Notes9:WANRoutingBGP - echadbourne/NET-330 GitHub Wiki

Routing Protocols

Exterior Gateway Protocol: Responsible for building and updating routing tables between organizations - across large internetworks like the internet

• Border Gateway Protocol (BGP): Is the EGP Used on the internet
• Used to help secure the WWI, if we used OSPF other routers could advertise other networks

BGP Overview

• Exchanges routing and reachability information among autonomous systems (AS) on the internet
• The protocol is classified as a path vector protocol
• Makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator
• Some very large organizations may also use BGP as an IGP

Autonomous Systems

• A collection of connected Internet Protocol (IP) routing prefixes
• Under the control of a single administrative entity (eg company, university, ISP)
• That presents a common, clearly defined routing policy to the Internet

AS Numbers

• Unique ASN is allocated to each AS for use in BGP routing
• AS numbers are important because the ASN uniquely identifies each (aggregated) network on the internet
• IANA handles AS number assignments
• Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65536 assignments
• Now registries can hand out 32 bit AS numbers (over 4 billion)

AS Number Growth

The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999

• 30,000 in late 2008
• 35,000 in mid 2010
• 42,000 in late 2012
• 54,000 in mid 2016
• 118,000 in Oct 2024

BGP Operation

• BGP neighbors, called peers, are established by manual configuration between routers
• Creates a TCP session on port 179
• A BGP speaker sends 19-byte keep-alive messages every 60 seconds to maintain the connection
• Among routing protocols, BGP is unique in using TCP as its transport protocol
  • Others are their own layer 4 protocols

BGP Peering

• So, organizations must "peer" with their ISP(s) and other networks with which they share an external link
• This allows for multi-homed networks (aka multiple connections to the internet)
• Your AS will be advertised through your multiple peers - this allows multiple paths to your IP networks from the internet
• Other routers pick the best "path" to your network

BGP - Path Vector Protocol

Distance Vector Protocols

• Like RIP - use "distances" as route selection metric
• Routing table includes how many hops/routers to a remote network
• Only knows IP of next router - not the full path to destination

Path Vector Protocols

• Routing table includes the "path" of AS numbers to get remote networks

BGP Best Path Algorithm

• BGP Best Path Selection Algorithm is used to choose and install the best routes into the router's routing table
• The full internet BGP routing table includes over 400,000 routes
• BGP router can receive numerous copies of those routes from various providers
• So it has to compare the multiple routing tables and choose the optimal route on the router

BGP Attributes for selecting path

BGP Update Message

• Sent to peers
• Includes AS Path and IPv4 nets
  • 10.0.0.0/8 in this example

Configuring BGP

A basic configuration of BGP is pretty simple

3 steps

1. Create BGP instance
2. Identify peer neighbors by IP Address
3. Identify any networks that the router is responsible 10 advertise

BGP on Cisco

1. Define router instance
   • router bgp *AS_Number_for_Router*
   • router bgp 1010
2. Identify peers
   • (config-router) neighbor *ip_of_peer* remote-as *as-number_of_peer*
   • (config-router) neighbor 192.168.1.2 remote-as 2352
3. Advertise Networks
   • (config-router) network 10.10.52.0 mask 255.255.255.0

Additional Notes

• Configuring redistribution of OSPF on border router
• To advertise networks shared through OSPF - the following command must be included in BGP config
  • redistribute ospf *instance_number*

with bgp the network between routers can only live on one AS number