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We also add a ``post_build`` method to launch tcpdump_ and capture the BGP packets exchanged by the routers. A BGP session runs over a TCP connection. Let us examine a few of the BGP messages exchanged on the BGP session between ``AS1`` and ``AS2``. The traces collected on the three routers are available from :download:`/exercises/traces/bgp-as1-trace.pcap`, :download:`/exercises/traces/bgp-as2-trace.pcap` and :download:`/exercises/traces/bgp-as2-trace.pcap`.
 
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IPMininet_ also allows to explore the dynamics of BGP by looking at the packets that the routers exchange. For this, we slightly modify the example above and add delays to the interdomain links as follows.
 
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The first of these two routes is preferred as indicated by the `>` character because it has a higher ``local-preference` (150) than the second one (100). For prefix ``2001:cafe:3::/48``, the route learned via ``fe80::3c81:2eff:fe19:465d`` is also preferred for the same reason.
 
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the eBGP session with ``fe80::c001:dcff:fe49:a512`` with an AS-Path of ``AS2`` (see last column)
 
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the eBGP session with ``fe80::3c81:2eff:fe19:465d`` with an AS-Path of ``AS3:AS2`` (see last column)
 
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It is interesting to look at the output of this command in details. Router `as1` has routes for three different IPv6 prefixes. The first prefix is its own prefix, ``2001:cafe:1::/48``. It has no nexthop since this prefix is originated by the router. Then, `as1` has received two paths for ``2001:cafe:2::/48``. In the BGP Loc-RIB, the `>` character indicates the best route according to the BGP decision process. ``2001:cafe:2::/48`` was learned over two different BGP sessions:
 
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We can now observe the BGP-Loc-RIB of the router with the ``show bgp ipv6 command`` command.
 
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This router (`as1`) has two BGP neighbors: ``2001:cafe:1:12::2`` and ``2001:cafe:1:13::3``. Both BGP sessions are established using the current version of the protocol (version 4). About 50 messages were sent/received over each session. These messages are mainly the BGP Keepalive messages that are exchanged every 30 seconds. The last column indicates that two prefixes were received over each session. We can see more details about these two eBGP sessions with the `show bgp neighbors` command.
 
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A useful command to start is `show bgp summary` which provides a summary of the state of the BGP daemon.
 
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In these exercises, we mainly consider the ``show`` that extracts information from the BGP daemon. We type `show bgp` and press the `tabulation` key to see the available commands in the `show bgp`.
 
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The password for the BGP daemon is `zebra`. The `noecho` command indicates that mininet_ does not need to echo the characters that you type. You then enter the Quagga VTY that enables you to type commands. The `help` commands gives you some information about the available commands as well as `?`.
 
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Besides :manpage:`ping6(8)` and :manpage:`traceroute6(8)`, it is also useful to interact with the BGP daemon that runs on each of our routers. This is done by connecting on the Command Line Interface of the BGP router using telnet.
 
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The same applies for the paths between ``h1`` and ``h2``
 
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Note that the path preferred by ``AS3`` to reach ``AS1`` is different.
 
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We can now explore the routes in this small Internet. Host ``h1`` can reach directly host ``h3``.
 
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We have configured the following addresses on the hosts.
 
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We can also use :manpage:`traceroute6(8)` to check the path followed by the packets. Before doing that, think about the configuration of the BGP routing policies and try to predict the output of :manpage:`traceroute6(8)`. This is a good exercise to check your understanding of BGP.
 
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Remember that BGP is a distributed protocol and that it takes some time to launch the daemons and exchange the messages. After some time, the same command will confirm that everything works as expected.
 
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If you launch the script and immediately type ``ping6all`` to check the connectivity, you might obtained the following result.
 
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We can now run this simple network.
 
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