA)PSNP (LSAck)

IS-IS Routing Process: UpdateDetermining if an LSP is valid:Receiving router uses three fields to help determine if the received LSP is more recent than the one in its LSDB.Remaining LifetimeUsed to age-out or delete LSPsLifetime is set to 0 and floodedReceiving routers recognize this means the route is bad and deletes the LSP from their LSDB, rerunning SPF algorithm, new SPT, new routing table.Note: LSPs have a maximum age of 20 minutes in an IS-IS LSDB, and are re-flooded (refreshed) every 15 minutes.Sequence NumberFirst LSP starts with a sequence number of 1, with following LSPs incremented by 1.ChecksumIf received LSPs checksum does not computer correctly, the LSP is flushed and the lifetime set to 0.The receiving router floods the LSP with the lifetime set to 0.When the originating router gets this LSP (lifetime = 0) it retransmits a new LSP.

IS-IS Routing Process: DecisionIS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

IS-IS Routing Process: DecisionThe Decision ProcessUses Dijkstras algorithm to build a SPT (Shortest Path Tree)The SPT is used to create the forwarding table, also known as the routing table.Several tables are used during this process:PATH tablePATH table is the SPT during the construction of the LSDBEach candidate route is placed in the PATH table while the metric is examined to determine if it is the shortest path to the destination.TENT is the tentative database (a scratchpad) during this process

IS-IS Routing Process: DecisionDetermining the best routeCriteria by which the lowest cost paths are selected and placed in the forwarding database are:Cisco allows up to six equal-cost paths, four by default.Cisco only supports the default metricInternal paths are chosen before external paths outside the routing domain, to prevent sub-optimal routes and routing loops.Level 1 paths within the area are more attractive than Level 2 paths outside the area, to prevent sub-optimal routes and routing loops.Longest match or most specific address in IP ensures that the closest router is chosen.ToS (Type of Service) in IP header is used, if configured.If there is no path, the forwarding database sends the packet to the nearest Level 2 router, which is the default router.

IS-IS Routing Process: ForwardingIS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

IS-IS Routing Process: Forwarding and ReceivingForwarding processAfter the SPT has been built the forwarding database can be created.The forwarding table is the lookup table for the longest match.The forwarding table for IS-IS is more relevant to CLNS than to IP, because the IP routing information is entered directly into the IP routing table.

Receive processIf the frame is valid, the receive process passes user data and error reports to the forwarding process.Whereas routing information: Hellos, LSPs, and SNPs are sent to the update process.Receive process is primarily concerned with CLNS routing and not IP.

Route SummarizationRules for IS-IS route summarization similar to that of OSPFLevel 1-2 routers (L1L2)Similar to OSPF ABR Configured at the L1L2 router at the edge of an area.L1L2 routers can summarize the routes within their area to L1L2 or L2 routers in another area.This is an efficient method of establishing prefix (network addresses) routing into other areas.If one edge L1L2 router in an area is summarizing routes for that area, other edge L1L2 routers in that area must also be summarizing routes.If other L1L2 routers are summarizing and one edge L1L2 router is not summarizing, all traffic destined for that area will be sent to the non-summarizing router because of longest match routing.Level 1 routes cannot be summarized within the area because it is not permitted by IS-IS (Level 1 routers cannot summary routes).Level 2 routers can summarize at the area boundary.

MetricsOdds and EndsCiscos IS-IS implementation will perform equal cost load balancing up to six paths.Supports VLSML1 routers calculate path to the nearest L2 router for inter-area routing (OSPF: Totally Stubby Area)When an L2 or L1L2 router is attached to another area, the router will advertise this fact The Decision Process in L1 routers will choose the metrically closest L1L2 router as the default router.An L1 0.0.0.0/0 route will be entered into the routing table.IS-IS command - summary-address network mask is used to configured summarization (Level 1, Level 2 or both).See summary-address command for more details.Level 2 routers are expected to know about all routes.ISIS command: default-information originate is used to advertise a default route into the backbone path.

Odds and EndsL1L2 routers run two IS-IS processes, one for its L1 LSDB and another for its L2 LSDB.Once an packet is accepted by a router the System ID and NSEL are stripped.

