WAN Routing BGP - caitlinmallen/TechWiki GitHub Wiki

WAN Routing - BGP

BGP Overview

  • Is the EGP used on the Internet
  • Exchanges routing and reachability information among autonomous systems on the Internet
  • Classified as a path vector protocol
  • Makes routing decisions based on paths, network policies, or rule sets configured by a network admin
  • Some large organizations may also use this as an IGP

Autonomous Systems

  • The AS is
    • A collection of connected IP routing prefixes
    • Under the control of a single administrative entity that presents a common, clearly defined routing policy
  • AS Numbers
    • A unique ASN is allocated to each AS for use in BGP routing
    • Important because the ASN uniquely identifies each network on the Internet
    • IANA handles ASN assignments
    • Until 2007, ASN were defined as 16-bit integers which allowed for a max of 65536 assignments
    • Registries can hand out 32 bit ASN (Over 4 billion)
  • ASN Growth
    • The unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999
    • 30k in late 2008
    • 35k in mid-2010
    • 42k in late 2012
    • 54k in mid-2016

BGP Operation

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

BGP Peering

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

Path Vector Protocol

  • Distance Vector
    • Like RIP - use "distance" as a route selection metric
    • Routing table includes how many hops to a remote network
    • Only knows IP of next router
  • Path Vector
    • Routing tables includes the "path" of ASNs to get to remote networks

BGP Best Path Algorithm

  • Best Path Algorithm 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 routers can receive numerous copies of those routes from various providers
    • So it has to compare the routing tables and choose the most optimal route on the router

BGP Attributes for Selecting Path

  1. Weight
    1. Cisco proprietary, to tell a router which of the multiple local paths to select for traffic leaving the AS. Highest weight preferred. Only has local significance
  2. Local Preference
    1. A value telling IBGP peers which path to select for traffic leaving the AS, default value is 100
  3. Shortest AS Path
    1. Ordered list of all the ASes through which this update has passed
  4. Lowest Origin
    1. How BGP learned of this network
  5. Lowest Multi-Exit Discriminator (MED)
    1. Suggests to a neighboring AS which of the multiple paths to select for traffic that is inbound to your AS. Lowest MED preferred.

BGP Update Message

  • Sent to peers
  • Includes the AS Path and IPv4 networks

Configuring BGP

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