ISIS Intermediate SystemtoIntermediate System Rick Graziani Cabrillo College

IS-IS Intermediate System-to-Intermediate System Rick Graziani Cabrillo College graziani@cabrillo. edu Fall 2009

Sources • Sources used in creating this IS-IS presentation: – Cisco Online curriculum, CCNP 1 version 3. 0 – IS-IS Network Design Solutions, Cisco Press • Very Good Reference – CCNP Self-Study (BSCI), Cisco Press – Routing TCP/IP Volume I by Jeff Doyle Rick Graziani graziani@cabrillo. edu 2

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

IS-IS versus OSPF • • OSPF – 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. Rick Graziani graziani@cabrillo. edu 4

IS-IS versus OSPF - Terminology IS-IS OSPF ES (End System) Host IS (Intermediate System) Router Circuit Link SNPA (Subnetwork Point of Attachment) Datalink Address PDU (Protocol Data Unit) Packet DIS (Designated Intermediate System) DR (Designated Router) N/A BDR IIH (IS-to-IS Hello Packet) Hello packet Rick Graziani graziani@cabrillo. edu Comments 5

IS-IS versus OSPF - Terminology IS-IS OSPF Comments LSP (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 Level 1 Area (nonbackbone) Level 2 Area Backbone area (Area 0) Rick Graziani graziani@cabrillo. edu The term routing domain is also used with OSPF. IS-IS uses a backbone path connected by contiguous L 2 routers. There is no backbone area in IS-IS 6

IS-IS versus OSPF – ISs (Routers) IS-IS OSPF Comments Level 1 IS (router) Internal Nonbackbone Router Internal, non-backbone router in a Totally Stubby Area Level 2 IS (router) Internal Backbone Router or ASBR Any 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) ABR System ID Router ID The System ID is the key for SPF calculations. Sometimes the NET address is thought of as the Router ID. AFI = 49 RFC 1918 Addresses AFI is part of the NSAP. Rick Graziani graziani@cabrillo. edu 7

IS-IS versus OSPF - Timers Interface IS-IS OSPF Point-to-Point Hello – 10 sec Holdtime – 30 sec Hello – 10 sec Dead – 40 sec Broadcast Hello – 10 sec Holdtime – 30 sec Hello – 10 sec Dead – 40 sec NBMA N/A Hello – 30 sec Dead – 120 sec Other IS-IS OSPF LS Aging 1, 200 sec or 20 min (counts down) 3, 600 sec or 60 min (counts up) LS Refresh Every 15 min Every 30 min NBMA N/A Hello – 30 sec Dead – 120 sec SPF Delay/Holdtime 5. 5 sec / 10 sec Rick Graziani graziani@cabrillo. edu 8

History of IS-IS and OSPF Just like the Mac. Intosh versus Windows debate

History of IS-IS and OSPF http: //www. nanog. org • • • 1985 – Originally called DECnet Phase V 1987 – 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 defined 1989 – 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 Rick Graziani graziani@cabrillo. edu 10

History of IS-IS and OSPF http: //www. nanog. org • • • 1991 – 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 Rick Graziani graziani@cabrillo. edu 11

History of IS-IS and OSPF http: //www. nanog. org • • 1996 -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 Rick Graziani graziani@cabrillo. edu 12

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 Rick Graziani graziani@cabrillo. edu • • NTT/Verio Qwest Sprint Verizon Business (UUNET) 13

Introduction to IS-IS Protocol Routers Areas

OSI: Two Network Services, Two Network Protocols • CMNS (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 established – CLNP (Connectionless Network Protocol) – OSI network layer protocol that carries upper layer data over connectionless links. (Similar to IP) Rick Graziani graziani@cabrillo. edu 15

IS-IS Protocol Options • IS-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 ISO • Purely IP (CCNP 1) • Both Rick Graziani graziani@cabrillo. edu 16

OSI Routing Protocols: ES-IS and IS-IS ISH ES-IS • Analogous to Address Resolution Protocol (ARP) in IP • Not technically a routing protocol • Sometimes referred to as Level 0 routing. • ESs (hosts) discover nearest IS (router) by listening to IS Hello (ISH) packets • ISs (routers) know which hosts are on their subnetwork by listening to ES Hello (ESH) packets. • Not applicable for IP networks Rick Graziani graziani@cabrillo. edu 17

