© 2006 Cisco Systems, Inc. All rights reserved. BSCI v The IS-IS Protocol Introducing IS-IS and Integrated IS-IS Routing

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Uses for IS-IS Routing Large ISPs Stable protocol Originally deployed by ISPs because U.S. government mandated Internet support of OSI and IP

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v IS-IS Routing IS = router. IS-IS was originally designed as the IGP for the Connectionless Network Service (CLNS), part of the OSI protocol suite. The OSI protocol suite layer 3 protocol is the Connectionless Network Protocol (CLNP). IS-IS uses CLNS addresses to identify routers and build the LSDB.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v IS-IS Features Link-state routing protocol Supports VLSM Uses Dijkstras SPF algorithm; has fast convergence Uses hellos to establish adjacencies and LSPs to exchange link-state information Efficient use of bandwidth, memory, and processor Supports two routing levels: –Level 1: Builds common topology of system IDs in local area and routes within area using lowest cost path. –Level 2: Exchanges prefix information (area addresses) between areas. Routes traffic to area using lowest-cost path.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Routers are identified as Level 1, Level 2, or Level 1-2: Level 1 routers use LSPs to build topology for local area. Level 2 routers use LSPs to build topology between different areas. Level 1-2 routers act as border routers between Level 1 and Level 2 routing domains. IS-IS Link-State Operation

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Integrated (or Dual) IS-IS Routing Integrated IS-IS is IS-IS for multiple protocols: –For IP, CLNS, or both Uses its own PDUs to transport IP routing information; updates not sent in IP packets Requires CLNS addresses, even if only routing for IP

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v IP and CLNP addresses must be planned. Use two-level hierarchy for scalability: –Limits LSP flooding –Provides opportunity for summarization Summarization: –Limits update traffic –Minimizes router memory and CPU usage Integrated IS-IS Design Principles

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Issues with Integrated IS-IS Default narrow metrics are limited to 6-bit interface and 10-bit path metric –In Cisco IOS Software Relase 12.0, wide metrics allow 24- bit interface and 32-bit path metric. Cisco IOS software has default metric of 10 on all interfaces.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v End System-to-Intermediate System ES-IS forms adjacencies between ESs and routers (ISs). –IP end-systems do not use ES-IS. ESs transmit ESHs to announce their presence to ISs. ISs transmit ISHs to announce their presence to ESs. ISs transmit IIHs to other ISs.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Four OSI Routing Levels

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Similarities Between IS-IS and OSPF Integrated IS-IS and OSPF are both open standard link-state protocols with the following similar features: –Link-state representation, aging timers, and LSDB synchronization –SPF algorithms –Update, decision, and flooding processes –VLSM support Scalability of link-state protocols has been proven (used in ISP backbones). They both converge quickly after changes.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Integrated IS-IS vs. OSPF: Area Design OSPF is based on a central backbone with all other areas attached to it. In OSPF the border is inside routers (ABRs). Each link belongs to one area.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Integrated IS-IS vs. OSPF: Area Design (Cont.) In IS-IS the area borders lie on links Each IS-IS router belongs to exactly one area. IS-IS is more flexible when extending the backbone.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Advantages of Integrated IS-IS Supports CLNP and IP More extensible through TLV design

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Advantages of OSPF OSPF has more features, including: –Has three area types: normal, stub, and NSSA –Defaults to scaled metric (IS-IS always 10) OSPF is supported by many vendors. Information, examples, and experienced engineers are easier to find.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary IS-IS is a popular routing protocol in the ISP industry. IS-IS is a stable, fast-converging IGP that is positioned to route IPv4, CLNS, or IPv6. All IS-IS interfaces have a default metric of 10. ES-IS (for CLNS routing only) provides discovery between host and routers using hello packets to form adjacencies. Hosts send ESHs, while routers send ISHs. OSI defines routing levels 0 through 3. Level 0 is between ES and IS. Levels 1 and 2 are between IS and IS to support intradomain routing. Level 3 supports interdomain routing. –Level 1 is intra-area –Level 2 is interarea. IS-IS and OSPF are both open-standard link-state routing protocols that support VLSM, scalability, and quick convergence.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v