IS-IS Link State Database Synchronization

LSDB Synchronization and Update ProcessIS-IS LSDB is accomplished by using special PDUs, known as SNPs (Sequence Number PDUs):CSNP (Complete Sequence Number PDU) (OSPF: DBD)List of LSPs held by the routerPSNP (Partial Sequence Number PDU) (OSPF: LSAck/LSR)Acknowledge the receipt of a LSPRequest a complete LSP for a missing entryXLSP (LSA)PSNP (LSAck)LSP (LSA)LSP (LSA)PSNP (LSAck)PSNP (LSAck)

Update ProcessPoint-to-Point networks:Once an LSP is sent, router sets a timer (minimumLSPTransmissionInterval) of 5 secondsIf PSNP (ACK) not received, resends LSP.

Update ProcessOn Broadcast networks:LSPs are not acknowledged by each receiving router.DIS periodically multicasts a CSNP (OSPF DBD) that describes every LSP in LSDB.Default is every10 secondsL1 CSNPs are multicast to AllL1ISsL2 CSNPs are multicast to AllL2ISsPSNP (OSPF LSR) goes to DIS. III. LSP 77 is sent by DIS to R1CSNP (DBD)PSNP (LSR)LSP 77 (LSA)PSNP (LSAck)

IS-ISIntermediate System-to-Intermediate System Rick GrazianiCabrillo Collegegraziani@cabrillo.eduFall 2009

IS-IS OSI distinguishes between Level 1, Level 2, and Level 3 routing.Level 1 RoutingIf DA is an ES on another subnetwork in the same area, the IS knows the correct route and forwards packet appropriately. Level 2 Routing If DA is an ES on another area, the Level 1 IS sends the packet to the nearest Level 2 IS. (coming)Level 3 Routing is between separate domains. Pure CLNS environment IDRP or ISO-IGRP can be used, in IP, BGP is used. (Not applicable to CCNP)

IS-IS Routers:Level 1 IS (L1 IS, router)Analogous to OSPF Internal non-backbone router (Totally Stubby)Responsible for routing to ESs inside an area.Level 2 IS (L2 IS, router)Analogous to OSPF Internal Backbone routerResponsible for routing between areasLevel 1 and Level 2 IS (L1-L2 IS, router)Analogous to OSPF ABR routerParticipate in both L1 intra-area routing and L2 inter-area routing.

Level 1 IS (L1 IS, router)Analogous to OSPF Internal non-backbone router (Totally Stubby)Responsible for routing to ESs inside an area.A contiguous group of Level 1 routers define an area.Level 1 routers maintain the Level 1 database for the area and exit points to neighboring areas.

Level 2 IS (L2 IS, router)Analogous to OSPF Internal Backbone routerResponsible for routing between areasAlso referred to as area routers.Interconnect the Level 1 areasStore separate database of only inter-area topologyAll interfaces in the same area

Level 1 and Level 2 IS (L1-L2 IS, router)Analogous to OSPF ABR routerParticipate in both L1 intra-area routing and L2 inter-area routing.Maintain both Level and Level 2 LSDBSupport Level 1 function communicating with other Level 1 routers in their areaInform other Level 1 routers that they are the exit point (default route) from the area.Support Level 2 function communicating with the rest of the backbone path. All interfaces in the same area

IS-IS does not share the concept of a backbone area 0 with OSPF.An IS-IS backbone can appear as a set of distinct areas interconnected by a chain of Level 2 routers, weaving their way through and between the Level 1 Areas.The IS-IS backbone (path) consists of a contiguous set of Level 1-2 and Level 2 routers.Where is the backbone (path)?

OSI network layer addressing is done through the NSAP (Network Service Access Point) consisting of:OSI Address of the deviceOSI network services provided to the transport layerConceptual boundary between network and transport layersA variety of NSAP formats exist, which we will not go into.Represented in hexadecimal (up to 40 hex digits)Cisco format: Area System ID NSEL (always 00 on ISs) 49.0001.2222.2222.2222.00

Format of the Cisco NSAP address consists of three parts. Area addressSystem IDNSAP selector byte

Area address is a variable length field The system ID is the ES or IS identifier in an area, similar to the OSPF router ID. The system ID has a fixed length of six bytes as engineered in the Cisco IOS. The NSAP selector byte is a service identifier. Analogous to that of a port or socket in TCP/IP.

NSAP prefixes are required for CLNS routing, including IP only networks.Even in IP only networks, IS-IS uses OSI addresses:Identify the router (IS)Build the topology table Build the SPF treeLSPsHello and other PDUsNSAP addresses are required for CLNS routing.