OSI Routing Protocols: ES-IS and IS-IS Boundary areas in ISIS 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 • OSI distinguishes between Level 1, Level 2, and Level 3 routing. • Level 1 Routing – If 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) Rick Graziani graziani@cabrillo. edu 18

IS-IS Areas IS-IS Routers: • Level 1 IS (L 1 IS, router) – Analogous to OSPF Internal non-backbone router (Totally Stubby) – Responsible for only routing to ESs inside an area. • Level 2 IS (L 2 IS, router) – Analogous to OSPF Internal Backbone router – Responsible for routing between areas • Level 1 and Level 2 IS (L 1 -L 2 IS, router) – Analogous to OSPF ABR router – Participate in both L 1 intra-area routing and L 2 inter-area routing. Rick Graziani graziani@cabrillo. edu 19

Level 1 Router • • Level 1 IS (L 1 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. Rick Graziani graziani@cabrillo. edu 20

Level 2 Router • • Level 2 IS (L 2 IS, router) – Analogous to OSPF Internal Backbone router – Responsible for routing between areas Also referred to as area routers. Interconnects the Level 1 areas Store separate database of only inter-area topology Rick Graziani graziani@cabrillo. edu 21

Level 1 – Level 2 Router • • Level 1 and Level 2 IS (L 1 -L 2 IS, router) – Analogous to OSPF ABR router – Participate in both L 1 intra-area routing and L 2 inter-area routing. Maintain both Level 1 and Level 2 LSDB Support Level 1 function communicating with other Level 1 routers in their area – Inform 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. Rick Graziani graziani@cabrillo. edu 22

IS-IS Backbone • • IS-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)? Rick Graziani graziani@cabrillo. edu 23

OSI Addressing NSAP (Area, System ID, NSEL) SNAP

OSI Addressing NSAP • • • IS-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. 00 Rick Graziani graziani@cabrillo. edu 25

NSAPs • • • Format of the Cisco NSAP address consists of three parts. 1. Area address 2. System ID 3. NSAP 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. Rick Graziani graziani@cabrillo. edu 26

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 tree – LSPs – Hello and other PDUs Rick Graziani graziani@cabrillo. edu 27

NSAPs – Cisco Format Area – System ID – NSEL 49. 0001. 2222. 00 Area • Addresses starting with 49 (AFI=49) are considered private IP address, analogous to RFC 1918. – Routed by IS-IS – Should not be advertised to other CLNS networks (outside this ISIS domain) • Additional 2 bytes (0001) added for the area ID • All routers in the same are must have the same area address Rick Graziani graziani@cabrillo. edu 28

NSAPs – Cisco Format Area – System ID – NSEL 49. 0001. 2222. 00 System ID • Cisco fixes the System ID at 6 bytes. • Customary to use one of the following: – MAC address from the router – IP address of loopback interface • 192. 168. 111. 3 -> 192. 168. 111. 003 -> 1921. 6811. 1003 • Each device (IS and ES) must have a unique System ID within the area. (Recommended to make them unique within the domain. ) Rick Graziani graziani@cabrillo. edu 29

NSAPs – Cisco Format Area – System ID – NSEL 49. 0001. 2222. 00 NSEL (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) Rick Graziani graziani@cabrillo. edu 30

NSAP (NETs) Area – System ID – NSEL 49. 0001. 2222. 00 Other Examples Example 1: NSAP 47. 0001. aaaa. bbbb. cccc. 00 • Area ID is: – 47. 0001 • System ID is: – aaaa. bbbb. cccc • NSAP selector byte is: – 00 Example 2: NSAP 39. 0 f 01. 0002. 0000. 0 c 00. 1111. 00 • Area ID is: – 39. 0 f 01. 0002 • System ID is: – 0000. 0 c 00. 1111 • NSAP selector byte is: – 00 graziani@cabrillo. edu Rick Graziani 31

Configuring IS-IS (so far) San. Jose 1 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 1 255. 0 ip router isis priority 100 router isis net 49. 0001. 1111. 00 San. Jose 2 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 2 255. 0 ip router isis net 49. 0001. 2222. 00 Area. System ID. NSEL • • • Area 49. 0001 San. Jose 3 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 3 255. 0 ip router isis net 49. 0001. 3333. 00 ip router isis: IS-IS must be enabled on the interface Note: 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 IPSec 56 (16 MB Flash/64 MB RAM) Rick Graziani graziani@cabrillo. edu 32

Configuring IS-IS (so far) Area 49. 0001 • • • 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. HDLC Type: Cisco routers default to L 1 -L 2 type routers. – We will see how to change this in a moment. Rick Graziani graziani@cabrillo. edu 33

IS-IS Adjacencies Are we half way yet?

