JN0-363 Intermediate System to Intermediate System (IS-IS)

Intermediate System to Intermediate System (IS-IS) Detailed Explanation

Overview

Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol used for large-scale networks, including service provider environments. Unlike OSPF, IS-IS operates at Layer 2 (Data Link Layer) of the OSI model. It does not depend on IP for its operation, making it flexible and scalable for multiprotocol environments, including IPv4 and IPv6.

Key Topics

1. Hierarchical Levels

IS-IS organizes networks into a two-level hierarchy to optimize routing and scalability.

• Level 1:

  • Handles intra-area routing within a single area.
  • Routers exchange information only with other Level 1 routers in the same area.
  • Level 1 routers require a default route to reach destinations outside their area.

• Level 2:

  • Handles inter-area routing, similar to OSPF's backbone area (Area 0).
  • Level 2 routers exchange topology information between different areas.

• Level 1-2 Routers:

  • Act as intermediaries between Level 1 and Level 2.
  • Maintain separate databases for Level 1 and Level 2 routing.

Example Scenario:

• A service provider network with multiple areas (e.g., Area 1, Area 2) uses Level 1 routing within each area and Level 2 routing for traffic between areas.

2. Protocol Data Units (PDUs)

IS-IS uses various types of Protocol Data Units (PDUs) to exchange information.

• Link-State PDU (LSP):

  • Contains detailed topology information, such as router IDs, metrics, and attached prefixes.
  • LSPs are flooded within a Level 1 or Level 2 area to maintain a synchronized database.

• Complete Sequence Number PDU (CSNP):

  • Provides a summary of all LSPs in the database.
  • Used to verify database consistency.

• Partial Sequence Number PDU (PSNP):

  • Requests missing or outdated LSPs.
  • Ensures database synchronization.

How PDUs Work Together:

• Routers use LSPs to advertise their topology.
• CSNPs summarize the known topology.
• PSNPs request missing or updated information.

3. TLVs (Type-Length-Value)

TLVs are the building blocks of IS-IS packets, allowing flexibility in carrying different types of information.

• What is a TLV?

  • A TLV consists of:
    • Type: The kind of information (e.g., area address).
    • Length: The size of the value field.
    • Value: The actual data.

• Common TLVs:

  1. TLV 1: Area Address:
     • Identifies the area to which a router belongs.
  2. TLV 2: IS Neighbors:
     • Lists neighboring routers and their metrics.
  3. TLV 128: IPv4 Prefix:
     • Advertises IPv4 routes.
  4. TLV 135: IPv6 Prefix:
     • Advertises IPv6 routes.

Configuration Example:

• To define an area in IS-IS:

  set protocols isis area 49.0001
  

4. IS-IS Metrics

IS-IS assigns a metric to each link, which represents the cost of using that link.

• Default Metrics:

  • Range: 0–63.
  • Often based on bandwidth or administrative preference.

• Wide Metrics:

  • Range: 0–16,777,215.
  • Allows for finer control and is required for MPLS Traffic Engineering (MPLS-TE).

• Configuration Example:

  • To enable wide metrics:

    set protocols isis level 1 wide-metrics-only
    

• Manual Metric Adjustment:

  • Example:

    set protocols isis interface ge-0/0/1 level 2 metric 10
    

    • Sets the metric for Level 2 routing on interface ge-0/0/1 to 10.

5. Route Redistribution

Route redistribution allows IS-IS to share routes between Level 1 and Level 2 or with external routing protocols.

• When to Use Redistribution?

  • To make external routes (e.g., BGP or static routes) visible within IS-IS.
  • To allow Level 1 routers to access routes from other areas.

• Configuration Example:

  • Redistributing static routes into IS-IS:

    set protocols isis redistribute static level 2
    

  • Explanation:

    • Injects static routes into Level 2 IS-IS routing.

6. IPv6 Support

IS-IS supports both IPv4 and IPv6 through integrated routing. This makes it efficient for dual-stack networks.

