EBGP vs. iBGP

Attribute	EBGP	iBGP
Routing Protocol	Exterior Gateway Protocol	Interior Gateway Protocol
Administrative Distance	20	200
Neighbor Relationship	Established between routers in different autonomous systems	Established between routers within the same autonomous system
Path Selection	Based on shortest AS path	Based on local preference
Next Hop	Next hop is modified to the router's IP address	Next hop remains unchanged
Loop Prevention	Uses AS Path attribute to prevent loops	Relies on the IGP (Interior Gateway Protocol) to prevent loops
Scalability	Less scalable due to full routing table exchange	More scalable as it only requires partial routing table exchange
Configuration	Requires manual configuration of neighbor relationships	Can use dynamic neighbor discovery protocols like BGP Confederations or Route Reflectors

Introduction

When it comes to routing protocols, Border Gateway Protocol (BGP) plays a crucial role in connecting different autonomous systems (AS) on the internet. BGP can be classified into two main categories: External BGP (EBGP) and Internal BGP (iBGP). While both protocols serve the purpose of exchanging routing information, they differ in their attributes and usage. In this article, we will explore the characteristics of EBGP and iBGP, highlighting their similarities and differences.

EBGP

EBGP is a routing protocol used to exchange routing information between different autonomous systems. It is typically implemented between routers in different ASs and is responsible for advertising and learning routes across AS boundaries. One of the key attributes of EBGP is that it uses a different autonomous system number (ASN) for each neighboring router. This ensures that routing information is exchanged between distinct ASs, allowing for inter-domain routing.

Another important characteristic of EBGP is that it prefers external routes over internal routes. This means that if an EBGP router receives multiple routes for the same destination, it will choose the route learned from an external AS rather than an internal one. This behavior helps in optimizing routing decisions and ensures efficient traffic flow across different ASs.

EBGP also employs a different administrative distance compared to iBGP. Administrative distance is a metric used to determine the trustworthiness of a routing protocol. In the case of EBGP, it typically has a lower administrative distance than iBGP. This means that if a router receives multiple routes for the same destination from both EBGP and iBGP, it will prefer the EBGP-learned route due to its lower administrative distance.

Furthermore, EBGP routers tend to be located at the edge of an AS, connecting to routers in neighboring ASs. This positioning allows EBGP to handle the exchange of routing information with external networks efficiently. The use of EBGP is crucial for maintaining connectivity and ensuring proper routing across the internet.

iBGP

iBGP, on the other hand, is a routing protocol used to exchange routing information within the same autonomous system. It is typically implemented between routers within the same AS and is responsible for propagating routing information learned from EBGP to other routers within the AS. Unlike EBGP, iBGP uses the same ASN for all routers within the AS.

One of the key attributes of iBGP is that it maintains full mesh connectivity between all routers within the AS. This means that each iBGP router establishes a direct peering session with every other iBGP router in the AS. This full mesh topology ensures that routing information is distributed uniformly across the AS, allowing for consistent and accurate routing decisions.

Another important characteristic of iBGP is that it does not prefer external routes over internal routes. If an iBGP router receives multiple routes for the same destination, it will choose the route learned from another iBGP router within the AS. This behavior ensures that routing decisions are made based on the best path within the AS, regardless of whether it is an external or internal route.

iBGP routers are typically located within the core of an AS, forming the backbone of the network. They are responsible for distributing routing information learned from EBGP to other routers within the AS. This internal distribution of routing information is crucial for maintaining consistent routing across the AS and ensuring that traffic is directed efficiently within the network.

Similarities

While EBGP and iBGP have distinct attributes, they also share some similarities. Both protocols use the same underlying BGP mechanisms for exchanging routing information, such as the BGP update messages. Additionally, both EBGP and iBGP rely on the concept of autonomous systems and use BGP attributes to make routing decisions.

Both EBGP and iBGP also support the use of BGP communities, which are tags attached to routes to provide additional information or instructions. BGP communities can be used to influence routing decisions, control traffic flow, or implement policies within an AS. The ability to manipulate BGP communities is valuable for network administrators to optimize routing and meet specific requirements.

Furthermore, both EBGP and iBGP can be configured to establish BGP peering sessions using different transport protocols, such as TCP or Multiprotocol Label Switching (MPLS). This flexibility allows network administrators to choose the most suitable transport protocol based on their network requirements and infrastructure.

Conclusion

EBGP and iBGP are two essential components of the Border Gateway Protocol, serving distinct purposes in the exchange of routing information. EBGP is used for inter-domain routing, connecting routers in different autonomous systems, while iBGP is used for intra-domain routing, distributing routing information within the same autonomous system. Understanding the attributes and differences between EBGP and iBGP is crucial for network administrators to design and maintain efficient and reliable routing infrastructures.