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What is the Loc-RIB of ``AS6`` for prefix `p9` ? Indicate which is the best route towards this prefix.
 
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Let us now consider another example. The network contains nine ASes with one host per AS. Assuming that ``AS9`` announces prefix `p9` and that ``AS2`` announces prefix `p2`.
 
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What are the BGP messages that will be exchanged when the link between ``AS3`` and ``AS4`` fails ? How does this affect the reachability of the different hosts ?
 
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What are the BGP messages that will be exchanged when the link between ``AS1`` and ``AS5`` fails ? How does this affect the reachability of the different hosts ?
 
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What are the BGP messages that will be exchanged when the link between ``AS1`` and ``AS3`` fails ? How does this affect the reachability of the different hosts ?
 
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What are the BGP messages that will be exchanged when the link between ``AS1`` and ``AS2`` fails ? How does this affect the reachability of the different hosts ?
 
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Draw the network and try to predict how it will react to a shutdown of any of the customer-provider links ?
 
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When this network starts, all hosts can reach all other hosts.
 
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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``.
 
9 months ago

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