• How IS-IS Supports IPv6:

  • IPv6 routes are advertised using specific TLVs (e.g., TLV 135).
  • There is no need for a separate IPv6 protocol instance.

• Configuration Example:

  set protocols isis interface ge-0/0/1 family inet6
  

Summary of Key Points

1. Hierarchical Levels:
   • Level 1: Intra-area routing.
   • Level 2: Inter-area routing (backbone).
2. PDUs:
   • LSPs, CSNPs, and PSNPs are used for topology sharing and database synchronization.
3. TLVs:
   • Flexible structure to carry different types of routing information.
4. Metrics:
   • Default (0–63) and wide (0–16,777,215) metrics control routing preferences.
5. Route Redistribution:
   • Shares routes between Level 1 and Level 2 or external sources.
6. IPv6 Support:
   • Fully integrated with IS-IS for dual-stack environments.

Intermediate System to Intermediate System (IS-IS) (Additional Content)

Overview

IS-IS is a link-state routing protocol widely used in service provider and large-scale enterprise networks. Unlike OSPF, IS-IS operates directly over Layer 2 (not IP) and supports both IPv4 and IPv6. It uses TLVs for flexible data handling and divides the routing domain into areas and levels for scalability.

In addition to core concepts, the following enhancements provide deeper understanding and are often tested in certification exams.

1. IS-IS Addressing: NSAP and NET Structure

IS-IS routers are not identified by IP addresses but by NET (Network Entity Title) addresses derived from NSAP (Network Service Access Point) formats. This unique identifier allows routers to participate in the IS-IS routing domain.

NET (Network Entity Title) Format:

• A NET consists of:

  1. Area ID: Identifies the IS-IS area.

  2. System ID: Uniquely identifies the router within the area (often derived from the loopback interface or MAC address).

  3. N-selector: Indicates the NSAP selector (typically 00 for routing).

Example:

49.0001.1921.6800.1001.00

• 49.0001 – Area ID

• 1921.6800.1001 – System ID (6-byte value)

• .00 – N-selector (indicates routing)

Every IS-IS router must have a unique NET, which is configured under the IS-IS protocol hierarchy.

Configuration Example in Junos:

set protocols isis net 49.0001.1921.6800.1001.00

2. IS-IS Neighbor State Formation

IS-IS routers establish neighbor relationships by exchanging Hello messages (IS-IS Hello PDUs, or IIHs). The adjacency state evolves in stages, and mismatches in parameters such as MTU or authentication can cause failures.

IS-IS Neighbor State Transitions:

State	Description
Down	No IIH messages received from neighbor.
Init	IIH message received, but adjacency not yet established.
Up	Full adjacency established; LSDBs can now be synchronized.

Common Reasons for Adjacency Failure:

• MTU mismatch between peers.

• Area ID mismatch.

• Authentication failure.

• Interface type mismatch (e.g., point-to-point vs broadcast).

Troubleshooting Tip:

Use this command to check adjacency status:

show isis adjacency

3. IS-IS Authentication

IS-IS supports authentication to secure control-plane exchanges, especially important in environments where spoofed PDUs could cause routing instability.

Key Points:

• Authentication can be applied to:

  • IIH (Hello PDUs)

  • LSP (Link State PDUs)

  • SNP (Sequence Number PDUs)

• Junos supports plaintext and MD5 authentication for IS-IS.

Configuration Example (MD5):

set protocols isis interface ge-0/0/0.0 hello-authentication md5 key 1 secretkey
set protocols isis interface ge-0/0/0.0 lsp-authentication md5 key 1 secretkey

• This ensures that only routers with matching keys can form adjacencies and exchange routing information.

Summary of Additions

Topic	Key Details
NET Addressing	Unique ID for each router; Area ID + System ID + N-selector.
Neighbor State Machine	Down → Init → Up; failure reasons include MTU or auth mismatch.
Authentication	Optional security for IIH, LSP, SNP PDUs; supports MD5 in Junos.