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—documentation:examples:ospf_inter-area_loop_prevention [2017/12/11 16:50] – external edit 127.0.0.1
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 +====== OSPF inter-area loop prevention ======
  
 +===== Presentation =====
 +
 +This lab show a non optimum route selection because of inter-area loop prevention feature of OSPF.
 +
 +==== Network diagram ====
 +
 +Here is the OSPF and logical view:
 +
 +{{:documentation:examples:ospf-loop-prevention.png|}}
 +
 +===== Preparing =====
 +
 +This chapter will describe how to start the lab.
 +
 +==== Downloading BSD Router Project images ====
 +
 +[[/downloads |Download a BSDRP image]].
 +
 +==== Download Qemu-KVM/Virtualbox Lab scripts =====
 +
 +More information on these BSDRP lab scripts available on [[documentation:examples:How to build a BSDRP router lab]].
 +
 +==== Starting the lab =====
 +
 +If you would to use Virtualbox, start this lab with:
 +
 +<code>virtualbox.sh -i BSDRP_0.35_full_amd64_vga.img -n 4 -c</code>
 +
 +The output should be like that:
 +
 +<code>
 +~>/usr/src/tools/tools/nanobsd/BSDRP/virtualbox.sh -n 4 -c
 +BSD Router Project: VirtualBox lab script
 +Creating lab with 4 router(s):
 +- 0 LAN between all routers
 +- Full mesh ethernet point-to-point link between each routers
 +- One NIC connected to the shared LAN with the host
 +
 +Router1 have the following NIC:
 +em0 connected to Router2.
 +em1 connected to Router3.
 +em2 connected to Router4.
 +em3 connected to shared-with-host LAN.
 +Router2 have the following NIC:
 +em0 connected to Router1.
 +em1 connected to Router3.
 +em2 connected to Router4.
 +em3 connected to shared-with-host LAN.
 +Router3 have the following NIC:
 +em0 connected to Router1.
 +em1 connected to Router2.
 +em2 connected to Router4.
 +em3 connected to shared-with-host LAN.
 +Router4 have the following NIC:
 +em0 connected to Router1.
 +em1 connected to Router2.
 +em2 connected to Router3.
 +em3 connected to shared-with-host LAN.
 +Connect to the router 1 by vnc client on port 5901
 +Connect to the router 2 by vnc client on port 5902
 +Connect to the router 3 by vnc client on port 5903
 +Connect to the router 4 by vnc client on port 5904
 +You need to configure an IP address in these range for communicating with the host:
 +        inet 192.168.56.1 netmask 0xffffff00 broadcast 192.168.56.255
 +        inet6 fe80::800:27ff:fe00:0%vboxnet0 prefixlen 64 scopeid 0x5
 +</code>
 +
 +===== Routers configuration =====
 +
 +==== Router 1 ====
 +
 +<code>
 +sysrc hostname=R1
 +sysrc frr_enable=yes
 +hostname R1
 +service frr start
 +vtysh
 +conf t
 +int em2
 + ip address 192.168.1.1/24
 + ip ospf cost 1
 +int em0
 + ip address 10.0.0.1/30
 + ip ospf cost 1
 +int em1
 + ip address 10.0.0.5/30
 + ip ospf cost 100
 +router ospf
 + router-id 1.1.1.1
 + network 192.168.1.0/24 area 1
 + network 10.0.0.0/30 area 0
 + network 10.0.0.4/30 area 0
 + exit
 +wr
 +exit
 +config save
 +</code>
 +==== Router 2 ====
 +Configure hostname:
 +
 +<code>
 +sysrc hostname=R2
 +sysrc frr_enable=YES
 +hostname R2
 +service frr start
 +vtysh
 +conf t
 +int em0
 + ip address 10.0.0.2/30
 + ip ospf cost 1
 +int em1
 + ip address 10.0.0.9/30
 + ip ospf cost 1
 +router ospf
 + router-id 2.2.2.2
 + network 10.0.0.0/30 area 0
 + network 10.0.0.8/30 area 2
 + exit
 +exit
 +wr
 +exit
 +config save
 +</code>
 +
 +==== Router 3 ====
 +
 +<code>
 +sysrc hostname=R3
 +hostname R3
 +sysrc frr_enable=YES
 +service frr start
 +vtysh
 +conf t
 +int em0
 + ip address 10.0.0.6/30
 + ip ospf cost 100
 +int em1
 + ip address 10.0.0.10/30
 + ip ospf cost 1
 +int em2
 + ip address 10.0.0.13/30
 + ip ospf cost 1
 +router ospf
 + router-id 3.3.3.3
 + network 10.0.0.4/30 area 0
 + network 10.0.0.8/30 area 2
 + network 10.0.0.12/30 area 2
 + exit
 +exit
 +wr
 +exit
 +config save
 +</code>
 +
 +==== Router 4 ====
 +
 +<code>
 +sysrc hostname=R4
 +hostname R4
 +sysrc frr_enable=YES
 +service frr start
 +vtysh
 +conf t
 +int em2
 + ip address 10.0.0.14/30
 + ip ospf cost 1
 +int em0
 + ip address 192.168.2.1/24
 + ip ospf cost 1
 +router ospf
 + router-id 4.4.4.4
 + network 10.0.0.12/30 area 2
 + network 192.168.2.0/24 area 2
 + exit
 +exit
 +wr
 +exit
 +config save
 +</code>
 +
 +===== The problem =====
 +
 +What route will R4 take for joining network 192.168.1.0/24 ?
