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