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Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet. The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol.

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How does BGP work?

This is a little beyond the scope of this document, however in simplest terms, BGP learns routes from your internal routing protocol (static routes, RIP, OSPF etc.) and announces them to a BGP-speaking neighbor or peer (the T-Mobile router in this case).

This means that if you are running BGP, you are responsible for your own routing and announcements. T-Mobile routers learn about your network from your router. Unless you announce the route to us, we cannot send you traffic for that CIDR block. A route announcement is sometimes referred to as a ‘prefix’. A prefix announced in BGP consists of the IPV4 or IPV6 address block being announced and also a path of AS numbers, indicating which ASNs the traffic must pass through to reach the announced address block. A BGP prefix would look something like (IPV4): 701 1239 42 The /24 part is referred to as a CIDR mask. The /24 indicates that the first 24 bits in the address block is the ?network? part of the address block.

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Basic BGP routing, lab description

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  • Basic BGP routing,
  • BGP-Split-Horizon avoidance using iBGP full mesh config in AS-100,
  • BGP-Split-Horizon avoidance using Route Reflectors configs in AS-125,
  • iBGP Next-hop behavior issues in AS100
  • Resolved iBGP Next-hop behavior using ‘next-hop-self’ cmd in AS-125
  • EBGP adjacency using WAN-LINKS between AS-2 & AS-100, AS-100 & AS-125
  • EBGP adjacency using LoopBackInterfaces between AS-125 & AS-50, AS-100 & 130
  • EBGP redundacy Links between AS-100 & 130
  • iBGP neighbor adjacencies using loopbacks in AS-100
  • iBGP neighbor adjacency using WAN-Links in AS-125
  • OSPF as underlay network reachability in AS-100 & AS-125
  • Network Automation

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