JN0-664 IS-IS

IS-IS Detailed Explanation

What is IS-IS?

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol originally designed for ISO (International Organization for Standardization) networks but widely adopted in IP networks. It is especially popular in large-scale networks, such as those managed by service providers and used in backbone and MPLS environments.

Comparison with OSPF:

• Like OSPF, IS-IS is a link-state protocol that maintains a database of the network topology.
• Unlike OSPF, IS-IS was not originally designed for IP, but it was adapted to support both IPv4 and IPv6.
• It uses TLVs (Type-Length-Value) structures for scalability and flexibility.

Key Concepts

1. Hierarchical Levels

IS-IS divides the network into two hierarchical levels to simplify routing:

1. Level-1 (Intra-Area Routing):

   • Routes traffic within a single area.
   • Routers at this level only exchange information about destinations in their own area.
   • Level-1 routers do not know about the entire network topology, similar to OSPF stub areas.

2. Level-2 (Inter-Area Routing):

   • Routes traffic between areas.
   • Acts as a backbone for inter-area communication, similar to OSPF Area 0.
   • Level-2 routers maintain a complete view of all areas in the network.

Why Levels Are Useful: Hierarchical levels reduce the size of the routing tables and limit the scope of updates, which enhances scalability and efficiency in large networks.

2. LSP (Link State PDU)

• LSP (Link State Protocol Data Unit) is the fundamental data structure IS-IS uses to exchange network topology information.
• Each router generates an LSP, which contains details about its neighbors, links, and metrics.
• These LSPs are flooded throughout the network to build a Link-State Database (LSDB), which all routers use to calculate the best paths.

Key Components of LSP:

• Router ID
• Neighbor IDs
• Link costs
• Attached prefixes (IP reachability information)

3. TLVs (Type-Length-Value)

IS-IS uses TLVs to carry information within LSPs. TLVs are extensible and allow IS-IS to support new features without redesigning the protocol.

Common TLVs:

1. IP Reachability TLV:

   • Advertises IPv4 and IPv6 routes.
   • Contains information about prefixes and their associated metrics.

2. Router Capability TLV:

   • Describes a router's capabilities, such as multi-topology support and other advanced features.

Advantages of TLVs:

• Highly extensible and modular.
• Enables IS-IS to support new technologies like MPLS, IPv6, and traffic engineering.

4. Multi-Topology IS-IS

• IS-IS can support multiple topologies for IPv4 and IPv6 simultaneously.
• This feature is called Multi-Topology IS-IS and allows the network to operate IPv4 and IPv6 independently.
• Multi-topology ensures that IPv4 changes do not impact IPv6 traffic and vice versa.

5. Authentication

To secure routing updates, IS-IS supports authentication at the area level and the interface level:

• MD5 Authentication: The most common method, which uses a secure hash to verify the integrity of LSPs.
• Plaintext Authentication: Simpler but less secure.

Authentication prevents unauthorized routers from participating in the network and tampering with routing information.

Advantages of IS-IS

1. Scalability:

   • Hierarchical levels and the use of TLVs make IS-IS highly scalable, suitable for large networks.

2. Flexibility:

   • It supports IPv4, IPv6, and MPLS.
   • TLVs enable easy extension for new features.

3. Fast Convergence:

   • IS-IS converges quickly after network changes, ensuring minimal downtime.

4. Simplicity in Design:

   • Since IS-IS operates directly over Layer 2 (not IP), there’s no need for transport configuration (like TCP/UDP for OSPF).

Use Cases

1. Backbone Networks:

   • Widely used by service providers to manage core networks and large-scale routing.

2. MPLS Core Networks:

   • Preferred for its scalability and ability to support MPLS-related features.

3. Dual-Stack Networks:

   • Useful in environments transitioning from IPv4 to IPv6 due to multi-topology support.

Junos Configuration

Here’s how to configure IS-IS on a Juniper router:

1. Enable Level-1 Routing on an Interface:

   set protocols isis level 1 interface ge-0/0/0.0
   

2. Enable Level-2 Routing on an Interface:

   set protocols isis level 2 interface ge-0/0/1.0
   

3. Configure MD5 Authentication:

   set protocols isis interface ge-0/0/0.0 authentication md5 key "password"
   

4. Enable IS-IS Globally:

   set protocols isis level 1 enable
   set protocols isis level 2 enable
   

Conclusion

IS-IS is a powerful and flexible routing protocol, particularly suited for large and complex networks. It provides:

• Scalability through hierarchical design.
• Extensibility via TLVs.
• Support for modern networking requirements, such as MPLS and IPv6.