Hello Messages • • 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 IS – ISH, sent by IS to an ES – IIH, used between two ISs (CCNP) Rick Graziani graziani@cabrillo. edu 35

Neighbors and Adjacencies • IS-IS discover neighbors and forms adjacencies using IS-IS Hello PDUs. – Transmitted every 10 seconds – Can be changed using the interface command, is hellointerval • Hold time defaults to 3 times the Hello time (30 seconds), before declaring a neighbor dead. – Changed using the interface command is hello-multiplier – Default is 3 Rick Graziani graziani@cabrillo. edu 36

LAN Representation and Adjacencies DIS • • • Similar to the DR in OSPF… DIS (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 topology LAN uses a virtual node called pseudonode. – Represents the LAN – Sent by the DIS Rick Graziani graziani@cabrillo. edu 37

LAN Representation and Adjacencies DIS • • • Election of DIS: – Router with highest priority (Cisco default is 64) – Router with highest MAC address No “BDR” No way to make a router ineligible from being DIS (no OSPF priority 0) New router (IS) can cause a new election, unlike OSPF Periodically broadcasts CSNPs (OSPF DBD) every 10 seconds Rick Graziani graziani@cabrillo. edu 38

Configuring IS-IS (so far) San. Jose 1 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 1 255. 0 ip router isis priority 100 router isis net 49. 0001. 1111. 00 San. Jose 2 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 2 255. 0 ip router isis net 49. 0001. 2222. 00 Area. System ID. NSEL • DIS Area 49. 0001 San. Jose 3 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 3 255. 0 ip router isis net 49. 0001. 3333. 00 isis priority: Sets DIS priority on a LAN interface, default 64 Rick Graziani graziani@cabrillo. edu 39

Adjacencies LAN Adjacencies • • • L 1 routers form L 1 adjacencies with L 1 and L 1 -L 2 routers in their area. L 2 routers form L 2 adjacencies with L 2 and L 1 -L 2 routers in their area or another area. L 1 router does not form an adjacency with an L 2 router Rick Graziani graziani@cabrillo. edu 40

Adjacencies • What are the adjacencies? L 1? L 2? L 1 L 2? None? • The adjacencies also determine what type of routes the IS (router) will have in its routing table. – L 1 – Intra-area routes (routes only within that area) – L 2 – Inter-area routes (routes from other areas) – Or both Rick Graziani graziani@cabrillo. edu 41

Adjacencies • For an adjacency to be formed and maintained, routers must agree on: – If they are both Level 1, them must be in the same area – The System ID must be unique to each router. – The Hello timers do not have to match. Rick Graziani graziani@cabrillo. edu 42

Metric and Calculate the Shortest Path • • Four types of metrics: 1. Default – Cisco only supports this metric. 2. Delay 3. Expense 4. Error Each metric expressed as an integer between 0 and 63. Rick Graziani graziani@cabrillo. edu 43

Metric • • Cisco 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. Rick Graziani graziani@cabrillo. edu 44

Multi-Area IS-IS Example

Note: 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. Levels L 1 L 2 San. Jose 1 interface Fast. Ethernet 0/1 ip address 192. 168. 10. 1 255. 252 ip router isis interface Fast. Ethernet 0/0 ip address 172. 16. 0. 1 255. 0 ip router isis interface Serial 0/0 ip address 10. 0. 0. 2 255. 252 ip router isis net 49. 0001. 1111. 00 is-type level-1 -2 San. Jose 2 interface Fast. Ethernet 0/0 ip address 172. 16. 0. 2 255. 0 ip router isis interface Fast. Ethernet 0/1 ip address 192. 168. 20. 1 255. 0 ip router isis net 49. 0001. 2222. 00 is-type level-1 Rick Graziani graziani@cabrillo. edu L 1 L 2 Phoenix interface Serial 0/0 ip address 10. 0. 0. 1 255. 252 ip router isis interface Fast. Ethernet 0/1 ip address 192. 168. 30. 1 255. 0 ip router isis net 49. 0002. 3333. 00 is-type level-2 -only • is-type {level-1 | level-1 -2 | level-2 -only} 46

