1、Introduction to IS-IS,NANOG 20, Washington DCOctober 22-24,Abe Martey ,Topics,1. IS-IS Overview 2. IS-IS Protocol Concepts 3. CLNP Addressing 4. IS-IS Database 5. MPLS TE Extensions for IS-IS 6. ISP Network Design Considerations 7. Configuration Examples 8. Basic Troubleshooting 9. Further Reading,1
2、. IS-IS Overview,Terminology and AcronymsIntermediate system (IS)- Router Designated Intermediate System (DIS) - Designated Router Pseudonode - Broadcast link emulated as virtual node by DIS End System (ES) - Network Host or workstation Network Service Access Point (NSAP) - Network Layer Address Sub
3、network Point of attachment (SNPA) - Datalink interface Packet data Unit (PDU) - Analogous to IP Packet Link State PDU (LSP) - Routing information packet,IS-IS Overview,The Intermediate Systems to Intermediate System Routing Protocol (IS-IS) was originally designed to route the ISO Connectionless Ne
4、twork 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). OSPF supports only IP,IS-IS Overview,IS-IS is an Interior Gateway Protocol (IGP) - for Intra-
5、domain (AS) routing BGP provides inter-domain routing for IP networks ISO-IGRP is a Cisco proprietary routing protocol that can be used between CLNP domains,IS-IS Overview,3 network layer protocols play together to deliver the ISO defined Connectionless Network Service - CLNP - IS-IS - ES- IS - End
6、System to Intermediate System Protocol All 3 protocols independently go over layer 2,IS-IS Overview,CLNP is the ISO equivalent of IP for datagram delivery services (ISO 8473, RFC 994) ES-IS is designed for routing between network hosts and routers (ISO9542, RFC 995). IS-IS for layer 3 routing betwee
7、n routers. (ISO 10589/RFC 1142). Integrated IS-IS (RFC 1195) works within the ISO CNLS framework even when used for routing only IP.,IS-IS Overview,End System Hellos (ESH) from Hosts and Intermediate System Hellos (ISH) from Routers used for ES-IS neighbor discovery Intermediate System to Intermedia
8、te Systems Hellos (IIH) are used for establishing IS-IS layer3 adjacencies ES-IS is somehow tied into IS-IS layer 3 adjacency discovery. ES-IS enabled automatically when IS-IS is configured on Ciscos,2. IS-IS Protocol Concepts,Nodes Links Areas and Adjacencies Level-1 and level-2 routing,IS-IS Proto
9、col Concepts Network Nodes,Hosts Level-1 Routers Level-2 Routers Level-1 and Level-2 Pseudonodes on broadcast links only,IS-IS Protocol Concepts Network Nodes,PSN,DIS,DIS,Broadcast link represented as virtual node, referred to as Pseudonode (PSN) PSN role played by the Designated Router (DIS) DIS el
10、ection is preemptive, based on interface priority with highest MAC address being tie breaker IS-IS has only one DIS. DIS/PSN functionality supports database synchronization between routers on a broadcast type link,IS-IS Protocol Concepts Network/Link Types,Point-to-Point Links (PoS, ptp FR, ptp ATM)
11、 Broadcast Links (Multi-point/multi-access, ie Ethernet, mpt FR, mpt ATM or Cisco DPT) Non-broadcast multi-access treated as broadcast or point-to-point by configuration,IS-IS Protocol Concepts Areas,L1L2,L1L2,L1,L1,L1,Area 49.001,Area 49.003,Area 49.0002,L1L2,Level-1 Area,Level-1 Area,Level-1 Area,
12、Level-2 Backbone,IS-IS Protocol Concepts IS-IS Adjacencies,IIH advertised for detecting neighbor and forming network layer adjacencies 3 types of IIHs:Point-to-pointLAN Level-1LAN Level-2,IS-IS Protocol Concepts Point-to Adjacencies,Pt-to-pt IIH used to establish level-1 or Level-2 pt-to-point adjac
13、ency Only two way communication required on pt-to-pt links by ISO 10589 3-way reliable process recently proposed for standardization in the IETF. Introduces pt-to-pt adjacency state TLV (Type 240),Intra-domain Routing Protocol Discriminator,Length Indicator,TLV Fields,Version/Protocol ID Extension,I
14、D Length,R,R,R,PDU Type,Version,Reserved,Maximum Area Addresses,Reserved (6 bits),Source ID,Holding Time,PDU Length,Local Circuit ID,Circuit Type,Bytes,1,1,1,1,1,1,1,1,2,1,ID Length,2,1,Variable Length,IS-IS Protocol Concepts LAN (Broadcast link) Adjacencies,LAN Level-1 and LAN Level-2 IIH multicast
