BGP

BGP uses a TCP connection to send routing updates using TCP port 179. BGP is therefore by definition a 'reliable' protocol. While BGP version 3 provides for the dynamic learning of routes, BGP 4 adds additional route dampening functionality, communities, MD5 and multicasting capability.

BGP is a PATH VECTOR protocol, which means that it does not keep track of internal routing within the AS, but rather keeps track of paths through other autonomous systems to reach destination networks. –

BGP uses the UPDATE message to advertise routes.

eBGP peers will advertise all known eBGP routes to all other eBGP peers. iBGP peers will only advertise their own internal routes to other iBGP peers. A BGP speaking router will never advertise another iBGP peer's routes to any other iBGP peer.

BGP selects a single best path to a destination, and inserts it in the IP routing table. IP datagrams are only forwarded based on routes in the IP table, NOT by the routes in the BGP table.

Packets:

OPEN, UPDATE, KEEPALIVE, NOTIFICATION

Route path selection (Sequence of selection)

“We Love Oranges AS Oranges Mean Pure Refreshment”
W	Weight (Highest)
L	LOCAL_PREF (Highest)
O	Originate (local) routes that are advertise through the "network" command or redistributed from an IGP.
AS	AS_PATH (shortest)
O	ORIGIN Code (IGP > EGP > Incomplete)
M	MED (lowest)
P	Paths (External > Internal)
R	RID (lowest)

1)      high Weight(Cisco proprietary)….if it is fail then go to next

2)      High Local preference(default : 100)

3)      Network(i) by bgp it self  and then followed to aggregate(re-distribute)? i.e. by other protocols  or directly connected.

4)       AS_PATH: Shortest hope count i.s. AS…Low Path

5)      Preferred Lowest origin type.  Igp<egp<incomplete(?).

6)      Low MED(multi-exit-discriminator)(i.e. Metric)…default : 0

7) Paths (external > internal)