BGP4 stuff: Local Preference Computation

[Cristina Radulescu-Banu <[e-mail]>]

	I have some problems in implementing BGP4, and if you could
clarify some of them , this would be great.

	I understood that LOCAL_PREF is used as degree of preference only for
the BGP4 speakers located in the SAME Autonomous System. The Degree of
Preference is something totally different, it is used both for the BGP
Peers in the same and in the neighbor AS (it can use in its
computation the LOCAL_PREF, that's true), and one of the devices I
tested computes the Degree of Prefence with formula:

	TotalASPolicyWeight+ PeerWeight + DefaultWeight

where DefaultWeight = the weight added to each route when computing 
the degree of preference for the route (LOCAL_PREF)-if you have
any-because it's an discretionary attribute-, and
if you consider it because it was received from a peer in the
same AS)
	You can assign weight to each AS in the AS_PATH => this way you have the
TotalASPolicyWeight; 

PeerWeight = the weight that is added to all routes received from the specified peer
 
	For me this is pretty clear, to take in account 
the AS and the Peer when you compute the Deegre of Preference.

	After the router has done the computation for this Degree of
Preference, the Local Pref  assigned to the route take the value of
the Degree of preference already computed. If an internal peer
received that route, it may or may not
take as Local Pref the LOCAL_PREF already received (it can compute
another Degree of Pref based on this Local Pref); If an external peer
received it, it ignores it and does the Degree of Pref computation for
the route again, without taking in account any Local Pref.

	Yes, sometimes the Local Pref can be computed (if the internal
peer choose this)but the one which is computed is always the DEGREE OF
PREFERENCE, which is different form Local Pref.

This is totally different from what I saw in the messages
exchanged, about the Local Pref Computation, which I think should be
the Degree of Preference Computation; what is the right thing to be?

	Thanks in advance,

	Cristina 
-- 


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