BGP

BGP

1. Path Attributes

   • Well known Mandatory
     • AS_PATH
     • Origin
     • Next Hop
   • Well known discretionary
     • Local Preference
     • Atomic Aggregate
   • Optional Transitive
     • Community
     • Aggregator
     • Extended Communites
   • Optional Non Transitive
     • Cluster List
     • MED
     • Originator
     • AIGP

2. Type

   • iBGP
   • eBGP

3. Messages

   • OPEN
     • - ASN number - Session number - Hold down timer (lower is used) - RID - Optional Parameters
   • Update
     • - Advertise / withdraw / update a route - can be used as keep-alive
   • Notification
     • - Used to report errorrs
   • Keepalive
     • - sent at 1/3 of the hold down time

4. States

   • Idle
   • Connect
     • - 3way hand shake
   • OpenSent
     • - sends OPEN message - RID / ASN / session / version / optional parameters - Lower hold down to be used
   • OpenConfirm
     • - waits for a keep alive or a notification
   • Established
     • - Connection established, prefixes are exchanged
   • Active
     • - Retries connection - it will go to this state if a paramater doesn't match

5. Tables

   • Adj-RIB-In
     • - Routes recieved from peers
   • Adj-RIB-Out
     • - Routes that will be advertised to peers
   • Loc-RIB
     • - All routes received, best ones will go to the routing table

6. Facts

   • NLRI
     • - Network layer reachability information - Length + Prefix + attributes of a path
   • TCP/179
   • Loop prevention
     • AS number (eBgp)
     • Full Mesh (iBGP)
       • Alternatives
         • Route Reflector
           • - Reflects the routes from other peers - Avoids having to do a full mesh -Only advertises the best route
           • Cluster
             • - RR with the same RID are part of the same cluster and are redundant to each other - Routes exchanged between RR with the same cluster list are discared
           • Rules
             • RR client -> RR
               • - Advertise routes to all RRClients and non RRClients - Even the originator will receive a copy of that NLRI, but will drop it
             • Non RRClient -> RR
               • - Advertise route to RRClients but not to the Non RRClient
             • eBGP -> RR
               • - Advertise route to RRClients and non RRclients
           • Loop Prevention
             • Originator_ID
               • - RID of the RR - discard if own originator ID is received
             • Cluster_list
               • - Appended to originator ID - discard if own cluster list is received
         • Confederations
           • - "Splits" the AS into smaller "sub-ASes" - those sub AS behave like eBGP neighbors
   • Only 1 route is installed in the Routing table (by default)
   • Private ASN (6451-65355)
   • Connected NW
     • 0.0.0.0 (next-hop)
     • 32,768 (Weight)
     • origin (i)
   • Metric
     • 20 (eBGP)
     • 200 (iBGP)
   • TTL
     • 1 (eBGP)
     • 255 (iBGP)

7. Path Selection

   • N WLLA OMNI
     • Next Hop
       • Reachable
     • Weight
       • Higher
     • Local Pref
       • Higher
     • Locally injected
       • Preferred over BGP
     • AS PATH
       • Shorter
     • Orign
       • i > e > ?
     • MED
       • Lower
     • Neighbor Type
       • eBGP > iBGP
     • IGP Metric
       • Lower
   • Influence Traffic
     • Outbound
       • WLLA
         • Weight
         • Local Preference
           • - Influence exit point - Received from eBGP and shared into iBGP - Can't cross AS
         • AS_PATH
           • - AS can be prepended to influence the length of the path and make it a less desirable route
     • Inbound
       • OMNI
         • Origin
           • - A prefix can be changed from i to e or incomplete to make it less desirable
         • MED
           • - Influences the entry point, usually advertised from a neighbor AS
         • IGP metric (aigp)
           • - used to allow bgp to calculate best path using IGP metrics - BGP AIGP - Packet Pushers