Inter-AS vs. Intra-AS

In the context of BGP (Border Gateway Protocol), "Inter-AS" and "Intra-AS" refer to routing and policy considerations within different types of networks. Here’s a detailed look at the distinctions between them:

Inter-AS (Inter-Autonomous System) Routing

Definition:

• Inter-AS routing deals with the exchange of routing information between different Autonomous Systems (ASes) on the internet. This is also known as inter-domain or external routing.

Key Characteristics:

• BGP Protocol: The primary protocol used for inter-AS routing is BGP (Border Gateway Protocol). BGP is designed to handle routing between different ASes and is crucial for internet-wide routing.
• AS Path: BGP uses the AS Path attribute to track the number of ASes a route has traversed. This helps in selecting the shortest path in terms of AS hops.
• MED (Multi-Exit Discriminator): MED is used to suggest to external ASes the preferred route into the advertising AS. It helps control which entry point is preferred for inbound traffic into an AS.
• Inter-AS Policies: Routing policies between ASes can be influenced by factors such as commercial agreements, traffic engineering, and network performance. These policies are generally set by network administrators to achieve specific business or technical objectives.
• Non-Transitive Attributes: Attributes like MED are not propagated beyond the AS that sets them, meaning their influence is limited to neighboring ASes.

Example: An AS that wants to prefer traffic coming in through one of its multiple connections from a neighboring AS might use MED to suggest the preferred entry point.

Intra-AS (Intra-Autonomous System) Routing

Definition:

• Intra-AS routing deals with routing within a single Autonomous System. This is also known as intra-domain or internal routing.

Key Characteristics:

• IGP Protocols: For intra-AS routing, Interior Gateway Protocols (IGPs) are used. Common IGPs include:
  • OSPF (Open Shortest Path First)
  • EIGRP (Enhanced Interior Gateway Routing Protocol)
  • IS-IS (Intermediate System to Intermediate System)
• Local Preference: In BGP, the local preference attribute is used to influence the choice of routes within the AS. Higher local preference values are preferred.
• IGP Metrics: Metrics such as OSPF cost or EIGRP metric are used to determine the best path within the AS.
• Routing Policies: Intra-AS routing policies are typically used to manage internal network performance and reliability, focusing on factors like link utilization and network topology.
• Transitive Attributes: Attributes used in IGPs, such as OSPF cost, are generally transitive within the AS, meaning they are used by all routers within the AS.

Example: Within an AS, OSPF or EIGRP might be used to route traffic between different routers based on metrics like bandwidth or latency.

Comparison

1. Scope:

   • Inter-AS: Involves routing information exchange between different ASes (external).
   • Intra-AS: Involves routing information exchange within a single AS (internal).

2. Protocols:

   • Inter-AS: BGP.
   • Intra-AS: IGPs (OSPF, EIGRP, IS-IS).

3. Routing Attributes:

   • Inter-AS: AS Path, MED, BGP Local Preference.
   • Intra-AS: IGP Metrics (cost, bandwidth), OSPF or EIGRP attributes.

4. Policy Influence:

   • Inter-AS: Policies can be influenced by commercial agreements, peering arrangements, and external routing preferences.
   • Intra-AS: Policies are generally focused on optimizing internal network performance and reliability.

5. Propagation:

   • Inter-AS: Attributes like MED are not propagated beyond the AS where they are set.
   • Intra-AS: Attributes and metrics are used internally and are generally visible to all routers within the AS.

Summary

Inter-AS routing (BGP) and Intra-AS routing (IGP) serve different but complementary roles in network routing. Inter-AS routing focuses on how data is exchanged between different ASes on the internet, while Intra-AS routing focuses on optimizing and managing traffic within a single AS. Understanding both is essential for effective network design and operation.