3.7.c Explain attributes and best-path selection

BGP is a path vector protocol. Among its noteworthy features:

Reliable updates

Triggered updates

Path attributes

Large networks; Internet centric

 

BGP can often be found in:

 

Customer networks connected to one or more ISP’s

Service Provider (transit networks)

Large Enterprise network cores

 

TCP Port 179

 

BGP is manually configured at both ends of a connection. TCP Port 179 is the session establishment facilitator. Its Finite State Machine indicators are Idle, Connect, Active, OpenSent, OpenConfirm and Established. It defaults to 60 second Keepalive’s for connection maintenance, and can use an MD5 shared secret between peers for security.

 

Best Path

 

In processing its routes, BGP has as its primary goal selecting the best path from among the choices made available from its peer establishments. All routes received from peers are retained in memory, however.

Best Path/Route Preference

 

Exclude route with an unreachable next hop

Weight (highest value, Cisco proprietary)

Local-preference (highest value within AS)

Locally originated (next hop 0.0.0.0, default weight value 32768 if applicable)

AS-Path (shortest)

Origin code (igp,egp,incomplete(?))

MED (mulri-exit discriminator, lowest value)

EBGP paths before IBGP

IBGP, closest IGP neighbor

EBGP oldest paths (age)

BGP router-id (lowest)

 

The best valid and reachable routes are propagated to BGP neighbors as well as placed into the IP route table after checking administrative distances

 

see also:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml