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<p><font face="Gentium">Well yes, the "prefer older" works only for
eBGP, as stated in RFC 5004. With iBGP, this isn't invoked as
you may run into some nasty routing loops. If you need this
feature, feel free to send in a patch.<br>
</font></p>
<p><font face="Gentium">Maria</font><br>
</p>
<div class="moz-cite-prefix">On 7/3/23 13:46, Mazur, Dariusz wrote:<br>
</div>
<blockquote type="cite"
cite="mid:E9D6F8C1-2043-415E-90D5-28B94C9B578F@akamai.com">
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<p class="MsoNormal">Hello Maria,<o:p></o:p></p>
<p class="MsoNormal">Thanks for response. Attributes look the
same. Maybe the problem is these routes are learnt via iBGP,
what is not visible in "show route all'<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Our simplified topology looks like below:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">1.Host_1 and Host_2 announces
172.232.160.0/19<o:p></o:p></p>
<p class="MsoNormal">2.Host_1 and Host_2 have different ASNs and
they use eBGP to peer with ToRs<o:p></o:p></p>
<p class="MsoNormal">3.All tors are connected to r01.leaf over
iBGP<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Host1----eBGP----r01a.tor----iBGP-----r01.leaf<o:p></o:p></p>
<p class="MsoNormal"> ----eBGP----r01b.tor----iBGP<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Host2----eBGP----r02a.tor----iBGP-----r01.leaf<o:p></o:p></p>
<p class="MsoNormal"> ----eBGP----r02b.tor----iBGP<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Show route all from r01.leaf<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.1__r01a.tor106 2023-06-30] * (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.1 on
vlan.201<o:p></o:p></p>
<p class="MsoNormal"> Type: BGP univ<o:p></o:p></p>
<p class="MsoNormal"> BGP.origin: Incomplete<o:p></o:p></p>
<p class="MsoNormal"> BGP.as_path: 4250627481<o:p></o:p></p>
<p class="MsoNormal"> BGP.next_hop: 192.168.196.1<o:p></o:p></p>
<p class="MsoNormal"> BGP.med: 0<o:p></o:p></p>
<p class="MsoNormal"> BGP.local_pref: 400<o:p></o:p></p>
<p class="MsoNormal"> BGP.atomic_aggr:<o:p></o:p></p>
<p class="MsoNormal"> BGP.aggregator:
23.219.179.225 AS4250627481<o:p></o:p></p>
<p class="MsoNormal"> BGP.community: (63949,1000)
(63949,1002) (63949,1004) (63949,1005) (65110,31107)
(65310,31107) (65518,31107)<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.3__r01b.tor106 2023-06-30] (100) [AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> <span lang="NL">via
192.168.196.3 on vlan.202<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="NL"> Type: BGP
univ<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="NL"> </span>BGP.origin:
Incomplete<o:p></o:p></p>
<p class="MsoNormal"> BGP.as_path: 4250627481<o:p></o:p></p>
<p class="MsoNormal"> BGP.next_hop: 192.168.196.3<o:p></o:p></p>
<p class="MsoNormal"> BGP.med: 0<o:p></o:p></p>
<p class="MsoNormal"> BGP.local_pref: 400<o:p></o:p></p>
<p class="MsoNormal"> BGP.atomic_aggr:<o:p></o:p></p>
<p class="MsoNormal"> BGP.aggregator:
23.219.179.225 AS4250627481<o:p></o:p></p>
<p class="MsoNormal"> BGP.community: (63949,1000)
(63949,1002) (63949,1004) (63949,1005) (65110,31107)
(65310,31107) (65518,31107)<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.5__r02a.tor106 2023-06-30] (100) [AS4250627482?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.5 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> Type: BGP univ<o:p></o:p></p>
<p class="MsoNormal"> BGP.origin: Incomplete<o:p></o:p></p>
<p class="MsoNormal"> BGP.as_path: 4250627482<o:p></o:p></p>
<p class="MsoNormal"> BGP.next_hop: 192.168.196.5<o:p></o:p></p>
<p class="MsoNormal"> BGP.med: 0<o:p></o:p></p>
<p class="MsoNormal"> BGP.local_pref: 400<o:p></o:p></p>
<p class="MsoNormal"> BGP.atomic_aggr:<o:p></o:p></p>
<p class="MsoNormal"> BGP.aggregator:
23.219.179.226 AS4250627482<o:p></o:p></p>
<p class="MsoNormal"> BGP.community: (63949,1000)
(63949,1002) (63949,1004) (63949,1005) (65110,31107)
(65310,31107) (65518,31107)<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.7__r02b.tor106 2023-06-30] (100) [AS4250627482?<o:p></o:p></p>
<p class="MsoNormal"> <span lang="NL">via
192.168.196.7 on vlan.204<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="NL"> Type: BGP
univ<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="NL"> </span>BGP.origin:
Incomplete<o:p></o:p></p>
<p class="MsoNormal"> BGP.as_path: 4250627482<o:p></o:p></p>
<p class="MsoNormal"> BGP.next_hop: 192.168.196.7<o:p></o:p></p>
<p class="MsoNormal"> BGP.med: 0<o:p></o:p></p>
<p class="MsoNormal"> BGP.local_pref: 400<o:p></o:p></p>
<p class="MsoNormal"> BGP.atomic_aggr:<o:p></o:p></p>
<p class="MsoNormal"> BGP.aggregator:
23.219.179.226 AS4250627482<o:p></o:p></p>
<p class="MsoNormal"> BGP.community: (63949,1000)
(63949,1002) (63949,1004) (63949,1005) (65110,31107)
(65310,31107) (65518,31107)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Dariusz<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div style="border:none;border-top:solid #B5C4DF
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span
style="font-size:12.0pt;color:black">From: </span></b><span
style="font-size:12.0pt;color:black">Maria Matejka via
Bird-users <a class="moz-txt-link-rfc2396E" href="mailto:bird-users@network.cz"><bird-users@network.cz></a><br>
<b>Reply-To: </b>Maria Matejka
<a class="moz-txt-link-rfc2396E" href="mailto:maria.matejka@nic.cz"><maria.matejka@nic.cz></a><br>
<b>Date: </b>Friday, June 30, 2023 at 12:53 PM<br>
<b>To: </b><a class="moz-txt-link-rfc2396E" href="mailto:bird-users@network.cz">"bird-users@network.cz"</a>
