BGP Theory and Concepts; BGP Attributes

Administrative distance - EBGP = 20, IBGP = 200

BGP neighbor relationships
Neighbors are manually configured.
Neighbors start in IDLE state
 -1st stage: Active
 -2nd stage: Open sent
 -3rd stage: Open Confirmed
 -4th stage: Established

Hello message sent once every 60 seconds with a holddown of 180 seconds
Neighbors are capable of MD5 authentication

Rule of Synchronization
Routes learned via BGP must be validated by the interior routing table before they can be advertised to remote peers.

If Synchronization is turned on at R1, it receives eBGP routes from AS4300 and AS2300 but R1 does not install them in routing table.

After Synchronization is turned off at R1, it installs eBGP routes in its routing table. It does not advertise to its iBGP peer due to Rule of Split-Horizon.

Rule of Split-Horizon (Per BGP)
Routes learned via IBGP will never be sent to another IBGP peer. It's to prevent loop and Split-horizon is good if all IBGP are fully mesh.
To overrule this rule, configure route reflector. Of course iBGP peer (R4) must know how to reach 10.12.1.2 and 10.13.1.2 via IGP or static routes.

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BGP Attributes

Well-known mandatory Attributes

AS-Path

Next-Hop; next-hop on shared media (if receiving router is on the same subnet, next-hop remains the same)

Origin - Tells how the route originated(IGP(I),EGP(E),Unknow(redistribution)).


Well-known optional Attributes

Local Preference - gives you control over preferred routes,higher is better.

Weight - cisco proprietary, gives you control of routes on the SAME router. It does not affect routing policy of other routers in the AS

Atomic aggregate - Informs router that a route has been summarized.

Multi-exit discriminator(MED) - used to suggest an entry point into your AS. Lower is better.

Aggregator - Designates the IP address of the router who performed summarization.

Community - Used for route tagging.