 +
 +<code>
 +R4.bsdrp.net# sh ip route 192.168.1.0/24
 +Routing entry for 192.168.1.0/24
 +  Known via "ospf", distance 110, metric 4, best
 +  Last update 00:02:12 ago
 +  * 10.0.0.13, via em2
 +</code>
 +
 +Notice the OSPF metric for this route: 4, this mean the route should be: R4 -> R3 -> R2 -> R1.
 +
 +Great, how about a traceroute:
 +
 +<code>
 +R4.bsdrp.net# traceroute ip 192.168.1.1
 +traceroute to 192.168.1.1 (192.168.1.1), 64 hops max, 52 byte packets
 + 1  10.0.0.13 (10.0.0.13)  0.668 ms  0.317 ms  0.209 ms
 + 2  192.168.1.1 (192.168.1.1)  1.169 ms  0.574 ms  0.742 ms
 +</code>
 +
 +**Where is R2 hop ??**
 +
 +Verifying the 192.168.1.0/24 route on R3:
 +
 +<code>
 +R3.bsdrp.net# sh ip route 192.168.1.0
 +Routing entry for 192.168.1.0/24
 +  Known via "ospf", distance 110, metric 101, best
 +  Last update 00:32:13 ago
 +  * 10.0.0.5, via em0
 +</code>
 +
 +**R3 didn't choose the "best" path** for reach 192.168.1.0/24: The installed route have a metric of 101.
 +How about its OSPF database state:
 +
 +<code>
 +R3.bsdrp.net# sh ip ospf database summary 192.168.1.0
 +
 +       OSPF Router with ID (3.3.3.3)
 +
 +
 +                Summary Link States (Area 0.0.0.0)
 +
 +  LS age: 1345
 +  Options: 0x2  : *|-|-|-|-|-|E|*
 +  LS Flags: 0x6
 +  LS Type: summary-LSA
 +  Link State ID: 192.168.1.0 (summary Network Number)
 +  Advertising Router: 1.1.1.1
 +  LS Seq Number: 80000004
 +  Checksum: 0x9456
 +  Length: 28
 +  Network Mask: /24
 +        TOS: 0  Metric: 1
 +
 +
 +                Summary Link States (Area 0.0.0.2)
 +
 +  LS age: 2456
 +  Options: 0x2  : *|-|-|-|-|-|E|*
 +  LS Flags: 0x6
 +  LS Type: summary-LSA
 +  Link State ID: 192.168.1.0 (summary Network Number)
 +  Advertising Router: 2.2.2.2
 +  LS Seq Number: 80000001
 +  Checksum: 0x8662
 +  Length: 28
 +  Network Mask: /24
 +        TOS: 0  Metric: 2
 +
 +  LS age: 1087
 +  Options: 0x2  : *|-|-|-|-|-|E|*
 +  LS Flags: 0x3
 +  LS Type: summary-LSA
 +  Link State ID: 192.168.1.0 (summary Network Number)
 +  Advertising Router: 3.3.3.3
 +  LS Seq Number: 80000002
 +  Checksum: 0x4838
 +  Length: 28
 +  Network Mask: /24
 +        TOS: 0  Metric: 101
 +</code>
 +
 +<note important>ABR (R3) expects summary LSAs from Area 0 only. Then R3 ignore summary LSA received from R2 (area 2)</note>
documentation/examples/ospf_inter-area_loop_prevention.txt · Last modified: 2019/08/10 16:44 by olivier
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