<a class="moz-txt-link-rfc2396E" href="mailto:bird-users@network.cz"><bird-users@network.cz></a><br>
<b>Subject: </b>Re: Bird does not prefer older eBGP route
- RFC5004 and "older prefer on"</span><span
style="font-size:12.0pt;color:black;mso-ligatures:none"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<p><span style="font-family:"Gentium",serif">Hello!</span><o:p></o:p></p>
<p><span style="font-family:"Gentium",serif">I suspect
that the routes either aren't all external, or are otherwise
compared different before it comes to breaking ties. Could
you please share the `show route all` output to see all the
relevant BGP attributes?</span><o:p></o:p></p>
<p><span style="font-family:"Gentium",serif">Maria</span><o:p></o:p></p>
<div>
<p class="MsoNormal">On 6/30/23 11:43, Mazur, Dariusz via
Bird-users wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal">Hello Bird users,<o:p></o:p></p>
<p class="MsoNormal">Have you ever use RFC 5004 and “older
prefer” knob. I am trying to use it but it seems not to
work:<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>1.Router learns the same route from
different ebgp peers, it prefers route from r01a and this
route is exported to BGP peers</b><o:p></o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.129__r01a.tor106 2023-06-14] * (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.129 on
vlan.201<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.131__r01b.tor106 2023-06-14] (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.131 on
vlan.202<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.133__r02a.tor106 2023-06-14] (100)
[AS4250627482?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.133 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>2.Once we lose links to r01a and r01b
route from r02a is preferred and exported to BGP peers.
It is expected</b><o:p></o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.133__r02a.tor106 2023-06-14] * (100)
[AS4250627482?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.133 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>3.When links to r01a and r01b. are
again online route, route from r01a is pricked as primary
and exported to BGP. It causes route oscillation</b><o:p></o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.129__r01a.tor106 09:14:19.982] * (100)
[AS4250627481?]
<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.129 on
vlan.201<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.131__r01b.tor106 09:14:19.896] (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.131 on
vlan.202<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.133__r02a.tor106 2023-06-14] (100)
[AS4250627482?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.133 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>4.I believe it is default behavior not
to prefer older path. According to documentation RFC 5004
and "prefer older on" should fix my problem, but it does
not work.</b><o:p></o:p></p>
<p class="MsoNormal">Bird doc says:<o:p></o:p></p>
<p class="MsoNormal"><i>prefer older switch</i><o:p></o:p></p>
<p class="MsoNormal"><i>Standard route selection algorithm
breaks ties by comparing router IDs. This changes the
behavior to prefer older routes (when both are external
and from different peer). For details, see RFC 5004.
Default: off.</i><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>5. According to documentation RFC 5004
and "prefer older on" should fix my problem, but it does
not work.</b><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>a) added "prefer older on", bgp
flapped and routes were re-learnt</b><o:p></o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.129__r01a.tor106 09:22:12.949] * (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.129 on
vlan.201<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.131__r01b.tor106 09:22:13.527] (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.131 on
vlan.202<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.133__r02a.tor106 09:22:12.683] (100)
[AS4250627482?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.133 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>b) shut links to r01a and r01b</b><o:p></o:p></p>
<p class="MsoNormal"><b>172.232.160.0/19 unicast
[192.168.196.133__r02a.tor106 09:22:12.683] * (100)
[AS4250627482?]</b><o:p></o:p></p>
<p class="MsoNormal"><b> via 192.168.196.133 on
vlan.203</b><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><b>c) unshut links to r01a and r01b,
route from r01a is again preferred, so looks like "older"
knob does not work</b><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">172.232.160.0/19 unicast
[192.168.196.129__r01a.tor106 09:27:55.841] * (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.129 on
vlan.201<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.131__r01b.tor106 09:27:54.448] (100)
[AS4250627481?]<o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.131 on
vlan.202<o:p></o:p></p>
<p class="MsoNormal"> unicast
[192.168.196.133__r02a.tor106 09:22:12.683] (100)
[AS4250627482?].
<b>---> this route is older</b><o:p></o:p></p>
<p class="MsoNormal"> via 192.168.196.133 on
vlan.203<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Dariusz<o:p></o:p></p>
</blockquote>
<pre>-- <o:p></o:p></pre>
<pre>Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.<o:p></o:p></pre>
</div>
</blockquote>
<pre class="moz-signature" cols="72">--
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.</pre>
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