If you’re learning IS-IS, focus on:

1. Understanding the relationship between Level-1 and Level-2 routing.
2. Learning how LSPs and TLVs work together to build the network topology.
3. Practicing configurations in a lab environment.

IS-IS (Additional Content)

1. IS-IS Independence from IP: Architectural Advantage

IS-IS was not originally designed for IP. Instead, it was built for the OSI protocol stack and uses:

CLNS (Connectionless Network Service):

• IS-IS packets are encapsulated using CLNS, a Layer 3 protocol in the OSI model.

• Unlike OSPF, IS-IS does not use IP for packet transport.

Key Implications:

• Interface IP addresses are not required to establish IS-IS neighbor relationships.

  • This allows adjacencies to form before IP is configured, which speeds up provisioning.

• IS-IS can run on unnumbered interfaces and point-to-point links with no IP address.

• It avoids potential IP-layer instability or delays during convergence.

Why service providers prefer it:

• In MPLS core networks where IP may not be configured on all links (e.g., label-only transport), IS-IS provides a lightweight and scalable IGP solution.

2. Level-1-2 Router Behavior: Core to Multi-Area Topology

IS-IS routers operate at either or both levels:

• Level-1 routers: Only have topology information within their own area.

• Level-2 routers: Form the backbone, forwarding traffic between areas.

• Level-1-2 routers: Operate at both levels simultaneously.

Level-1-2 Routers are similar to OSPF ABRs:

• Act as border routers between areas.

• Can perform route leaking (importing Level-2 routes into Level-1 or vice versa) under policy.

• Maintain two separate LSDBs: one for each level.

• Can forward traffic between different areas when properly configured.

In practice, Level-1-2 routers help bind the IS-IS topology together, especially in large provider networks with multiple areas.

3. DIS: Designated Intermediate System (vs OSPF DR)

On broadcast multi-access networks (e.g., Ethernet), IS-IS uses DIS, not DR/BDR like OSPF.

Key Differences:

• Only one DIS is elected per LAN segment; there is no Backup DIS.

• DIS is selected based on priority (highest wins), with the highest MAC address as tiebreaker.

• The DIS performs two key functions:

  1. Generates a pseudo-node LSP on behalf of the LAN.

  2. Ensures proper flooding of LSPs across the shared segment.

Pseudo-node LSP:

• Represents the LAN as a virtual node in the topology.

• Reduces the number of LSPs required on multi-access links.

Unlike OSPF’s DR, the DIS role is less control-plane intensive and does not involve a full topology centralization.

4. LSP Aging and Reliable Flooding in IS-IS

LSP Aging:

• Each LSP has an aging timer, with a default value of 1200 seconds (20 minutes).

• When an LSP reaches MaxAge, it is removed from the LSDB.

Update Verification:

• Each LSP includes a sequence number and checksum.

• When a router receives an LSP, it checks:

  • If the sequence number is higher → accept and flood.

  • If lower or same → discard or acknowledge.

Flooding Behavior:

• IS-IS uses reliable flooding:

  • All LSPs are acknowledged.

  • Retransmissions are done if acknowledgments are missing.

• Note: Flooding is not TCP-based; it uses its own acknowledgment and retransmit logic over CLNS.

Understanding LSP lifetime, checksum, and flooding is vital for troubleshooting convergence or inconsistent topology views.

5. Area Definition in IS-IS: Based on NET

Unlike OSPF, IS-IS does not use an "area 0" backbone or area configuration commands.

IS-IS Area is Defined in the NET Address:

• The Network Entity Title (NET) is the ISO address used to identify the router.

• It consists of:

  • Area ID (variable length, typically up to 13 bytes).

  • System ID (6 bytes, like a MAC).

  • NSEL (Selector Byte): Always 00 for routers.

Example:

NET: 49.0001.1921.6800.1001.00

• 49.0001 → Area ID

• 1921.6800.1001 → System ID

• 00 → NSEL

• Routers are considered in the same area if their Area IDs match.

• There is no explicit "area" configuration like in OSPF — the NET determines the area.

This design gives IS-IS flexibility in address planning and reduces configuration complexity.

Summary Table

Topic	Key Detail
CLNS & Layer 2 Operation	IS-IS runs over CLNS, independent of IP, forms adjacencies on unnumbered links
Level-1-2 Routers	Bridge inter-area routing, like OSPF ABRs, maintain dual LSDBs
DIS vs DR	No BDR; DIS generates pseudo-node LSPs to represent LANs
LSP Aging & Flooding	Default age: 1200s; uses seq/checksum; reliable but not TCP
Area Definition via NET	Area ID is part of NET address; no area 0 backbone required