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LAB: What is BGP

LAB: What is BGP

April 6, 2025·Ryan
Ryan

So we are kicking the tires a bit and going right into a lab that I’ve got on my Cisco_CML repository. When I was looking through the CCIE exam topics this lab that I did about ~3 weeks ago does not really fit into the CCIE docs, but I think its a good way to kick the tires. If you got Cisco Modeling Labs for the first time this is a good lab to try it out.

Let me know what you think if you have comments or discussions you can discuss them in Github, Cisco CML Lab Discussions

Requirements & Topology:

Cisco Modeling Labs 2.8+ Free-Tier

  flowchart LR
    n1(("R1")) <--> n2(("R2"))
    n2 <--> n4(("R3"))

Lab Download (Github): What_is_BGP.yaml

In this Lab:

We are using a three (3) IOL routers with eBGP and iBGP config. This is a simple lab to get you started into the world of BGP.

  • Verify that BGP is Established
Answer
  • So for this question running the command that is under the helpful commands: show ip bgp neighbors | include BGP would be a helpful command to verify if BGP is established.
    R1#show ip bgp neighbors | include BGP
BGP neighbor is 192.168.12.2,  remote AS 65001, internal link
BGP version 4, remote router ID 192.168.23.1
BGP state = Established, up for 00:15:54
BGP table version 1, neighbor version 1/0

In the command output we can see for this example a neighbor has been established.

  • Which routers are using iBGP and which ones are using eBGP?
Answer

  • For this question running the command that is under the helpful commands: show ip bgp summary would be be helpful but you have to remember about the AS. (autonomous system)

    R2#show ip bgp summary
BGP router identifier 192.168.23.1, local AS number 65001
BGP table version is 1, main routing table version 1

    Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.12.1    4        65001      26      25        1    0    0 00:21:20        0
192.168.23.2    4        65002      26      26        1    0    0 00:21:12        0
R2#

    In the command output notice what (AS) R2 (192.168.23.1) is in? Hint: local AS number AS 65001

    We also see 192.168.12.1 is also in (AS) 65001. If both routers are using the same (AS) number they are in a (iBGP). If routers are using a different (AS) number they would be eBGP. So for this example R2 is in an iBGP and a eBGP neighbor.

  • Why don’t we have any routes in our routing table besides directly connected routes if BGP is established?
Answer
  • If you look at the configuration BGP has no network statements, BGP does not just automatically start sending routes, you have to tell BGP what routes you want to advertise.

Helpful Commands: 

show ip bgp neighbors | include BGP
show ip bgp summary

History of BGP

The Border Gateway Protocol (BGP) has evolved significantly since its inception. Here are some key milestones in its history:

  • 1989: BGP was first introduced in RFC 1105 by Yakov Rekhter and Kirk Lougheed. This initial version, known as BGP-1, was designed to replace the Exterior Gateway Protocol (EGP) and provide a more robust and scalable routing protocol for the internet.
  • 1990: BGP-2 was introduced in RFC 1163, which included improvements and bug fixes from the initial version.
  • 1991: BGP-3 was specified in RFC 1267, further refining the protocol and addressing additional issues identified in earlier versions.
  • 1994: BGP-4, the current version of the protocol, was introduced in RFC 1654. BGP-4 added support for Classless Inter-Domain Routing (CIDR), which allowed for more efficient use of IP address space and improved scalability.
  • 1995: RFC 1771 replaced RFC 1654, providing further clarifications and updates to BGP-4.
  • 2006: RFC 4271 was published, which is the current standard for BGP-4. This RFC consolidated and updated previous BGP-4 specifications, providing a comprehensive and detailed description of the protocol.

Over the years, numerous extensions and enhancements have been proposed and implemented to address emerging requirements and challenges in internet routing. BGP continues to be a critical component of the internet’s infrastructure, enabling efficient and reliable routing between Autonomous Systems.

Key Features of BGP

  • Path Vector Protocol: BGP uses a path vector mechanism to maintain the path information that gets updated dynamically as the network topology changes.
  • Scalability: BGP is designed to handle a large number of routes, making it suitable for the global internet.
  • Policy-Based Routing: BGP allows for complex routing policies based on various attributes like AS path, next-hop IP address, and more.
  • Loop Prevention: BGP uses the AS path attribute to prevent routing loops.

Types of BGP

  • External BGP (eBGP): Used for routing between different Autonomous Systems.
  • Internal BGP (iBGP): Used for routing within the same Autonomous System.

BGP Attributes

  • AS Path: Lists the ASes that routing information has passed through.
  • Next Hop: Indicates the next hop IP address to reach a destination.
  • Local Preference: Used to prefer an exit point from the AS.
  • MED (Multi-Exit Discriminator): Suggests the preferred route into an AS when multiple entry points exist.
  • … FYI there are more but theses are the most common.

BGP Operations

  • Establishing BGP Sessions: BGP peers establish a TCP connection on port 179 to exchange routing information.
  • Route Advertisement: BGP routers advertise routes to their peers.
  • Route Selection: BGP uses various attributes to select the best route to a destination.

Common BGP Issues

  • Route Flapping: Frequent changes in route availability can cause instability.
  • BGP Hijacking: Malicious actors can advertise incorrect routes, leading to traffic misdirection.
  • Configuration Errors: Misconfigurations can lead to routing issues and network outages.

Conclusion

BGP is a critical protocol for the functioning of the internet, enabling efficient and scalable routing between different networks. Understanding its features, operations, and potential issues is essential for network engineers and administrators.

Sources

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