JN0-351 IS-IS

IS-IS Detailed Explanation

IS-IS (Intermediate System to Intermediate System) is a robust and scalable link-state routing protocol widely used in service provider and enterprise networks. It plays a crucial role in dynamic routing, similar to OSPF, but with some unique features and design principles.

1. Basic Concepts

What is IS-IS?

• IS-IS is a link-state routing protocol designed for Interior Gateway Protocol (IGP) use.
• It operates at Layer 3 and uses the Type-Length-Value (TLV) format to exchange routing information efficiently.
• It was originally developed for routing in the OSI protocol suite but later adapted for IP routing.

Key Features:

• Scalability: Supports large networks with complex topologies.
• Protocol Independence: Works with both IPv4 and IPv6.
• Hierarchical Design: Divides networks into levels and areas for efficient routing.

2. Detailed Knowledge

Levels and Areas

IS-IS organizes the network into levels and areas to create a hierarchical topology.

1. Level-1:

   • Used for intra-area routing (routing within an area).
   • Routers in the same area exchange routing information to build a complete topology of that area.
   • Level-1 routers only know about destinations within their own area.

2. Level-2:

   • Used for inter-area routing (routing between areas).
   • Level-2 routers act as the backbone and maintain connectivity between different areas.
   • They do not have detailed knowledge of intra-area routes, only summaries.

3. Hierarchical Structure:

   • Routers can be configured as:
     • Level-1 Router: Only routes within its own area.
     • Level-2 Router: Acts as a backbone router between areas.
     • Level-1/Level-2 Router: Participates in both levels and facilitates routing between areas.

Designated Intermediate System (DIS)

• What is DIS?

  • DIS is the IS-IS equivalent of OSPF’s Designated Router (DR).
  • It is elected to coordinate routing information and reduce overhead in a broadcast network (e.g., Ethernet).

• Key Features:

  • Unlike OSPF, IS-IS does not have a Backup DIS.
  • DIS election is based on priority, and if priorities are equal, the router with the highest MAC address wins.

• Responsibilities:

  • Generate a pseudo-node (virtual node) to represent the broadcast segment.
  • Simplify routing by consolidating link-state information.

Link-State Protocol Data Units (PDU)

• IS-IS exchanges routing information using Link-State Protocol Data Units (PDUs), which are similar to OSPF’s LSAs (Link-State Advertisements).

• Types of PDUs:

  1. Hello PDUs:
     • Establish and maintain neighbor relationships.
  2. LSP (Link-State PDUs):
     • Carry link-state information about a router’s connectivity.
  3. CSNP (Complete Sequence Number PDUs):
     • Provide a complete list of LSPs in the LSDB to ensure synchronization.
  4. PSNP (Partial Sequence Number PDUs):
     • Acknowledge specific LSPs and request missing information.

TLV Usage

• IS-IS uses the Type-Length-Value (TLV) format to make its protocol flexible and extensible.

• What is TLV?

  • Type: Indicates the kind of information (e.g., IP address, metric).
  • Length: Specifies the size of the information.
  • Value: Contains the actual information.

• Common TLVs:

  1. Metric TLV: Describes the cost of a link.
  2. IP Address TLV: Provides IP routing information.
  3. Extended TLVs: Support additional features like IPv6, Traffic Engineering, and MPLS.

3. Key Takeaways

• IS-IS is a link-state protocol: It builds a complete map of the network using PDUs and TLVs.
• Levels and Areas: Hierarchical design improves scalability by separating intra-area and inter-area routing.
• DIS Role: Reduces routing overhead in broadcast networks by centralizing link-state updates.
• TLV Flexibility: Makes IS-IS adaptable to new technologies and protocols.

IS-IS (Additional Content)

1. Router Types vs Interface Configuration

Unlike OSPF, which uses area-level configuration to define behavior, IS-IS in Junos is enabled on a per-interface basis, and the router’s level (Level-1, Level-2, or both) is determined by this configuration.

In Junos, a router becomes Level-1 or Level-2 based on how IS-IS is enabled on its interfaces.

Example CLI:

set protocols isis interface ge-0/0/1 level 1
set protocols isis interface ge-0/0/2 level 2

If interfaces are configured for both levels, the router becomes a Level-1/Level-2 router, enabling both intra-area and inter-area routing.

Exam Tip:
You may be asked:

“What determines whether a router participates in Level-2 routing?”
Correct answer: The interface-level configuration of IS-IS.

2. IS-IS Addressing – NET, System ID, and NSEL

IS-IS uses a hierarchical identifier format known as the Network Entity Title (NET). While not tested in-depth, the format and key components frequently appear in exam options.

Structure of the NET:

• Area ID: Variable length (up to 13 bytes); identifies the IS-IS area.

• System ID: 6 bytes (48 bits); uniquely identifies the router.

• NSEL (N-Selector): 1 byte, always set to 00 for routing.

Example NET:

49.0001.1921.6800.1001.00

• 49.0001 = Area ID

• 1921.6800.1001 = System ID

• 00 = NSEL

Exam Tip:
Expect a question like:

“Which field in the NET uniquely identifies a router?”
Correct answer: System ID

3. Metric Types – Wide vs Narrow Metrics

IS-IS supports two types of metrics: narrow (6-bit) and wide (32-bit). Juniper platforms default to wide metrics, which offer more granularity and scalability.

• Wide metrics are critical in MPLS, IPv6, and multi-area designs.

• The metric style must match among peers to avoid adjacency failures.

Configuration Example:

set protocols isis level 1 metric-style wide
set protocols isis level 2 metric-style wide

Exam Tip:
You might be asked:

“What is a key advantage of wide metrics in IS-IS?”
Correct answer: Supports higher metric values for modern networks.

4. DIS Election Behavior

In broadcast or multi-access segments, IS-IS elects a Designated Intermediate System (DIS) to manage pseudonode creation and LSP flooding efficiency. However, there is no Backup DIS.

IS-IS does not use a backup DIS; if the DIS fails, a new election occurs immediately.

Key Concepts:

• The DIS is elected based on priority (default is 64).

• If multiple routers have the same priority, the one with the highest MAC address becomes the DIS.

• The process is non-preemptive: A new router with a higher priority will not take over unless the current DIS fails.

Exam Tip:
You may see true/false or scenario questions that test this exact point.

5. IS-IS vs OSPF – Structural Comparison

Though the exam won’t explicitly ask you to compare IS-IS and OSPF, understanding their conceptual differences helps clarify protocol behaviors, especially in topology and packet formats.

Characteristic	IS-IS	OSPF
Layer of Operation	OSI Layer 3 (CLNS)	IP Layer
Routing Info Format	TLVs (Type-Length-Value)	LSAs (Link-State Advertisements)
Area Hierarchy	Level-1 / Level-2	Area 0 (Backbone) + others
Hello Message Name	IIH (IS-IS Hello)	OSPF Hello
DR Equivalent	DIS (no backup)	DR + BDR

Exam Tip:
This comparison helps you eliminate incorrect assumptions, such as expecting a BDIS to exist in IS-IS.

Summary – Key Enhancements for IS-IS Exam Mastery

Focus Area	Key Takeaway
Router Type Determination	Based on interface-level IS-IS configuration
NET Address Structure	Know the roles of Area ID, System ID, and NSEL
Metric Style	Junos uses wide metrics by default; critical for modern networks
DIS Election	No Backup DIS; new election triggered on failure
Protocol Comparison	TLV vs LSA, DIS vs DR/BDR, OSI Layer 3 vs IP