BGP (Border Gateway Protocol) Cheat Sheet

BGP Cheat Sheet

Basic Concepts

- BGP (Border Gateway Protocol): The protocol used to exchange routing information between different autonomous systems (AS).
- Autonomous System (AS): A collection of IP networks and routers under a single organization's control.
- BGP Peers: Routers that exchange BGP routing information.
- BGP Session: TCP connection between BGP peers (port 179).

BGP States

1. Idle
2. Connect
3. Active
4. OpenSent
5. OpenConfirm
6. Established

BGP Attributes

- AS_PATH: List of ASs a route has traversed.
- NEXT_HOP: Next hop IP address for the route.
- LOCAL_PREF: Indicates the preferred exit point from the AS.
- MED (Multi-Exit Discriminator): Suggests preferred entry point to an AS.
- COMMUNITY: A way to group routes for routing decisions.

BGP Configuration Commands (Cisco)

Basic Configuration:
router bgp 
  neighbor  remote-as 
  neighbor  description 

Example:
router bgp 65001
  neighbor 192.168.1.1 remote-as 65002
  neighbor 192.168.1.1 description "Peer Router"

Advertise a Network:
network  mask 

Example:
network 10.0.0.0 mask 255.255.255.0

Viewing BGP Information (Cisco)

- Show BGP Summary:
show ip bgp summary

- Show BGP Routes:
show ip bgp

- Show BGP Neighbor Details:
show ip bgp neighbors 

BGP Route Filtering

- Disallow Specific Routes:
neighbor  filter-list  in

- Using Route Maps:
route-map  permit 
  match 
  set 

BGP Timers

Adjust BGP Keepalive and Hold Timers:
neighbor  timers  

Example:
neighbor 192.168.1.1 timers 30 90

BGP Best Path Selection Criteria

1. Highest Weight (Cisco-specific)
2. Highest Local Preference
3. Shortest AS Path
4. Lowest Origin Type (IGP < EGP < Incomplete)
5. Lowest MED
6. EBGP over IBGP
7. Lowest IGP metric to NEXT_HOP
8. Oldest route (if all else is equal)
9. Lowest BGP router ID (if all else is equal)

Common BGP Commands for Troubleshooting

- Check BGP Neighbors:
show ip bgp neighbors

- BGP Route Information:
show ip bgp 

- Check BGP Updates:
show ip bgp updates