15、 independently to AllL1ISs (01-80-C2-00-00-14) and AllL2ISs (01-80-C2-00-00-14) 3-way reliable adjacency formation can be enforced due to IS neighbors TLV (Type 2) appended to LAN IIHs,Intradomain Routing Protocol Descr.,Lenth Indicator,Version/Protocol ID Extension,ID Length,PDU Type,R,R,R,Version,
16、Reserved,Maximum Area Addresses,Reserved/Circuit type,Source ID,Holding Time,PDU Length,Priority,TYPE LENGTH FIELDS,bytes1 1 1 1 1 1 1 1 1 ID Length 2 2 1 ID Length + 1 Variable,R,Lan ID,Holding Time = Hello Interval x Hello Multiplier Priority - Interface Priority for DIS arbitration LAN ID - Syste
17、m ID of DIS + PSN number,IS-IS Protocol Concepts Hierarchical Routing,Area 49.001,Area 49.0002,Level-1 Routing,Level-2 Routing,Level-1 Routing,IS-IS supports 2-level routing hierarchy Routing domain is carved into areas. Routing in an area is level-1. Routing between areas is level-2 All ISO 10589/R
18、FC1195 areas are stubs,Backbone,IS-IS Protocol Concepts IS-IS Packet Types,IS-IS Hello Packets (IIH) Level 1 LAN IS-IS Hello Level 2 LAN IS-IS Hello Point-to-point Hello Link State Packets (LSP) Level 1 and Level 2 Complete Sequence Number packets (CSNP) Level 1 and Level 2 Partial Sequence Number P
19、ackets (PSNP) Level 1 and Level 2,3. CLNS Addressing,NSAP Format AFI Values Requirements and Caveats Examples Globally unique NSAPs,CLNS Addressing NSAP Format,System ID,NSEL,AFI,Variable length Area address,6 bytes,1 byte,1 byte,1 - 12 bytes,NSAP format has 3 main componentsArea IDSystem IDN-Select
20、or (NSEL) - value is 0x00 on a router NSAP of a router is also called a NET,Area ID,Sys ID,NSEL,CLNS Addressing AFI Values,X.121 - Intl plan for public data networks ISO DCC - Data country code IS0 6523 ICD - Telex Local - For local use within network domain only,Address Domain,AFI Value,X.121 ISO D
21、CC ISO 6523 Local,37 39 47 49,CLNS Addressing Requirements and Caveats,At least one NSAP is required per node All routers in the same area must have a common Area ID Each node in an area must have a unique System ID All level 2 routers in a domain must have unique System IDs relative to each other A
22、ll systems belonging to a given domain must have System IDs of the same length in their NSAP addresses,CLNS Addressing Requirements and Caveats,Multiple NSAPs allowed on Cisco routers for merging, splitting or renumbering All NSAPs on the same router must have the same system ID. The maximum size of
23、 an NSAP is 20 bytes Minimum of 8 bytes allowed on Ciscos. 1 byte for area, 6 bytes for system ID and 1 byte for N-selector. AFI prefix recommended to make minimum of 9 bytes,CLNS Addressing NSAP Examples,Example 147.0001.aaaa.bbbb.cccc.00 Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00Example 239
24、.0f01.0002.0000.0c00.1111.00 Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00Example 3.49.0002.0000.0000.0007.00 Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00,CLNS Addressing How do most ISP define System IDs?,Router A# Interface Loopback 0 IP address 192.168.3.25Router isis Net 49.0001.19
25、21.6800.3025.00IP Address conversion process:192.168.3.25 - 192.168.003.025 |V1921.6800.3025|V49.001.1921.6800.3025.00,CLNS Addressing Globally Unique NSAPs,AFI 47 (ISO 6523 ICD) is allocated via national sponsoring authority of the International Registration Authority (RA), usually a national stand
26、ards body NIST - allocated IDI 0005 and 0006 BSI subsidiary IOTA allocated 0124 for assignment of ATM End Systems Addresses AFI 39 also administered through national institutions IDI 0840 allocated to ANSI,NIST - United States National Institute of Standards BSI - British Standards Institute IOTA -I
27、dentifiers for Organizations for Telecommunications Addressing,47 0124 XXXXX YYYYYYYYYYYYYY AABBCCDDEEFF 00,AFI,IDI,ORG ID,ORG ASSIGNED,SYS ID,NSEL,CLNS Addressing Globally Unique NSAPs,Globally unique NSAPs make sense for interconnectedtelecommunications systems such as ATM switches, SONET/SDH ADMs
