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RFC 4456 / START HERE

Route reflection, before the details

BGP is the Border Gateway Protocol. A route reflector shares selected routing information among BGP routers inside one autonomous system, or AS.

Ordinary internal BGP, or IBGP, needs a session between every pair of routers to distribute routes. A reflector can share selected routes between configured peers, reducing the number of control connections.

Start the guide See the 6 chapters ↓

The idea in one picture

  1. Learn a route

  2. Reflect the selected path

  3. Check the loop history

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

RFC 4456 is an April 2006 Draft Standard explaining route reflection, its client relationships, loop-prevention attributes and route-selection effects. It replaces two earlier reflection RFCs; a later error-handling update changes how malformed attributes are treated.

A familiar example

Imagine colleagues sharing route notes through a coordinator instead of every colleague maintaining a separate conversation with everyone else. The coordinator shares its selected note, so some alternatives can stay hidden. Sharing the note does not mean travelers must pass through the coordinator.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

BGPRFC 4271 · beginner overview ↗CIDRRFC 4632 · beginner overview ↗TCPRFC 9293 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Replace a full mesh of internal sessionsA route reflector changes how internal routers distribute routing information, without becoming every packet’s next hop.
  2. Reflect according to the source peer’s roleClients and non-clients are configured relationships to a reflector, not different router hardware.
  3. Carry the originator and reflection historyTwo optional non-transitive attributes carry a router identifier and a sequence of cluster identifiers.
  4. Stop reflected loops and handle malformed inputOriginator checks, cluster checks and malformed-attribute handling solve different problems.
  5. Place reflection tie-breakers in orderAn originator and a cluster history affect late tie-breakers, after earlier BGP decisions.
  6. Deploy the roles without changing their meaningRedundancy, attributes and network topology still need deliberate configuration.
Start with chapter one →

This introduction explains RFC 4456 (2006). Read the original RFC ↗