Integrated Intermediate Systems-to-Intermediate System

• The IS-IS protocol is part of the Open System Interconnection (OSI) suite of protocols.
• The OSI suite uses the Connectionless Network Service (CLNS) for data delivery.
  • Connectionless Network Protocol (CLNP) is the actual Layer 3 protocol, similar to the Internet Protocol (IP) of the TCP/IP suite.
  • IS-IS uses CLNS address to identify the routers and built the link-state database.
• IS-IS operates strictly in CLNS.
• Integrate IS-IS support CLNS as well as IP routing.
• The ISO calls routers Intermediate Systems (IS)
  • IS-IS is a protocol that allows routers to communicate with other routers.
• In OSI terminology, hosts area called End Systems (ES)

IS-IS Routing Levels

There are two routing levels in IS-IS:

1. Level 1 (L1)
   • L1 routing occurs within an IS-IS area and is responsible for  routing inside an area.
   • All devices (ISs and ESs) in the same area have the same area address.
   • Two route within the same area, the system ID of the devices is considered.
2. Level 2 (L2)
   • L2 routing occurs between different IS-IS areas.
   • Two route from one area to the next, the area address is considered. The System ID is ignored.

Three types of IS-IS Routers:

1. Level 1 (L1) Routers
   • L1 routers learn about paths within the areas they connect to (intra-area) by use of Link State PDUs (LSP) – the equivalent of LSAs in the OSPF world.
   • These routers are equivalent to OSPF internal non-backbone routers.
   • Intra-area (L1) routing enables ESs to communicate. An L1 area is a collection of L1and L1/L2 routers.
2. Level 2 (L2) Routers
   • L2 routers learn about paths between areas (inter-area) with the use of LSPs.
   • These routers are similar to OSPF backbone routers.
3. Level 1-2 (L1-L2) Routers
   • Learn about paths both within and between areas.
   • They are the equivalent of ABRs in OSPF.

• The path of L2 and L1/L2 routers is called the backbone
• All areas and the backbone must be contiguous.

OSI Routing Levels

1. Level 0 (L0) Routing
   • When an ES needs to send a packet to another ES, it finds the nearest IS on the same subnet and sends the packet there.
   • This is conducted by the ES-IS protocol.
     • ES-IS forms adjacencies between ESs (hosts) and ISs (routers)
       • IP end-systems do not use ES-IS
     • ESs transmit End System Hellos (ESHs) to announce their presence to ISs.
     • ISs transmit Intermediate System Hellos (ISH) to announce their presence to ESs.
     • ISs transmit IS-IS Hellos (IIHs) to other ISs.
2. IS-IS Level 1 (L1) Routing
   • Traffic exchanges between ISs in the same area
   • Also called intra-area routing.
3. IS-IS Level 2 (L2) Routing
   • If a destination address is in another area, the L1 finds the nearest L1/L2 IS and sends packet there.
   • Using the area address, packets are sent through other L2 and L1/L2 ISs until the packet reaches an L1/L2 IS in the destination area.
   • Within the destination area, ISs forward the packet using the best route, based on the sytem ID.
   • Also called inter-area routing.
4. Level 3 (L3) Routing
   • Passed traffic between different autonomous system.
   • Comparable to BGP
   • Not supported in Cisco routers.
   • Uses Interdomain Routing Protocol (IDRP) to conduct L3 routing.

IS-IS and OSPF Comparison

• Both are open standard link-state routing protocols. They maintain a link-state database from which Dijkstra-based SPF algorithm computes a shortest path tree.
• They both use similar mechanisms (such as LSA/LSP, link-state aging timers, and links-state database synchronization) to maintain the health of the LSDB.
• They both use Hello packets for establishing and maintaining adjacencies.
• Both use areas to form a two-level hierarchical topology.
• They are both classless protocols, and therefore support VLSM.
• Both have the capability of providing address summarization between areas.
• Both elect designated router to represent broadcast networks.
• Both have authentication capabilities
• Both converge quickly after network changes.

Area Design

• In OSPF, the border between OSPF areas is inside the ABRs. Some interfaces are in one area, and other interfaces are in another area.
• With this design, all areas have to connect to an area backbone. A consistent IP addressing is a must in order to properly summarize address into the backbone.

Figure 1: OSPF Area Topology

• IS-IS areas, in comparison, have all their routers completely within an area.
• The area borders are on links, not in the routers.
• IS-IS has a hierarchy of L1, L1/L2, and L2 routers.
• Extending the backbone is much more flexible. To extend, simply add another L1/L2 or L2 routers.

Figure 2: IS-IS Area Topology

OSPF and IS-IS Side-by-Side Comparison

Integrated IS-IS Advantages

• IS-IS updates for a certain group of routers are sent with very few LSPs, whereas, OSPF sends many small LSA updates.
• The relative small number of LSPs that IS-IS routers send adds to the effiecient and faster use of CPU resources for IS-IS.
• NET addresses that are used by IS-IS routers are already summarized, therefore, installing and removing prefixes are also less resource intensive.
• Based on default timers, IS-IS detects failures faster than OSPF. This helps with faster convergence.
• Extending the capability of IS-IS require only the addition of new TLVs, which is much simpler than creating new LSAs with OSPF.

OSPF Advantages

• OSPF is designed and optimized for use with IP.
• Finding support personnel and equipment is relatively much easier with OSPF.
• Documentation for OSPF is also abundant and readily available.

Resources:

1. Intermediate System-to-Intermediate System Protocol

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.