Lec33.1- Border Gateway Protocol;eBGP variant|Computer Networks

Key Concepts

• Border Gateway Protocol (BGP)
• Path Vector Routing Algorithm
• Autonomous Systems (AS)
• Intra-domain Routing
• Inter-domain Routing
• External Border Gateway Protocol (EBGP)
• Internal Border Gateway Protocol (IBGP)
• Border Routers (EBGP Speakers)
• Stub Autonomous System
• Transient Autonomous System
• TCP Protocol (Port 179)
• Update Messages

Border Gateway Protocol (BGP) Overview

The video discusses the Border Gateway Protocol (BGP), an inter-domain routing protocol crucial for communication between different Autonomous Systems (AS) on the internet. It highlights BGP's foundation in the path vector routing algorithm and contrasts it with intra-domain routing protocols like RIP and OSPF.

Unicast Routing Algorithm Classification

• Intra-domain Routing: Used within an AS (e.g., RIP, OSPF).
• Inter-domain Routing: Used between ASes (BGP).

Autonomous Systems (AS)

• The internet is divided into ASes, each managed by a single administrator.
• An AS consists of smaller networks.
• BGP facilitates routing packets between these ASes.

External Border Gateway Protocol (EBGP)

The video focuses primarily on EBGP, the variant of BGP used for communication between different ASes.

EBGP Functionality

• Enables routers in one AS to communicate with networks in other ASes.
• The current version in use is version 4.
• It is the only inter-domain protocol currently used on the internet.

EBGP Speakers (Border Routers)

• Within each AS, certain routers are designated as border routers or EBGP speakers.
• These routers are responsible for exchanging routing information with border routers in neighboring ASes.

Stub vs. Transient Autonomous Systems

• Stub AS: Connected to only one other AS. (e.g., AS2, AS3, AS4 in the example)
• Transient AS: Connected to multiple ASes. (e.g., AS1 in the example)

Example Network Configuration

The video uses a diagram with four ASes (AS1, AS2, AS3, AS4) to illustrate EBGP.

• AS1: Contains four routers (R1, R2, R3, R4) and is a transient AS.
• AS2: Contains one router (R5) and is a stub AS.
• AS3: Contains three routers (R6, R7, R8) and is a stub AS.
• AS4: Contains one router (R9) and is a stub AS.

Establishing EBGP Sessions

• EBGP speakers establish point-to-point connections over networks (e.g., N5, N6, N7).
• A logical connection is established using the TCP protocol on port 179.
• These logical connections are referred to as EBGP sessions.
• In the example, there are three EBGP sessions: R1-R5, R2-R6, and R4-R9.

Message Exchange

• EBGP speakers exchange update messages containing network reachability information.
• Each router advertises the networks it can reach within its own AS.
• The update messages include:
  • Networks reachable within the AS (e.g., N1, N2, N3, N4 in AS1).
  • The next-hop router to reach those networks (e.g., R1 for networks in AS1 when advertising to AS2).
  • The autonomous system number (e.g., AS1).

Example Message Exchange Scenario

• R1 (AS1) sends a message to R5 (AS2): "You can reach networks N1, N2, N3, and N4 via R1 in AS1."
• R5 (AS2) sends a message to R1 (AS1): "You can reach networks N8 and N9 via R5 in AS2."
• R2 (AS1) sends a message to R6 (AS3): "You can reach networks N1, N2, N3, and N4 via R2 in AS1."
• R6 (AS3) sends a message to R2 (AS1): "You can reach networks N10, N11, and N12 via R6 in AS3."
• R4 (AS1) sends a message to R9 (AS4): "You can reach networks N1, N2, N3, and N4 via R4 in AS1."
• R9 (AS4) sends a message to R4 (AS1): "You can reach networks N13, N14, and N15 via R9 in AS4."

Intra-domain Routing within ASes

• Routers within each AS use intra-domain routing protocols (RIP or OSPF) to learn how to reach networks within their own AS.
• This information is then shared with neighboring ASes via EBGP.

Limitations of EBGP and Introduction to IBGP

• EBGP, by itself, only allows directly connected ASes to learn about each other's networks.
• In the example, AS1 can reach all networks in AS2, AS3, and AS4, but AS2, AS3, and AS4 can only reach networks in AS1.
• The video hints at the need for Internal Border Gateway Protocol (IBGP) to address this limitation and enable all ASes to reach all networks.
• IBGP will be covered in a subsequent video (Part 2).

Conclusion

The video provides a detailed explanation of EBGP, its role in inter-domain routing, and how it enables communication between different ASes on the internet. It emphasizes the importance of identifying border routers, establishing EBGP sessions, and exchanging update messages to share network reachability information. The video also introduces the concept of IBGP as a solution to the limitations of EBGP in achieving full network reachability across all ASes.