BGP Concepts Overview

Lecture Overview

• Presenter: David Voss (CCIE 11372)
• Topics covered:
  • BGP Basics
  • BGP Path Attributes
  • eBGP vs iBGP
  • Public vs Private AS Numbers
  • BGP Updates and Route Advertisement

BGP Basics

• BGP (Border Gateway Protocol): An exterior gateway protocol using Autonomous System (AS) numbers.
• Communicates using TCP 179.
• Requires manual configuration for most operations.
• Path Attributes: Key for decision making in route selection.

Key Path Attributes to Know for CCNP Route Exam

1. Weight: Influences best route for the local router (manually configured).
2. Local Preference: Influences best route for all routers in an AS (shared attribute).
3. AS Path: Lists the number of ASs in the path; crucial for decision making.
4. Origin: Indicates if the route is from an IGP or EGP.
5. MED (Multi-Exit Discriminator): Influences best route for routers in another AS.

Internal vs External BGP

• iBGP (Internal BGP): Used within the same AS, routers do not update AS Path.
  • Routers must be fully meshed.
• eBGP (External BGP): Connects different ASs, routers update the AS Path.

Example BGP Routing Decisions

• Prefer routes with fewer ASs in the AS Path more than hops.
• Traffic flows can be manipulated by AS Path and MED attributes.

Public vs Private ASN

• Understand the distinction between public and private Autonomous System Numbers (ASN).

BGP Updates

• Types of updates from providers:
  • Default route only
  • Full BGP routing table
  • Partial updates
• Advertising Routes: Can be done via:
  1. Manual network command.
  2. Redistribution of BGP and IGP.
  3. Propagation of existing BGP routes.
  4. Using aggregate address command.

Synchronization Rule in BGP

• BGP will not advertise a route unless it knows about it via an IGP.
• Can disable synchronization with no synchronization command.

BGP Configuration

• Enabling BGP:
  • Requires an ASN and neighbors' IP addresses.
• Example configuration: router bgp 10 neighbor 10.1.1.2 remote-as 2
• Neighbors must match AS numbers and establish a TCP connection.

Advertising Networks

• Use network command for manual advertisement: network 192.168.1.0 mask 255.255.255.0
• Redistribute routes from IGP into BGP.

Verification and Troubleshooting BGP

Important Show Commands

1. show ip bgp: Displays the BGP routing table.
2. show ip bgp neighbor advertise: Shows routes advertised to a specific neighbor.
3. show ip bgp neighbor routes: Displays received routes from a specific neighbor.
4. show ip bgp summary: Displays the status of all BGP connections.

BGP Neighbor States

• Active State: Indicates an issue (no BGP messages sent).

Clear Commands

• Hard Clear: Tears down and rebuilds the neighbor relationship.
• Soft Clear: Resets the neighbor relationship without tearing it down.

BGP Path Control

• Learn about BGP best path decision process focusing on:
  • Weight, Local Preference, AS Path, MED.

Best Path Decision Process Order

1. Highest Weight
2. Highest Local Preference
3. Shortest AS Path
4. Lowest Origin Type
5. Lowest MED
6. eBGP preferred over iBGP

Manipulating BGP Attributes

• Use route maps to influence path selection through weight and MED settings.
• Prepend AS numbers to manipulate AS Path preferences.

BGP Troubleshooting Techniques

• Common issues include mismatched AS numbers, TCP port 179 being blocked, and incorrect IP addresses.
• Monitor neighbor relationships and routing tables using show commands.
• Employ next-hop-self command to manage routing updates effectively.

Final Tips

• Test configurations in a lab environment, simulate failures, and troubleshoot to reinforce learning.