Date: 8-08-03

Lecture Number: 06

Author: Umardand Shripad Manikrao


         Border Gateway Protocol is EGP which routes information between AS.
         It is path vector protocolas BGP routing updates can be thought of as providing list of paths.
         It selects loop free path.
         It uses  incremental state update.
         It do not deal with congestion.
         It uses incremental state update.

Operation of BGP:

                                The Router with which a BGP router X exchange information is called BGP peer.
                                It sends BGP Open message to its peer.If peer wishes it exchanges routing information by collection of BGP Update message.
                                They exchange BGP keep alive to keep BGP session alive.


Two Types of state update Approaches:
                      The update information is convey only in case of topology change.
                       The state update infomation is conveyed periodically.
                        Large amount of information need to be sent even if not needed.
                       The period should be set correctly.

BGP Path Attributes:

1.MED(Multi Exit Descriminator):
                       When two neighbouring ASes are connected at multiple points , one AS informs other which is better entry point.
                        The entry with lower MED value is favored.

2.Local REF:
                        BGP router uses to tell its IBGP neighbour whether it should use exit point.

BGP instability:
                        It is defined as the rapid change of routes /reachability & topology information.
                        It causes
                            a. Affects End -End Performance .
                            b.Packet Loss.
                            c.Additional Resources Needed (i.e memory , CPU overhead)

IGP Instability:
                         Easy to maintain because of small size .
                         OSPF uses periodic updates & converges in miliseconds.

BGP Updates:
                        It contain multiple route announcements & withdrawls.
                                                It indicates router has  learned of new network attachment or  made policy decision to prefer another path .
                                                1.Implicit: Occurs when an existing route is replaced by announcement of new route to destination prefix
                                                without withdrawl message.
                                                2.Explicit:Assosited with withdrawl message.

Types of Instability:
1.Forward Instability:              Reflect topological changes.

2.Policy Fluctuation:               Changes in routing policy information  i.e LOCAL REF, MED.

3.Pathological Instability:        Redundant BGP information.

Core Of Internet:                     i.e Tier 1.                                             

Effects of Instability:
                                                1.Load on CPU if cache entry missing.

                                                2.Route Flap storm.
                                                    Keep alive Packet Delayed & its link treated as down.

Ways to Reduce Instability:
                                                1.Address aggregation:  Aggregation of prefixes.
                                                As shwn below IP prefixes 128.2.4/24 ,128.5.6/24 & 128.5.2/24 are aggregated to IP 128.5/16.
                                                So outside world look it as only one network.


                                                This is not possible because of following reasons.

                                                   End sites obtain redundant connectivity via diff ISP .This requires that each router maintain more specific or longer prefix in
                                                   addition to less specific aggregate address block prefixes covering multihomed sites.  

                                                b.No cooperation between ISP.
                                                   Customer want different ISP connection but do not want to change IP address for network.

                                                2.Route Dampening:
                                                   It refuses to believe updates about routes that exceed certain parameter of instability such as exceed in no of updates in hour.
                                                   This can introduce connectivity problem.