BGP

Packet Type = Layer 4 protocol, Runs on tcp, port 179
Neighbors
1. tcp connectivity required - routers may be multiple hops away
2. neighbors are statically configured
3. Full mesh required, unless using route reflector because routers will not re-advertise routes learned via iBGP to another internal neighbor
4. External neighbor - a neighbor in a different AS
5. Internal neighbor - neighbor in the same AS
6. keep-alives sent every 60 seconds by default
7. states
  1. idle - router is looking for a route to the neighbor
  2. connect - 3-way handshake completed
  3. open sent - self explanatory
  4. open confirm - an agreement on session parameters received
  5. established - self explanatory
  6. active - no response to the open message
Tables
1. BGP table
2. neighbor table
3. topology table
4. topology database
5. routing table
6. forwarding database
Messages
1. Open
  1. Version
  2. AS
  3. hold time - 2 neighbors use the shortest configured hold-time
  4. router id
  5. optional parameters in TLV fields
2. Keepalive
3. Update
  1. contains info on one path
  2. includes
    1. withdrawn routes
    2. path attributes
    3. IP prefixes that can be reached on the path
4. Notification - sent to report errors or special conditions
Attributes
1. Well-known mandatory - must appear in all BGP updates
  1. as-path - list of all AS's a route has traversed
  2. next hop - entry point for the next AS in the path
    1. iBGP - next hop is carried into iBGP from eBGP
    2. eBGP - IP of the neighbor who sent the advertised route is used as next hop
  3. origin
    1. IGP
    2. EGP (no longer used)
    3. Incomplete
  Well-known discretionary - must be supported by all BGP speakers, but doesn't have to be included in an update
  1. local preference
    1. describes preferred exit point of the AS
    2. higher is better
    3. defaults to 100
  2. atomic aggregate
2. Optional transitive
  1. does not have to be understood by every bgp speaker
  2. routers that do not implement an optional transitive attribute should pass it on to other routers and mark it as partial
  3. examples
    1. aggregator - the router id of the router that summarized a route
    2. community
3. Optional non-transitive
  1. not implemented by all speakers
  2. should not be passed on if not implemented
  3. example: MED - preferred path into an AS
Route selection
1. highest weight (cisco only, is not advertised to other routers)
2. highest local preference - factory default of 100
3. local routes (next hop of 0.0.0.0) - generated by this router
4. shortest AS-path
5. lowest origin code (IGP, EGP, then incomplete)
6. MED - lower MED prefered
7. prefer eBGP over iBGP
8. closest IGP neighbor, metric-wise
9. oldest route
10. lowest neighbor router id
11. lowest neighbor IP address
configuring
1. router bgp as#
2. configure at lease one neighbor - neighbor ip-add remote-as as router command
3. list networks - network network-add mask netmask
4. Peer groups
  1. create a set of policies applied to multiple peers
  2. neighbor group-name peer-group router config command
  3. neighbor ip-add peer-group group-name router command assigns a neighbor to a peer group
5. Authentication (optional)- neighbor ip-add password string router config command
6. manipulating traffic
  1. bgp default local-preference value router config command
  2. neighbor ip-add weight weight
  3. you can also use route-maps to apply these attributes to select routes
Verifying and troubleshooting
1. show ip bgp
2. sh ip bgp rib-failure - shows routes that were not installed in the routing information base and why
3. sh ip bgp neighbors
4. sh ip bgp summary
5. debug ip bgp
  1. dampening - info about dampened routes
  2. events
  3. keepalives
  4. updates
  5. ipv4 unicast - view the progression through neighbor states
References
1. Cisco Press BSCI Authorized Self-Study Guide
2. cisco.com

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Updated:
23 Aug 07

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