show clns neigh • • • 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. L 1 L 2 L 1 L 2 San. Jose 1#show clns neighbors System Id Interface SNPA San. Jose 2 Fa 0/0 0003. 6 be 9. d 480 Phoenix Se 0/0 *HDLC* State Up Up Holdtime 26 28 Type Protocol L 1 IS-IS L 2 IS-IS San. Jose 2#show clns neighbors System Id Interface SNPA San. Jose 1 Fa 0/0 0002. b 9 ee. 5 ee 0 State Up Holdtime 4 Type Protocol L 1 IS-IS Phoenix#show clns neighbors System Id Interface SNPA San. Jose 1 Se 0/0 *HDLC* State Up Holdtime 27 Type Protocol L 2 IS-IS Rick Graziani graziani@cabrillo. edu 47

show isis data • San. Jose 1 has a L 1 LSDB and a L 2 LSDB. L 1 L 2 L 1 L 2 San. Jose 1#show isis database IS-IS Level-1 Link State Database: LSPID LSP Seq Num LSP Checksum San. Jose 1. 00 -00 * 0 x 00000005 0 x. DE 15 San. Jose 1. 01 -00 * 0 x 00000003 0 x. BBFE San. Jose 2. 00 -00 0 x 00000006 0 x. BDFB IS-IS Level-2 Link State Database: LSPID LSP Seq Num LSP Checksum San. Jose 1. 00 -00 * 0 x 0000001 B 0 x. AB 48 San. Jose 1. 01 -00 * 0 x 0000001 B 0 x 5526 Phoenix. 00 -00 0 x 0000001 E 0 x. A 3 D 3 Phoenix. 01 -00 0 x 00000002 0 x 54 A 6 Rick Graziani graziani@cabrillo. edu LSP Holdtime 916 1165 470 ATT/P/OL 1/0/0 0/0/0 LSP Holdtime 914 480 912 826 ATT/P/OL 0/0/0 48

show isis data • • San. Jose 2 only has a L 1 LSDB. Phoenix only has a L 2 LSDB. L 1 L 2 L 1 L 2 San. Jose 2#show isis database IS-IS Level-1 Link State Database: LSPID LSP Seq Num San. Jose 1. 00 -00 0 x 00000005 San. Jose 1. 01 -00 0 x 00000003 San. Jose 2. 00 -00 * 0 x 00000006 LSP Checksum 0 x. DE 15 0 x. BBFE 0 x. BDFB LSP Holdtime 907 1155 464 ATT/P/OL 1/0/0 0/0/0 Phoenix#show isis database IS-IS Level-2 Link State Database: LSPID LSP Seq Num San. Jose 1. 00 -00 0 x 0000001 B San. Jose 1. 01 -00 0 x 0000001 B San. Jose 2. 00 -00 0 x 00000024 Phoenix. 00 -00 * 0 x 0000001 E Phoenix. 01 -00 * 0 x 00000002 LSP Checksum 0 x. AB 48 0 x 5526 0 x 2 FD 3 0 x. A 3 D 3 0 x 54 A 6 LSP Holdtime 893 459 0 (932) 895 808 ATT/P/OL 0/0/0 0/0/0 Rick Graziani graziani@cabrillo. edu 49

show ip route • Because San. Jose 1 is an L 1 L 2 router, it contains both: Ø L 1 routes for Area 49. 0001 and Ø L 2 routes for the other area 49. 0002. L 1 L 2 L 1 L 2 San. Jose 1#show ip route i L 2 192. 168. 30. 0/24 [115/20] via 10. 0. 0. 1, Serial 0/0 C 192. 168. 10. 0/24 is directly connected, Fast. Ethernet 0/1 172. 16. 0. 0/24 is subnetted, 1 subnets C 172. 16. 0. 0 is directly connected, Fast. Ethernet 0/0 i L 1 192. 168. 20. 0/24 [115/20] via 172. 16. 0. 2, Fast. Ethernet 0/0 10. 0/30 is subnetted, 1 subnets C 10. 0 is directly connected, Serial 0/0 Rick Graziani graziani@cabrillo. edu 50

