- Single homed customers use static routes
- Multi-homed customers usually use static routes or policy routing
- Multi-homed customers who host networks might run BGP to advertise.
BGP currently in version 4, successor of BGP v2/3 and replaced EGP
Designed to route IP through Autonomous Systems.
By default uses the shorted AS-Path, routing polices are configured using BGP Attributes.
BGP does not enable one AS to send traffic to a neighbor AS intending that the traffic take a different route from that taken by traffic orginating in the neighbor AS - RFC 1771
Routes learned via BGP must be validated by the interior routing table before they can be advertised to remote peers.
Routes learned via iBGP will never be sent to another iBGP peer - BGP Split-Horizon rule. Loop protection assumes a full mesh.
BGP Path Attributes
- Well-Known, Mandatory
- Origin (i, e, ?)
- AS-Path
- Next-hop
- Well-Known, Discretionary
- Local Preference
- Atomic Aggregate (Who summarised routes)
- Optional, Transitive (will pass)
- Aggregator
- Community
- Optional, non-transitive
- MED (Metric)
- Originator ID
- Cluster
BGP Sessions and Neighbors
Two types of neighbours, eBGP iBGP
Neighbours do not need to be directly connected
Neighbours need to be manually formed
Periodic DBP keepalives, no periodic updates
Uses TCP port 179
Open, Keepalive, Update, Notification
Batch updates sent once every 5 seconds of iBGP and once every 30 seconds for eBGP.
Hello messages sent every 60 seconds with a holddown of 180 seconds
Capable of MD5 Authentication
Six states to form peer
- Idle (Configured, not able to reach)
- Connect
- Active
- OpenSent
- OpenConfirm
- Established
Mutliprotocol BGP
- Address-family command active extensions
- BGP can run over IPv4 or IPv6
- Can Carry: IPv4, IPv6, CLNS, VPNv4, L2VPN
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