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Another interesting utilization IPMininet_ is to explore the impact of a link failure. We start from a small variant of the above topology.
 
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When this network is launched, `ping6all` reports connectivity problems. Hosts `h1` and `h4` cannot exchange packets. Can you fix the problem by changing the routing policy used on only one interdomain link ? Justify your answer
 
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Our first example is a small Internet with 5 ASes. A subset of the script that configures this network is shown below. There is one host attached to each AS and this host has the same number as its AS. The entire script is available from :download:`/exercises/ipmininet_scripts/ebgp-bug.py`.
 
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We can use IPMininet_ to prepare some networks with problems that need to be analyzed and corrected.
 
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Exercises
 
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We can fix them by enabling again the BGP session with the `no neighbor 2001:cafe:2:23::3 shutdown` command.
 
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Without a BGP session between ``AS2`` and ``AS3``, there are reachability problems in this simple Internet.
 
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We start indicate that we will use the terminal to change the router configuration with `configure terminal`. We then enter the BGP part of the configuration with `router bgp 2` (`2` is the AS number of `as2`). Then we use the `neighbor 2001:cafe:2:23::3 shutdown` that takes as parameter the IP address of the peer of the session that we want to stop. We then leave the BGP part of the configuration (first `exit`) and the configuration menu (second `exit` command). At this point, the BGP session between ``AS2`` and ``AS3`` is down.
 
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We can now disable one of the BGP sessions on router `as2` as follows.
 
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We first connect to the BGP daemon on router `as2`. In addition to the `show` commands that have been described earlier, the router also supports privileged commands that can change its configuration. Before executing these commands, we must enter the privileged mode with the `enable` command. On production routers, this command requires a password to verify the credentials of the network administrator. The `#` prompt indicates that we are allowed to execute privileged commands. We first check the state of the BGP sessions with the `show bgp summary` commands. There are two BGP sessions configured on this router.
 
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Another interesting utilization of IPMininet_ is to explore how routers react to link failures. We start from the same network as with the previous example and disable the link between ``AS2`` and ``AS3``. For this, we log on one of the two routers and issue the following commands.
 
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The ``UPDATE`` message can be used to withdraw and advertise routes. The packet below is sent by ``AS2`` to advertise its route towards `2001:cafe:2::/48` on the BGP session with ``AS1``.
 
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The BGP session starts with a TCP three-way handshake. Once the session has been established, both BGP daemons send an ``OPEN`` message describing their capabilities and the BGP extensions that it supports. The details of these extensions go beyond the scope of this book. However, it is important to note that the ``OPEN`` message contains the ``AS`` number of the router that sends the message and its identifier as a 32 bits IPv4 address. This router identifier uniquely identifies the router. The last mandatory parameter of the ``OPEN`` message is the `Hold Time`, i.e. the maximum delay between two successive messages over this BGP session. A BGP router should send ``KEEPALIVE`` messages every one third of the `Hold Time` to keep the session up.
 
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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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