show ip route • Because San. Jose 2 is only an L 1 router, it only contains: Ø L 1 routes for Area 49. 0001 and Ø The 0. 0/0 default route sent by San. Jose 1. Ø L 1 -only routers, always learn a default route from a neighboring L 1 -L 2 router L 1 L 2 L 1 L 2 San. Jose 2#show ip route i L 1 192. 168. 10. 0/24 [115/20] via 172. 16. 0. 1, Fast. Ethernet 0/0 172. 16. 0. 0/24 is subnetted, 1 subnets C 172. 16. 0. 0 is directly connected, Fast. Ethernet 0/0 C 192. 168. 20. 0/24 is directly connected, Fast. Ethernet 0/1 10. 0/30 is subnetted, 1 subnets i L 1 10. 0 [115/20] via 172. 16. 0. 1, Fast. Ethernet 0/0 i*L 1 0. 0/0 [115/10] via 172. 16. 0. 1, Fast. Ethernet 0/0 Rick Graziani graziani@cabrillo. edu 51

show ip route • Because Phoenix is an L 2 router, it contains: Ø L 2 routes which are L 1 routes from Area 49. 0001. Ø No L 1 routes from Area 49. 0002. L 1 L 2 L 1 L 2 Phoenix#show ip route Gateway of last resort is not set C 192. 168. 30. 0/24 is directly connected, Fast. Ethernet 0/1 i L 2 192. 168. 10. 0/24 [115/20] via 10. 0. 0. 2, Serial 0/0 172. 16. 0. 0/24 is subnetted, 1 subnets i L 2 172. 16. 0. 0 [115/20] via 10. 0. 0. 2, Serial 0/0 i L 2 192. 168. 20. 0/24 [115/30] via 10. 0. 0. 2, Serial 0/0 10. 0/30 is subnetted, 1 subnets C 10. 0 is directly connected, Serial 0/0 Rick Graziani graziani@cabrillo. edu 52

Some final notes… • • IS-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 protocols IS-IS forms adjacencies with all neighbors. By default, Cisco IS-IS routers are Level 1 -2 routers. Rick Graziani graziani@cabrillo. edu 53

Additional Information

Sub-optimal Routing • Sub-optimal routing is not necessarily a disadvantage. Rick Graziani graziani@cabrillo. edu 55

IS-IS vs OSPF • IS-IS is “more scalable” than OSPF because: – More routers can reside in an area – Fewer link-state PDUs are used Rick Graziani graziani@cabrillo. edu 56

Authentication • • IS-IS authentication Interface configuration – isis password Rick Graziani graziani@cabrillo. edu 57

Stuck in INIT • Routers may have difficulty forming an adjacency, stuck in INIT if: – Mismatched authentication configurations – Mismatched MTU configurations Rick Graziani graziani@cabrillo. edu 58

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

Terminology • 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. Rick Graziani graziani@cabrillo. edu 60

IS-IS Routing Process • IS-IS Routing Process is divided into four stages: – Update – Decision – Forwarding – Receive Rick Graziani graziani@cabrillo. edu 61

IS-IS Routing Process: Update The Update Process • Routers 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) Rick Graziani graziani@cabrillo. edu 62

IS-IS Routing Process: Update Sending and Receiving an LSP • Receiving an LSP – If 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 area • Level 2 LSPs are sent across all L 2 adjacencies. Rick Graziani graziani@cabrillo. edu 63

IS-IS Routing Process: Update Sending and Receiving an LSP: • Propagating (sending) LSPs on a Point-to-Point Interface – When 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. CSNP You are missing LSP 3 CSNP LSP 3 Rick Graziani graziani@cabrillo. edu PSNP (Ack) 64

IS-IS Routing Process: Update Sending and Receiving an LSP: • Propagating (sending) LSPs on a Point-to-Point Interface – Likewise, 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. CSNP PSNP 3 LSP 3 PSNP (Ack) Rick Graziani graziani@cabrillo. edu I am missing LSP 3 65

IS-IS Routing Process: Update Sending and Receiving an LSP: • Propagating (sending) LSPs on a Broadcast Interface – The 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 adjacencies • Creating and updating the pseudonode LSP • Flooding the LSPs over the LAN. Rick Graziani graziani@cabrillo. edu 66

IS-IS Routing Process: Update Sending and Receiving an LSP: • Propagating (sending) LSPs on a Broadcast Interface – On receiving a CSNP the router compares it with its LSDB… – If 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) I have a newer version of LSP 88 Receipt of LSP 88 is acknowledged by all routers with a PSNP. Rick Graziani graziani@cabrillo. edu 67