AreaAddresses starting with 49 (AFI=49) are considered private IP address, analogous to RFC 1918.Routed by IS-ISShould not be advertised to other CLNS networks (outside this IS-IS domain)Additional 2 bytes (HODSP) added for the area IDAll routers in the same are must have the same area address

System IDOSI requires it must be the same number of bytes throughout the domain.Cisco fixes the System ID at 6 bytes.Customary to use one of the following:MAC address from the routerIP address of loopback interface192.168.111.3 -> 192.168.111.003 -> 1921.6811.1003Each device (IS and ES) must have a unique System ID within the area. (Recommended to make them unique within the domain.)

NSEL (NSAP Selector)NSEL is a service identifier. Loosely equivalent to that of a port or socket in TCP/IP.Must be specified by a single byte preceded by a period (.)Not used in routing decisions.NETWhen NSEL = 00, it identifies the device itself, the network level address.The NSAP with a NSEL = 00 is known as a Network Entity Title (NET)A NET is an NSAP with the NSEL set to (00)

Example 1: NSAP 47.0001.aaaa.bbbb.cccc.00Area ID is 47.0001System ID is aaaa.bbbb.ccccNSAP selector byte is 00Example 2: NSAP 39.0f01.0002.0000.0c00.1111.00Area ID is 39.0f01.0002System ID is 0000.0c00.1111NSAP selector byte is 00

ip router isis: IS-IS must be enabled on the interfaceIOS: Cisco IOS 12.2(12) with Enterprise Plus (16 MB Flash/48 MB RAM) or Enter Plus IPSec56 (16 MB Flash/64 MB RAM) Note: IS-IS routing cannot be enabled on an interface until an IP address has been configured on the interface.

To display both ES and IS neighbors.SNPA is the MAC address of the remote router.If serial, would show encapsulation, I.e. HDLCCisco routers default to L1-L2 type routers.We will see how to change this in a moment.

IS-IS uses Hello PDUs to establish adjacencies with other routers (ISs) and ESs.IS-IS has three types of Hello PDUs:ESH, sent by ES to an ISISH, sent by IS to an ESIIH, used between two ISs (CCNP 1)Hello Level 1 LANHello Level 2 LANHello Point-to-Point

In OSI there are only two main types of physical links:Broadcast: Multiaccess media types, usually LANsNonbroadcast: Point-to-Point, Multipoint, and dynamically established links (WAN links)Thus, IS-IS supports only two media representations:Broadcast for LANsPoint-to-Point for all other mediaIS-IS has no concept of an NBMA network.Recommended that point-to-point links be used for native ATM, Frame Relay, or X.25

IS-IS discover neighbors and forms adjacencies using IS-IS Hello PDUs.Transmitted every 10 secondsCan be changed using the interface command, is hello-intervalHold time defaults to 3 times the Hello time (30 seconds), before declaring a neighbor dead.Changed using the interface command is hello-multiplierDefault is 3

Similar to the DR in OSPFDIS (Designated IS) is elected to generate the LSP (Link State Packet, ie. LSA) representing the virtual router connecting all attached routers to a star-shaped topologyFor SPF, the whole network must look like a collection of nodes and point-to-point linksMulti-access networks are differentLAN uses a virtual node called pseudonode.Represents the LANSent by the DIS

Election of DIS:Router with highest priority (Cisco default is 64)Router with highest MAC addressNo BDRNo way to make a router ineligible from being DIS (no OSPF priority 0)New router (IS) can cause a new election, unlike OSPFPeriodically broadcasts CSNPs (OSPF DBD) every 10 secondsFrame Relay: DIS is highest local DLCI if priorities are equal

isis priority: Sets DIS priority on a LAN interface, default 64

L1 routers form L1 adjacencies with L1 and L1-L2 routers in their area.L2 routers form L2 adjacencies with L2 and L1-L2 routers in their area or another area.L1L2 routers form L1 and L2 adjacencies with each other in their area or another area.L1 router does not form an adjacency with an L2 router

What are the adjacencies? L1? L2? L1L2? None?

The adjacencies also determine what type of routes the IS (router) will have in its routing table. L1 Intra-area routes (routes only within that area)L2 Inter-area routes (routes from other areas)Or both

For an adjacency to be formed and maintained, routers must agree on:If they are both Level 1, them must be in the same areaThe System ID must be unique to each router.The Hello timers do not have to match.

Once the update process has built the LSDB, the Decision Process uses the LSDB to calculate the SPF.Separate SPF for L1 routes and L2 routes.Four types of metrics:Default Cisco only supports this metric.DelayExpenseErrorEach metric expressed as an integer between 0 and 63.Separate route is calculated for each metric.SPF must be run for each metric, for both L1 and L2 routes.Because of these and other reasons, Cisco only supports the Default.