28、 that use CLNS based applications for management. For ISP networks, the local AFI (49) would just be fine! Why?,4. IS-IS Link State Database,Link State Packets Sequence Number Packets More on IS- IS Metrics Flooding Database Parameters, Flags and Timers SPF Algorithm,IS-IS LS Database Link State Pac
29、kets,LSP: RTC.00-00,RTD.,LSP: RTE.00-00,RTC,RTE,LSP: RTA.00-00,RTA,LSP: RTD.00-00,RTB.,LSP: RTB.00-00,IS-IS LS Database IS-IS Packet Format,A Fixed Header Contains generic packet information and other specific information about the packet Type, Length, Value (TLV) Fields TLVs are blocks of specific
30、routing-related information in IS-IS packets,IS-IS Packets are made of the following:,IS-IS LS Database Generic Packet Format,IS-IS LS Database LSP Format,Intradomain Routing Proto Descriminator,Lenth Indicator,Version/Protocol ID Extension,ID Length,PDU Type,R,R,R,Version,Reserved,Maximum Area Addr
31、esses,PDU Length,Remaining Lifetime,LSP ID,Sequence Number,Checksum,LSPDBOL,IS Type,P,ATT,TYPE LENGTH VALUE FIELDS,Octets1 1 1 1 1 1 1 1 2 2 ID Length + 2 4 2 1 Variable,IS-IS LS Database LS Packet Identifier (LSPID),00c0.0040.abcd.02-01,Sys ID,PSN ID,LSPNo.,Sys ID - System ID of IS or DIS in case o
32、f PSN LSP. PSN ID - Zero for Non-PSN LSP and Non-zero for PSN LSP LSP Number - Fragmentation number,Complete Sequence Number Packets (CSNPs),Describes all known LSPs in the LS Database Standard IS-IS packet format Fixed Header and TLV Fields Each TLV includes the following info about each known LSP:
33、 LSP ID Remaining Lifetime Sequence number Checksum,Partial Sequence Number Packets (PSNPs),PSNPs are similarly constructed as CSNPs but have summaries of only specific LSPs PSNPs are used in 2 ways: To acknowledge receipt of LSP on pt-to-pt links To request transmission of the latest version of an
34、LSP,Level-1 TLVs,Level-2 TLVs,New TLVs,Extended IS Reachability Information,Router-Id,Extended IP Reachability Information,Dynamic Hostname Information,TLV Name,Type,Comments,22,134,135,137,TE Extension. Replaces TLV 2,TE Extension,TE extension used in place of TLV 128 or 130,For dynamic distributio
35、n of Hostname to NET mapping via LSP Flooding,Point-to-Point Adjacency State,240,Reliable Pt-to-pt adjacency formation,Old IS-IS Metrics,ISO 10589 specifies 4 metric types Default - Supported by all routers. Delay - measures transit delay Expense - Measures the monetary cost of link utilization Erro
36、r - measures error probability Default metric type must be supported by all implementations Other metric types specified for QoS routing. Not supported in Cisco IOS and most other vendor implementations,Old IS-IS Metrics,Metric Information is carried in the following TLVs:IS Neighbors ES NeighborsPr
37、efix NeighborsIP Internal ReachabilityIP External Reachability Extended IP Reachability Extended IS Reachability,Old IS-IS Metrics,Maximum metric per interface is 63 Maximum path metric is 1023 There is no automatic assignment based on interface bandwidth Cisco uses default of 10 on all interfaces r
38、egardless of bandwidth,0,S,S,S,I/E,I/E,I/E,I/E,Default Metric (6 bits),Delay Metric,Expense Metric,Error Metric,1,Byte(s),1,1,1,IP Address,Subnet Mask,4,4,Flooding and LSDB synchronization,Acknowledgement-based reliable mechanism on pt-to-point links Flooding on broadcast links depends on periodic a
39、dvertisements of CSNPs for reliable flooding and synchronization,SRM and SSN Flags,Send Routing Message flag (SRMflag) and Send Sequence Number flag (SSNflag) are used by the Update Process to control delivery and acknowledgement of LSPs SRMFlag - If set for a LSP on a specific link, indicates the L
40、SP should be flooded on that link SSNFlag - If set for a LSP on a link, means send a PSNP over that link to acknowledge receipt of LSP,.,Flooding on Pt-to-pt links,RTA,RTB,RTC,Interface 1,Interface 2,Interface 3,Interface 4,PSNP RTA.00-00 SEQ#100,PSNP RTA.00-00 SEQ#100,LSP RTA.00-00 SEQ#100,LSP RTA.
