BGP

Short for Border Gateway Protocol. Path vector EGP routing protocol of the Internet. Runs over TCP port 179.

Each route advertisement carries the full AS-PATH, an ordered list of ASes the route has traversed.

Path Vector Algorithm

Each border router (speaker node) advertises to peers the full sequence of ASes on the path to a destination.

If a router $W$ accepts a path advertised by $X$, $W$ prepends itself and re-advertises to its own peers:

A router discards any route whose AS-PATH contains its own AS number to prevent loops.

Session Types

iBGP

Sessions between routers within the same AS.

eBGP

Sessions between routers in different ASes.

Route Selection

Attributes evaluated in order:

• LOCAL-PREF
  Preferred exit for outbound traffic within the AS. Higher value preferred.
• AS-PATH
  Shorter path preferred after LOCAL-PREF ties.
• MED
  Multi-Exit Discriminator. Hint to neighboring ASes for inbound traffic. Lower value preferred.
• NEXT-HOP
  IP address of the next-hop router for the advertised prefix.

Traffic Control

An AS controls incoming traffic by selectively advertising routes. Withholding a route advertisement to a peer AS blocks traffic from or through that AS.

Properties

• Does not compute shortest paths. Selects routes based on policy.
• Convergence is deliberately slow. Stability preferred over speed.
• A full Internet routing table carries over 900,000 IPv4 prefixes.