Cisco assigns a default metric of 10 to every interface regardless of interface type.Left to the default, IS-IS metric becomes a simple measure of hop count.Interface command isis metric changes the default value.The total cost of any route is a sum of the individual metrics of the outgoing interfaces.This is known as the narrow metric, which uses 6 bits for the interface metric and 10 bits for the total path metric.The maximum interface metric value is 63.The maximum total path metric value is 1023.Extended MetricCisco IOS software addresses this issue with the support of a 24-bit metric field for the interface 16,777,215 (224 - 1) 32 bit metric for the total path, called the wide metric, 4,261,412,864 (232 - 225).

System ID Cisco IOS uses the hostname.SNPA is the MAC of the remote router or HDLC on the serial link.The Type is the is-type of the remote router.

SanJose1 has a L1 LSDB and a L2 LSDB.Email me if you want the detail of this output or I can add it to the presentation.Use detail option for complete LSDB.

SanJose2 only has a L1 LSDB.Phoenix only has a L2 LSDB.

Because SanJose1 is an L1L2 router, it contains both:L1 routes for Area 49.0001 and L2 routes for the other area 49.0002.

Because SanJose2 is only an L1 router, it only contains:L1 routes for Area 49.0001 andThe 0.0.0.0/0 default route sent by SanJose1.

Because Phoenix is an L2 router, it contains:L2 routes which are L1 routes from Area 49.0001.No L1 routes from Area 49.0002.

IS-IS is more scalable than OSPF because:More routers can reside in an areaFewer link-state PDUs are used

IS-IS authenticationInterface configurationisis password password

Routers may have difficulty forming an adjacency, stuck in INIT if:Mismatched authentication configurationsMismatched MTU configurations

CLV (Code/Length/Value) and TLV (Type/Length/Value)Same thing, CLV is more of the OSI term.There are variable length fields in a PDU.Code or Type specifies the type of information.Length specifies the size of the Value field.Value is the information itself.Example CLV or TLV 128 defines the capability to carry IP routes in IS-IS packets, in essence TLV 128 is Integrated IS-IS.

IS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

The Update ProcessRouters can only forward data packets if they have an understanding of the network topology.

LSPs are generated and flooded throughout the network whenever:An adjacency comes up or down (example: a new router comes online).An interface on a router changes state or is assigned a new metric.An IP route changes (example: because of redistribution)

Sending and Receiving an LSP

Receiving an LSPIf the LSP is already present in the database (LSDB), the router (IS) acknowledges (PSNP) and ignores it.The router sends the duplicated LSP it its neighbors.Level 1 LSPs are flooded throughout the areaLevel 2 LSPs are sent across all L2 adjacencies.

Sending and Receiving an LSP:

Propagating (sending) LSPs on a Point-to-Point InterfaceWhen an adjacency is established both routers send a CSNP (OSPF DBD) summary of their LSDB.If the receiving router has any LSPs that were not present in the CSNP it received, it sends a copy of the missing LSP to the other router.

Sending and Receiving an LSP:Propagating (sending) LSPs on a Point-to-Point InterfaceLikewise, if the receiving router is missing any LSPs received in the CSNP, the receiving router sends a PSNP (OSPF LSR) requesting the full LSP to be sent.LSPs are acknowledges with a PSNP (OSPF LSAck)When the LSP is sent, the router sets a timer.If the acknolwedgement (PSNP) is not received within 5 seconds (Cisco default), the LSP is resent.

Sending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceThe DIS (OSPF DR) takes on much of the responsibility for synchronizing the databases on behalf of the pseudonode.DIS has three tasks:Creating and maintaining adjacenciesCreating and updating the pseudonode LSPFlooding the LSPs over the LAN.

Sending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceOn receiving a CSNP the router compares it with its LSDBIf the receiving router has a newer version of the LSP then what was sent in the CSNP, or if the CSNP did not contain one of its LSPs, the router multicasts the LSP to all routers on the LANSending and Receiving an LSP:Propagating (sending) LSPs on a Broadcast InterfaceOn receiving a CSNP the router compares it with its LSDBIf the database is missing an LSP that was in the CSNP, it sends a PSNP requesting the full LSP.The DIS sends the LSP.

