Interior Gateway Protocols (IGP) in ISP Networks: A Deep Dive

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At Exascale, we’ve implemented and optimised Interior Gateway Protocols (IGPs) for numerous Internet Service Providers (ISPs) across the UK. In this post, we’ll explore how IGPs, particularly OSPF and IS-IS, form the backbone of efficient ISP network routing.

IGPs in ISP Networks

IGPs are crucial for routing within an ISP’s autonomous system (AS). They enable routers to exchange information about network topology, facilitating optimal path selection for internal traffic. In ISP networks, IGPs lay the foundation for seamless packet forwarding and provide the underlying structure for services like BGP.

Loopbacks and Passive Interfaces

A key aspect of IGP configuration in ISP networks is the use of loopback interfaces. These virtual interfaces are always up, providing stable router identification. We typically configure loopbacks as passive interfaces, meaning they don’t send or receive routing protocol messages but are still advertised to other routers. This approach reduces unnecessary protocol traffic whilst maintaining reachability.

Network Advertisement

In ISP deployments, we carefully control what’s advertised via IGPs. Generally, only two types of networks are included:

  1. Loopback interfaces
  2. Link networks (connections between routers)

This focused approach keeps routing tables lean and efficient, which is crucial in large-scale ISP environments.

OSPF vs IS-IS: The ISP Perspective

While both OSPF and IS-IS serve similar purposes, their differences can be significant in ISP contexts:

  1. Protocol foundation: OSPF is IP-based, while IS-IS is protocol-agnostic, based on OSI.
  2. Addressing: OSPF uses IP addresses for router identification; IS-IS uses OSI NSAP addresses.
  3. Hierarchy: OSPF has a two-level hierarchy with a mandatory backbone area. IS-IS offers more flexible multi-level hierarchy.
  4. Extensibility: IS-IS is more easily extended to support new features or protocols, which can be advantageous for ISPs planning for future network evolution.

BGP and Loopback Interfaces

In ISP networks, we typically build BGP sessions to loopback interfaces rather than physical Layer 2 interfaces. This practice enhances stability – if a physical interface fails, the BGP session remains active as long as there’s an alternate path to the loopback.

IS-IS Addressing in ISP Networks

IS-IS uses the OSI NSAP format for addressing, which includes:

  1. Area ID: Identifies the IS-IS area
  2. System ID: Uniquely identifies the router (often derived from a loopback IP)
  3. NSEL: Usually set to 00 for routing purposes

This addressing scheme allows IS-IS to operate independently of IP addressing, offering flexibility in network design.

OSPF Areas in ISP Deployments

For ISPs using OSPF, area design is crucial for scalability. Key concepts include:

  1. Backbone Area (Area 0): The central area connecting all others
  2. Standard Areas: Non-backbone areas with full link-state information
  3. Stub Areas: Areas with reduced routing table size
  4. Totally Stubby Areas: Further simplified areas
  5. Not-So-Stubby Areas (NSSAs): Areas that can import external routes with limitations

Proper area design helps manage large ISP networks by compartmentalising routing information.

IS-IS Metrics in ISP Networks

IS-IS uses a straightforward metric system based on link cost. In ISP networks, we often leverage wide metrics for more granular control:

  1. Default metric range: 1-63 (narrow metrics)
  2. Wide metric range: 1-16777215

The flexibility of IS-IS metrics allows us to fine-tune routing decisions based on various factors beyond just bandwidth, which is particularly useful in complex ISP topologies.


At Exascale, our experience has shown that understanding and properly implementing IGPs is crucial for building robust and efficient ISP networks. Whether using OSPF or IS-IS, careful configuration of loopbacks, thoughtful network advertisement, and strategic use of areas or levels can significantly enhance network performance and scalability.

For ISPs looking to optimise their routing infrastructure or plan for future growth, a deep understanding of IGP mechanics is invaluable. As always, we’re here to help navigate these complex waters and ensure your network is built on a solid foundation.