IS-IS Routing Process: Update Sending and Receiving an LSP: • Propagating (sending) LSPs on a Broadcast Interface – On receiving a CSNP the router compares it with its LSDB… – If 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) LSP 77 (LSA) PSNP (LSR) PSNP (LSAck) Rick Graziani graziani@cabrillo. edu 68

IS-IS Routing Process: Update • 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 Lifetime • Used to age-out or delete LSPs • Lifetime is set to 0 and flooded • Receiving 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 Number • First LSP starts with a sequence number of 1, with following LSPs incremented by 1. – Checksum • If received LSP’s 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. Rick Graziani graziani@cabrillo. edu 69

IS-IS Routing Process: Decision • IS-IS Routing Process is divided into four stages: – Update – Decision – Forwarding – Receive Rick Graziani graziani@cabrillo. edu 70

IS-IS Routing Process: Decision • The Decision Process – Uses Dijkstra’s 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 table – PATH table is the SPT during the construction of the LSDB – Each 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 Rick Graziani graziani@cabrillo. edu 71

IS-IS Routing Process: Decision Determining the best route • Criteria 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 metric – Internal 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. – To. S (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. Rick Graziani graziani@cabrillo. edu 72

IS-IS Routing Process: Forwarding • IS-IS Routing Process is divided into four stages: – Update – Decision – Forwarding – Receive Rick Graziani graziani@cabrillo. edu 73

IS-IS Routing Process: Forwarding and Receiving • Forwarding process – After 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 process – If 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. Rick Graziani graziani@cabrillo. edu 74

Route Summarization • • • Rules for IS-IS route summarization similar to that of OSPF Level 1 -2 routers (L 1 L 2) – Similar to OSPF ABR – Configured at the L 1 L 2 router at the edge of an area. – L 1 L 2 routers can summarize the routes within their area to L 1 L 2 or L 2 routers in another area. – This is an efficient method of establishing prefix (network addresses) routing into other areas. If one edge L 1 L 2 router in an area is summarizing routes for that area, other edge L 1 L 2 routers in that area must also be summarizing routes. – If other L 1 L 2 routers are summarizing and one edge L 1 L 2 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. Rick Graziani graziani@cabrillo. edu 75

Metrics Odds and Ends • Cisco’s IS-IS implementation will perform equal cost load balancing up to six paths. • Supports VLSM • L 1 routers calculate path to the nearest L 2 router for inter-area routing (OSPF: Totally Stubby Area) – When an L 2 or L 1 L 2 router is attached to another area, the router will advertise this fact – The Decision Process in L 1 routers will choose the metrically closest L 1 L 2 router as the default router. – An L 1 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. Rick Graziani graziani@cabrillo. edu 76

Odds and Ends • • L 1 L 2 routers run two IS-IS processes, one for its L 1 LSDB and another for its L 2 LSDB. Once an packet is accepted by a router the System ID and NSEL are stripped. Rick Graziani graziani@cabrillo. edu 77

IS-IS Link State Database Synchronization

LSDB Synchronization and Update Process LSP (LSA) PSNP (LSAck) • LSP (LSA) PSNP (LSAck) X PSNP (LSAck) 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 router – PSNP (Partial Sequence Number PDU) – (OSPF: LSAck/LSR) • Acknowledge the receipt of a LSP • Request a complete LSP for a missing entry Rick Graziani graziani@cabrillo. edu 79

Update Process • Point-to-Point networks: – Once an LSP is sent, router sets a timer (minimum. LSPTransmission. Interval) of 5 seconds – If PSNP (ACK) not received, resends LSP. Rick Graziani graziani@cabrillo. edu 80

Update Process PSNP (OSPF LSR) goes to DIS. III. LSP 77 is sent by DIS to R 1 • CSNP (DBD) LSP 77 (LSA) PSNP (LSR) PSNP (LSAck) 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 every 10 seconds – L 1 CSNPs are multicast to All. L 1 ISs – L 2 CSNPs are multicast to All. L 2 ISs Rick Graziani graziani@cabrillo. edu 81

IS-IS Intermediate System-to-Intermediate System Rick Graziani Cabrillo College graziani@cabrillo. edu Fall 2009