<div dir="ltr"><div><br></div>Hello list,<div><br></div><div>I am trying to understand where I might be going wrong in my implementation or understanding of the following setup.</div><div><br></div><div>I have two servers in different locations, both of which are using exabgp to advertise the same /32 loopback to their nearest bird router over eBGP. The network is logically just a ring of 6 bird routers running OSPF and an iBGP full mesh. These /32 routes are then distributed via the iBGP mesh to the rest of the network.</div><div>Both routes are identical in their attributes apart from the next-hop and router-id. They have the same MED and localpref. The next-hop routes are of course known from OSPF.</div><div><br></div><div>The documentation states that "BGP depends on existing IGP routing table with AS-internal routes to determine immediate next hops for routes and to know their internal distances to boundary routers for the purpose of BGP route selection". However:</div><div>When inspecting the available routes from one of my bird routers elsewhere on the network, I see one for which the next-hop OSPF cost is 20, and one where the cost is 30. Both routes still show the same MED, the same localpref, same as-path etc, but the route with the higher OSPF cost is always being installed.</div><div><br></div><div>The only reason I can see for this route to be preferred is that the originating router has the lower router-id, but this check comes after the supposed IGP distance check in the route selection rules. So am I completely misunderstanding how this process works, or have I just done something wrong? Any advice would be greatly appreciated.</div><div><br></div><div><br></div><div><br></div><div>Available routes for the /32: -------------------------</div><div><br></div><div><div>root@bird1:~# birdc show route all for 10.13.83.6</div><div>BIRD 1.4.5 ready.</div><div><a href="http://10.13.83.6/32">10.13.83.6/32</a> via 10.13.83.246 on eth1.10 [BIRD3 2015-09-08 from 10.13.83.2] * (100/?) [AS65100i]</div><div><span class="" style="white-space:pre"> </span>Type: BGP unicast univ</div><div><span class="" style="white-space:pre"> </span>BGP.origin: IGP</div><div><span class="" style="white-space:pre"> </span>BGP.as_path: 65100</div><div><span class="" style="white-space:pre"> </span>BGP.next_hop: 10.13.82.65</div><div><span class="" style="white-space:pre"> </span>BGP.med: 100</div><div><span class="" style="white-space:pre"> </span>BGP.local_pref: 100</div><div> via 10.13.83.233 on eth0.14 [BIRD6 2015-09-08 from 10.13.83.5] (100/?) [AS65100i]</div><div><span class="" style="white-space:pre"> </span>Type: BGP unicast univ</div><div><span class="" style="white-space:pre"> </span>BGP.origin: IGP</div><div><span class="" style="white-space:pre"> </span>BGP.as_path: 65100</div><div><span class="" style="white-space:pre"> </span>BGP.next_hop: 10.13.81.73</div><div><span class="" style="white-space:pre"> </span>BGP.med: 100</div><div><span class="" style="white-space:pre"> </span>BGP.local_pref: 100</div></div><div><br></div><div><br></div><div><br></div><div>Installed routes to the next-hops: -------------------------</div><div><br></div><div><div>root@bird1:~# birdc show route all primary for 10.13.82.65 </div><div>BIRD 1.4.5 ready.</div><div><a href="http://10.13.82.64/27">10.13.82.64/27</a> via 10.13.83.246 on eth1.10 [PackOSPF 2015-08-05] * E2 (150/30/10000) [10.13.83.2]</div><div><span class="" style="white-space:pre"> </span>Type: OSPF-E2 unicast univ</div><div><span class="" style="white-space:pre"> </span>OSPF.metric1: 30</div><div><span class="" style="white-space:pre"> </span>OSPF.metric2: 10000</div><div><span class="" style="white-space:pre"> </span>OSPF.tag: 0x00000000</div><div><span class="" style="white-space:pre"> </span>OSPF.router_id: 10.13.83.2</div><div><br></div><div>root@bird1:~# birdc show route all primary for 10.13.81.73 </div><div>BIRD 1.4.5 ready.</div><div><a href="http://10.13.81.64/27">10.13.81.64/27</a> via 10.13.83.233 on eth0.14 [PackOSPF 2015-08-24] * E2 (150/20/10000) [10.13.83.5]</div><div><span class="" style="white-space:pre"> </span>Type: OSPF-E2 unicast univ</div><div><span class="" style="white-space:pre"> </span>OSPF.metric1: 20</div><div><span class="" style="white-space:pre"> </span>OSPF.metric2: 10000</div><div><span class="" style="white-space:pre"> </span>OSPF.tag: 0x00000000</div><div><span class="" style="white-space:pre"> </span>OSPF.router_id: 10.13.83.5</div></div><div><br></div><div><br></div><div><br clear="all"><div><div class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><pre><br>Adam Priestley
Network Administrator
Packnet Limited
<font face="monospace, monospace" size="1">t: +44 (0)161 660 2350</font>
f: <a value="+441616609825">+44 (0)161 660 9825</a>
e: <a href="mailto:adam@pack-net.co.uk" target="_blank">adam@pack-net.co.uk</a>
w: <a href="http://www.pack-net.co.uk/" target="_blank">http://www.pack-net.co.uk/</a>
Registered Address: PACKNET LIMITED, Duplex 2, Ducie House
37 Ducie Street
Manchester, M1 2JW
COMPANY REG NO. 04920552</pre></div></div></div></div></div></div></div></div></div></div></div>
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