Determining if an LSP is valid:Receiving router uses three fields to help determine if the received LSP is more recent than the one in its LSDB.Remaining LifetimeUsed to age-out or delete LSPsLifetime is set to 0 and floodedReceiving routers recognize this means the route is bad and deletes the LSP from their LSDB, rerunning SPF algorithm, new SPT, new routing table.Note: LSPs have a maximum age of 20 minutes in an IS-IS LSDB, and are re-flooded (refreshed) every 15 minutes.Sequence NumberFirst LSP starts with a sequence number of 1, with following LSPs incremented by 1.ChecksumIf received LSPs checksum does not computer correctly, the LSP is flushed and the lifetime set to 0.The receiving router floods the LSP with the lifetime set to 0.When the originating router gets this LSP (lifetime = 0) it retransmits a new LSP.

IS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

The Decision ProcessUses Dijkstras algorithm to build a SPT (Shortest Path Tree)The SPT is used to create the forwarding table, also known as the routing table.Several tables are used during this process:PATH tablePATH table is the SPT during the construction of the LSDBEach candidate route is placed in the PATH table while the metric is examined to determine if it is the shortest path to the destination.TENT is the tentative database (a scratchpad) during this process

Determining the best routeCriteria by which the lowest cost paths are selected and placed in the forwarding database are:Cisco allows up to six equal-cost paths, four by default.Cisco only supports the default metricInternal paths are chosen before external paths outside the routing domain, to prevent sub-optimal routes and routing loops.Level 1 paths within the area are more attractive than Level 2 paths outside the area, to prevent sub-optimal routes and routing loops.Longest match or most specific address in IP ensures that the closest router is chosen.ToS (Type of Service) in IP header is used, if configured.If there is no path, the forwarding database sends the packet to the nearest Level 2 router, which is the default router.

IS-IS Routing Process is divided into four stages:UpdateDecisionForwardingReceive

Forwarding processAfter the SPT has been built the forwarding database can be created.The forwarding table is the lookup table for the longest match.The forwarding table for IS-IS is more relevant to CLNS than to IP, because the IP routing information is entered directly into the IP routing table.

Receive processIf the frame is valid, the receive process passes user data and error reports to the forwarding process.Whereas routing information: Hellos, LSPs, and SNPs are sent to the update process.Receive process is primarily concerned with CLNS routing and not IP.

Rules for IS-IS route summarization similar to that of OSPFLevel 1-2 routers (L1L2)Similar to OSPF ABR Configured at the L1L2 router at the edge of an area.L1L2 routers can summarize the routes within their area to L1L2 or L2 routers in another area.This is an efficient method of establishing prefix (network addresses) routing into other areas.If one edge L1L2 router in an area is summarizing routes for that area, other edge L1L2 routers in that area must also be summarizing routes.If other L1L2 routers are summarizing and one edge L1L2 router is not summarizing, all traffic destined for that area will be sent to the non-summarizing router because of longest match routing.Level 1 routes cannot be summarized within the area because it is not permitted by IS-IS (Level 1 routers cannot summary routes).Level 2 routers can summarize at the area boundary.

Odds and EndsCiscos IS-IS implementation will perform equal cost load balancing up to six paths.Supports VLSML1 routers calculate path to the nearest L2 router for inter-area routing (OSPF: Totally Stubby Area)When an L2 or L1L2 router is attached to another area, the router will advertise this fact The Decision Process in L1 routers will choose the metrically closest L1L2 router as the default router.An L1 0.0.0.0/0 route will be entered into the routing table.IS-IS command - summary-address network mask is used to configured summarization (Level 1, Level 2 or both).See summary-address command for more details.Level 2 routers are expected to know about all routes.ISIS command: default-information originate is used to advertise a default route into the backbone path.

L1L2 routers run two IS-IS processes, one for its L1 LSDB and another for its L2 LSDB.Once an packet is accepted by a router the System ID and NSEL are stripped.

IS-IS LSDB is accomplished by using special PDUs, known as SNPs (Sequence Number PDUs):CSNP (Complete Sequence Number PDU) (OSPF: DBD)List of LSPs held by the routerPSNP (Partial Sequence Number PDU) (OSPF: LSAck/LSR)Acknowledge the receipt of a LSPRequest a complete LSP for a missing entry

Point-to-Point networks:Once an LSP is sent, router sets a timer (minimumLSPTransmissionInterval) of 5 secondsIf PSNP (ACK) not received, resends LSP.

On Broadcast networks:LSPs are not acknowledged by each receiving router.DIS periodically multicasts a CSNP (OSPF DBD) that describes every LSP in LSDB.Default is every10 secondsL1 CSNPs are multicast to AllL1ISsL2 CSNPs are multicast to AllL2ISs