BGP route selection

[Ondrej Zajicek]

On Wed, Apr 18, 2018 at 03:35:43AM +0000, Arvin Gan wrote:
> Hi all,
>    Sorry, it's my ambiguous description. What I said default value is Cisco's admin distance value, eBGP is 20 and iBGP is 200. We plan to use preference to implement  admin distance. Actually ,in bird,  the preference of channel for BGP is  BGP protocol default preference DEF_PREF_BGP ( = 100). 
>      1)  Since the configuration of preference is in channel ,if the protocol have more than one channel, does mean different channel can configure different preference  in the same protocol ?

Yes

>      2) I also notice that " new->pref = c->preference; " in rte_update2, does mean  the preference of route in route table get from  preference of channel ?    

Yes, the channel preference is just a default value for route preference (could be changed even per-route by filters).

>      3)  When route selection, does mean algorithm selection compare preference of every route firstly , whether compare  the preference of  routes coming from same protocol ?  

Yes

> The global best route selection algorithm is (roughly) as follows:  ---> Does routes selection be processed as below sequence ?
> 
>    *   Preferences of the routes are compared.  ----> what's preference of the route, is preference in channel-config ?
>    *   Source protocol instance preferences are compared. --> what's protocol instance preference, is preference in channel-config for BGP protocol instance?

This is a bit confusing in documentation, there is just route preference
(which is compared as a first step). Protocol/channel preference is just
a default value for route preference.

>    *   If source protocols are the same (e.g. BGP vs. BGP), the protocol's route selection algorithm is invoked. --->source protocols, iBGP and eBGP are treated as same source protocol ?

Yes

>    *   If source protocols are different (e.g. BGP vs. OSPF), result of the algorithm is undefined.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."