41、00-00 SEQ#100,Flooding on Broadcast Links,RTA (DIS),RTB,PSNP RTX.00-00 SEQ#1,LSP RTX.00-00 SEQ#1,RTC,CSNP RTA.00 RTB.00 RTC.00 RTX.00,RTA,RTB,RTA,RTX,PSNP RTX.00-00 SEQ#1,RTC,LSP RTX.00-00 SEQ#1,RTA,X,LSP RTX.00-00 SEQ#1,RTX,PSNP RTX.00-00 SEQ#1,RTA,IS-IS Database Timers,Timer,Default Value,Cisco IO
42、S Command,Maxage,LSP Refresh Interval,LSP Transmission Interval,LSP Retransmit Interval,CSNP Interval,1200s,900s,33ms,5s,10s,isis max-lsp-interval,isis refresh-interval,isis lsp-interval,isis retransmit-interval,isis csnp-interval,SPF Algorithm,Shortest Path First Algorithm (SPF), also known as Dijk
43、stra (named after Dutch Mathematician) is used by link state protocols to obtain best paths through network Works by building shortest path tree from calculating router (root) to all destinations in the network SPF Process runs over level-1 and level-2 databases separately,SPF Algorithm,In default m
44、ode, SPF process runs no frequent than every 5s Full SPF is run when topology changes When leaf elements such as IP prefixes change, routing table is adjusted with Partial Route Calculation (PRC) PRC evaluates only routes that changed hence less CPU intensive and relatively fast,SPF Algorithm,Durati
45、on of SPF depends on many factors such as: Number of nodes Number of links Number of IP prefixes Degree of mesh (especially for NBMA) Speed of Route Processor,5. IS-IS Extensions for MPLS TE,Wide Metrics Extended IS Reachability Information Extended IP Reachability Information Sub TLVs,IS-IS Extensi
46、ons for MPLS TE Wide Metric,Traffic engineering extensions to IS-IS introduces reuse fields for non-implemented QoS metrics for larger Default metric values in TLV 22 and TLV 135 3 byte metric field in TLV 22 4 byte metric field in TLV 135,IS-IS Extensions for MPLS TE TLVs 22,Type (1 byte): 22 Lengt
47、h (1 byte): Total Length of Value Field Value: 3 bytes default metric information 7 bytes of system ID + pseudonode number 1 byte of length of sub-TLVs 0-244 bytes of sub-TLVs,IS-IS Extensions for MPLS TE TLVs 135,Type (1 byte): 135 Length (1 byte): Total Length of Value Field Value: 4 bytes default
48、 metric information 1 byte of control information 1 bit up/down status1 bit sub-TLV presence indication 6 bits prefix length 0-4 bytes of IPV4 Prefix 0-250 bytes of optional sub-TLVs1 byte to specify length of sub-TLVs0-249 bytes of sub-TLVs,IS-IS Extensions for MPLS TE Sub-TLVs,Administrative Group
49、 ( color ),IPV4 Interface Address,Sub-TLV Name,Type,Length (bytes),3,6,IPV4 Neighbor Address,Sub-TLVs are used to flood MPLS TE resource information and link attributes,Maximum Link Bandwidth,Reservable Link Bandwidth,Unreserved Link Bandwidth,8,9,10,11,4,4,4,4,4,32,6. ISP Network Design Considerations,Sub-Optimal Level 1 Routing 2 Layer Domain-wide Prefix distribution Running IS-IS over NBMA Routing Convergence,
