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clambert
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ROSv6 BGP Best Path Selection Algorithm problem

Wed May 04, 2022 6:04 pm

Hi, I'm having trouble selecting the active default route learned by iBGP. If I understand the following document correctly (https://wiki.mikrotik.com/wiki/Manual:B ... _Algorithm), the chosen route will be determined in the first instance by the weight assigned to that prefix. However, I'm not being able to select the preferred route using this parameter:

/ip route print detail where routing-mark=INET
Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, B - blackhole, U - unreachable, P - prohibit 
 0 ADb  dst-address=0.0.0.0/0 gateway=192.168.1.1 gateway-status=192.168.1.1 recursive via 192.168.100.100 ether1 distance=200 scope=20 target-scope=10 
        routing-mark=INET bgp-weight=0 bgp-local-pref=100 bgp-med=0 bgp-origin=incomplete bgp-ext-communities="RT:65000:10,RT:65000:110" 

 1  Db  dst-address=0.0.0.0/0 gateway=192.168.1.1 gateway-status=192.168.1.1 recursive via 192.168.100.100 ether1 distance=200 scope=20 target-scope=10 
        routing-mark=INET bgp-as-path="65010,65011" bgp-weight=1 bgp-local-pref=100 bgp-med=0 bgp-origin=igp bgp-ext-communities="RT:65000:20,RT:65000:120"

It can be seen that the active route has a lower weight than the other route present. Is there an error in my interpretation of the route selection process?
 
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netzwerghh
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Sat May 21, 2022 12:28 am

Have you tried to reverse the weight between the two routes? BGP in RouterOS 6 is sometimes just broken. For example: We found that the MED value behavior is reversed. Not the route with the lowest MED is selected but that with the highest MED. Although per RFC and also per Mikrotik documentation the route with the lowest MED should be selectet when everything else in the rules further up the decision tree is the same. Very annoying. So check if behavior is also reversed for weight. May be the devs just mixed up those two parameters.
 
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clambert
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Mon May 23, 2022 7:00 pm

@netzwerghh, thanks for the reply.
I tried your suggestion, but the result is the same. It seems like weight is not taken into account in route selection.
 
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mrz
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Mon May 23, 2022 8:30 pm

both routes are received from peers belonging to the same instance?
 
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clambert
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Mon May 23, 2022 9:11 pm

Hi @mrz, both routes are learned from a route reflector, and are filtered using route-targets.
 
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loloski
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Tue Jun 21, 2022 5:29 pm

hi,

any update? we are also interested on this
 
buset1974
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Re: ROSv6 BGP Best Path Selection Algorithm problem

Wed Jun 22, 2022 3:53 am

it's a bad habit that mikrotik support ask some question then leave the customer without answer

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