Border Gateway Protocol

BGP is the only routing protocol in widespread use which facilitates inter-domain routing (between autonomous systems).

BGP is path-vector; routes are tracked in terms of which autonomous systems they pass through.

Message Types

• Open
• Keepalive
• Update
• Notification

Connections between BGP peers are maintained via Open and Keepalive messages on TCP/179.

BGP Tables

• Neighbor table
• BGP table
• IP Routing table

BGP doesn't use a route metric; it relies on a complex path selection process.

EBGP is used between autonomous systems; IBGP is used within an AS.

IGP Synchronization

The synchronization requirement mandates that BGP must learn a route from an IGP for it to be propagated, to ensure consistency. BGP synchronization is often disabled for autonomous systems which do not act as a transit AS.

BGP States

• Idle (active) - Searching for neighbors
• Connect (active) - TCP connection established
• Open Sent (active) - Open message sent
• Open Confirm (active) - Response received
• Established - BGP neighborship established

Neighbors still displayed as "active" after some time has passed have not correctly peered.

BGP Attributes

• Aggregator - ID and AS of the advertising router
• AS_Path - List of the autonomous systems the route has traversed
• Atomic aggregate - Summary includes multiple AS
• Cluster ID - Originating cluster
• Community - Route tag
• Local preference - External path metric for internal neighbors (prefer highest)
• Multiple Exit Discriminator (MED) - Informs external peers which path to take into the AS (prefer lowest)
• Next Hop - External peer in neighboring AS
• Origin - Lowest origin type preferred (IGP < EGP < unknown)
• Originator ID - Identifies route reflector
• Weight (Cisco proprietary) - Administrative Cisco attribute (prefer highest)

Path Selection Process

Routes must meet the synchronization requirement (if enforced) to be considered.

1. Weight (highest) - Provided for administrative override
2. Local Preference (highest) - Used internally to select path out of AS
3. Self-originated (TRUE) - Prefer paths originated by self
4. AS_Path (shortest) - Minimize AS hops
5. Origin (lowest type) - native < EGP < redistributed; stability
6. MED (lowest) - Used to determine entrance into an AS
7. External (eBGP < iBGP) - External path preferred over internal path
8. IGP cost (lowest) - Used to consider additional information about the route
9. eBGP peering (oldest) - Stability
10. Router ID (lowest) - Lowest router ID breaks a tie