'Re: [j-nsp] set MED only on empty'

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List:       juniper-nsp
Subject:    Re: [j-nsp] set MED only on empty
From:       Alexander Arseniev via juniper-nsp <juniper-nsp () puck ! nether ! net>
Date:       2020-09-20 18:26:29
Message-ID: mailman.59.1600626394.1403.juniper-nsp () puck ! nether ! net
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From: "Alexander Arseniev" <arseniev@btinternet.com>
To: "Baldur Norddahl" <baldur@gigabit.dk>, "Juniper List"
 <juniper-nsp@puck.nether.net>
Subject: Re[2]: [j-nsp] set MED only on empty
Date: Sun, 20 Sep 2020 18:26:29 +0000
Message-Id: <em4821ea78-3a97-4d19-b045-501d29b26387@laptop-t1tojcr7>
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Reply-To: "Alexander Arseniev" <arseniev@btinternet.com>
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Hello,
Last time I tested it, "from metric 0" works only if MED is present and 
equals 0.
Back then (around JUNOS 17) I developed a policy to detect missing MED 
which exploits metric2 rollover (basically, subtracting 1 from missing 
MED results in MED==4294967295)
Example below:
regress@R1# show policy-statement CHECK_MED_IMPORT
term CHECK_IF_MED_EXISTS_AND_0 {
     from {
         metric 0; {## accept if MED present && 0
     }
     then accept;
}
term MED_DECREMENT {
     then {
         metric {
             expression {
                 metric multiplier 1 offset -1; ## turns absent MED into 
2^32-1
             }
         }
         next term;
}}
term CHECK_IF_MED_ROLLED_OVER {
     from metric 4294967295;
     then accept; ## sets missing MED to 4294967295 but You can set it to 
smthg else
}
term RESTORE_ORIGINAL_MED {
     then {
         metric add 1;
     }
}

Thanks
Alex

------ Original Message ------
From: "Baldur Norddahl" <baldur@gigabit.dk>
To: "Juniper List" <juniper-nsp@puck.nether.net>
Sent: 19/09/2020 22:49:07
Subject: Re: [j-nsp] set MED only on empty

> Hello
> 
> Metric zero should work, I do not know why I did not think of that.
> 
> Thanks,
> 
> Baldur
> 
> Den lÃ¸r. 19. sep. 2020 kl. 21.13 skrev Eduardo Schoedler <listas@esds.com.br
> > > 
> 
> > Hi Baldur,
> > 
> > Try match metric, with some tests you can define empty (maybe zero?).
> > 
> > 
> > https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/policy-configuring-match-conditions-in-routing-policy-terms.html
> >  
> > 
> > 
> > Regards.
> > 
> > 
> > Em sÃ¡b., 19 de set. de 2020 Ã s 12:59, Baldur Norddahl <baldur@gigabit.dk>
> > escreveu:
> > 
> > > Hello
> > > 
> > > 
> > > 
> > > I want to set MED (metric) only when the peer did not specify a MED value.
> > > 
> > > The idea being that peers that set MED have a reason for their choice, but
> > > 
> > > if no MED value is set, I am going to set it to MED=(latency_to_peer_in_ms
> > > 
> > > + 1000). That way we will prefer the route with lowest latency.
> > > 
> > > 
> > > 
> > > But I can not figure out how to match on no MED value in a policy. There
> > > 
> > > must be a way to do this?
> > > 
> > > 
> > > 
> > > Thanks,
> > > 
> > > 
> > > 
> > > Baldur
> > > 
> > > _______________________________________________
> > > 
> > > juniper-nsp mailing list juniper-nsp@puck.nether.net
> > > 
> > > https://puck.nether.net/mailman/listinfo/juniper-nsp
> > > 
> > > --
> > Eduardo Schoedler
> > 
> _______________________________________________
> juniper-nsp mailing list juniper-nsp@puck.nether.net
> https://puck.nether.net/mailman/listinfo/juniper-nsp

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