cnp3-ebook/exercises/routing-protocols

Exit Zen

48 ../../exercises/routing-protocols.rst:569

	English Note that the path preferred by ``AS3`` to reach ``AS1`` is different. 70/10000 · 70
49 ../../exercises/routing-protocols.rst:581

	English The same applies for the paths between ``h1`` and ``h2`` 56/10000 · 56
50 ../../exercises/routing-protocols.rst:598

	English 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. 231/10000 · 231
51 ../../exercises/routing-protocols.rst:619

	English 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 `?`. 297/10000 · 297
52 ../../exercises/routing-protocols.rst:649

	English 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`. 197/10000 · 197
53 ../../exercises/routing-protocols.rst:663

	English A useful command to start is `show bgp summary` which provides a summary of the state of the BGP daemon. 104/10000 · 104
54 ../../exercises/routing-protocols.rst:685

	English 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. 496/10000 · 496
55 ../../exercises/routing-protocols.rst:812

	English We can now observe the BGP-Loc-RIB of the router with the ``show bgp ipv6 command`` command. 92/10000 · 92
56 ../../exercises/routing-protocols.rst:838

	English 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: 479/10000 · 479
57 ../../exercises/routing-protocols.rst:840

	English the eBGP session with ``fe80::3c81:2eff:fe19:465d`` with an AS-Path of ``AS3:AS2`` (see last column) 100/10000 · 100
58 ../../exercises/routing-protocols.rst:841

	English the eBGP session with ``fe80::c001:dcff:fe49:a512`` with an AS-Path of ``AS2`` (see last column) 96/10000 · 96
59 ../../exercises/routing-protocols.rst:843

	English 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. 277/10000 · 277
60 ../../exercises/routing-protocols.rst:845

	English 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. 205/10000 · 205
61 ../../exercises/routing-protocols.rst:854

	English 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`. 463/10000 · 463
62 ../../exercises/routing-protocols.rst:856

	English 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. 754/10000 · 754
63 ../../exercises/routing-protocols.rst:861

	English 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``. 187/10000 · 187
64 ../../exercises/routing-protocols.rst:866

	English 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. 282/10000 · 282
65 ../../exercises/routing-protocols.rst:898

	English 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. 620/10000 · 620
66 ../../exercises/routing-protocols.rst:900

	English We can now disable one of the BGP sessions on router `as2` as follows. 70/10000 · 70
67 ../../exercises/routing-protocols.rst:911

	English 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. 535/10000 · 535

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