Date: 8-08-03
Lecture Number: 06
Author: Umardand Shripad Manikrao
BGP:
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:
1.Incremental:
The update information is convey only in case of topology change.
2.Periodic:
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.
Announcement:
It indicates
router has learned of new network attachment or made policy
decision to prefer another path .
Withdrawls:
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.
a.Multihoming:
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.