Module 9 Notes

WAN Routing - BGP

BGP Overview

• Border Gateway Protocol (BGP): Is the EGP used on the internet
• Exchanges routing and reachability information among autonomous systems (AS) on the internet.
• path vector protocol
• Makes routing descriptions based on paths, network policies or rule-sets configured by a network administrator.
• Some large orgs may use BGP as an IGP

Autonomous Systems

• A collection of connected Internet Protocol (IP) routing prefixes
• Under the control of a single administrative entity (company, university, ISP)
• AS Numbers
  • A unique ASN is allocated to each AS to use in BPG routing
  • ASNs are important because the ASN uniquely identifies each network on the internet
  • Registries hand out 32 bit AS numbers

BGP Operation

• BGP neighbors (peers) are established by manual configuration between routers
• TCP session on port 179
• BGP speaker sends 19-byte keep-alive messages every 60 sections to keep the connection
• BGP is the oney routing protocol that uses TCP as its transport protocol - the rest are their own
• BGP Peering
  • Organizations must "peer" with their ISP(s) and other networks that they are externally linked to.
  • Allows for multiple connections to the internet
  • AS will be advertised through multiple peers which allows multiple paths -> routers choose the best path.
• Path Vector Protocol
  • Distance Vector Protocols
    • Uses distance as route selection metric
    • Routing table includes how many hops/routers to a remote network
    • Only knows IP of next router not full path
  • Path vector protocols - BGP
    • Routing table includes the "path" of AS numbers to get to remote networks
• Best Path Algorithm
  • Choose and install best routes into router routing table
  • Full internet BGP routing table includes 400,000 routes
  • BGP router receives numerous copies of routes from various providers
  • compares multiple routing tables and chooses optimal route
    
    

Configuring BGP

• Basic Configuration
  • 3 Steps
    1. Create BGP instance
    2. Identify peer neighbors by IP address
    3. Identify any networks that the router is responsible to advertise
• On Cisco
  1. Define router instance
  • router bhp 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_numberI”