Introduction to the Border Gateway Protocol (BGP) Part-2

Multi-Exit Discriminator (MED)

  • Non-transitive
  • Used to convey the relative preference of entry points
  • Influences best path selection
  • Comparable if paths are from same AS
  • IGP metric can be conveyed as MED
  • Multi-Exit Discriminator

  • AS 201
  • AS 200
  • A
  • C
  • B
  • 68.1.0/24
  • 68.1.0/24 1000
  • 68.1.0/24 2000

Applying Policy with BGP

  • Policy based on AS path, community or the network
  • Rejecting/accepting selected routes
  • Set attributes to influence path selection

Communities

  • BGP attribute
  • Used to group destinations
  • Represented as an integer
  • Each destination could be member of multiple communities
  • Community attribute carried across AS
  • Useful in applying policies
  • Community Attribute
  • RFC1997, RFC1998
  • Optional attribute
  • Range: 0 to 4,294,901,760
  • Method to group destinations into communities and apply routing decisions (accept, prefer, redistribute, etc.) using route-maps

Community attribute (cont.)

  • Format — aa:nn
  • aa’ -- ASN
  • nn’ — 1 to 4,294,901,760

BGP Communities

  • Community 10:200
  • Community 10:300
  • Community 10:200
  • Community 10:300

Load Balancing

  • BGP does not load-balance traffic; it chooses & installs a “best” route.

BGP Load Balancing Problem

  • “Since BGP picks a ‘best’ route based upon most specific prefix and shortest AS_PATH,
    it becomes non-trivial to figure out how to manually direct specific portions of internal traffic (prefixes) in a distributed fashion across multiple external gateways.”

eBGP Multihop

BGP Multipath

Difficulties in Load Balancing

Internal BGP (iBGP)

  • Same routing protocol as BGP, different application
  • iBGP should be used when AS_PATH information must remain intact between multiple eBGP peers
  • All iBGP peers must be fully meshed, logically; an iBGP peer will not advertise a route learned by one iBGP peer to another iBGP peer (readvertisement restriction to prevent looping)

PPT Slide

  • AS 1
  • AS 2
  • eBGP
  • eBGP
  • eBGP
  • iBGP
  • iBGP
  • Upstream 
  • Provider B
  • AS200
  • Upstream 
  • Provider A
  • AS100