Inter-domain routing and BGP


This is an unpolished draft of the second edition of this ebook. If you find any error or have suggestions to improve the text, please create an issue via


  1. The interdomain topology below is composed of four domains. For this exercise, we assume that there are no routing policies, i.e. each domain advertise all its best paths to its peers. We focus on the prefix p advertised by AS1.
graph foo {
   AS1 -- AS3;
   AS1 -- AS2;
   AS3 -- AS4;
   AS1 -- AS4;

Assume that the BGP sessions are activated in the following order :

  • AS1-AS2
  • AS3-AS4
  • AS1-AS3
  • AS1-AS4

At each step of this activation sequence, show the BGP messages for prefix p that are exchanged and provide the BGP routing table of the different ASes. Assume that BGP always prefers the short AS-Path.

Once the interdomain network has fully converged, analyze the consequence of a failure of the following BGP sessions on the routes towards prefix p :

  • AS1-AS2
  • AS1-AS4
  1. Consider the network shown in the figure below. In this network, each AS contains a single BGP router. Assume that R1 advertises a single prefix. R1 receives a lot of packets from R9. Without any help from R2, R9 or R4, how could R1 configure its BGP advertisement such that it receives the packets from R9 via R3 ? What happens when a link fails ?
  1. Consider the network shown in the figure below where R1 advertises a single prefix. In this network, the link between R1 and R2 is considered as a backup link. It should only be used only when the primary link (R1-R4) fails.
  • Can you implement this in R2 ? How ?
  • Assuming that R1-R2 is a backup link, what are the paths used by all routers to reach R1 ?
  • Assume now that the link R1-R4 fails. Which BGP messages are exchanged and what are now the paths used to reach R1 ?
  • Link R1 - R4 comes back. Which BGP messages are exchanged and what do the paths used to reach R1 become ?
  1. Consider the small Internet show in the figure below.
digraph foo {
  AS2 -> AS1 [label=<<font color="red">$</font>>, color=red];
  AS2 -> AS3 [label=<<font color="red">$</font>>, color=red];
  AS1 -> AS4 [label=<<font color="red">$</font>>, color=red];
  AS3 -> AS4 [label=<<font color="red">$</font>>, color=red];
  AS5 -> AS4 [label=<<font color="red">$</font>>, color=red];
  AS1 -> AS3 [dir=none,label=<<font color="blue">=</font>>, color=blue];
  AS3 -> AS5 [dir=none,label=<<font color="blue">=</font>>, color=blue];
  AS2 -> AS5 [dir=none,label=<<font color="blue">=</font>>, color=blue];

In this small Internet, AS5 advertises prefix 2001:db8:5/48. Which BGP messages are exchanged concerning this prefix when the following events occur (one after the other) :

  • the link between AS3 and AS5 fails
  • the link between AS1 and AS2 fails
  • the link between AS2 and AS5 fails
  1. Consider the small Internet show in the figure below.
digraph foo {
  AS3 -> AS2 [label=<<font color="red">$</font>>, color=red];
  AS5 -> AS1 [label=<<font color="red">$</font>>, color=red];
  AS1 -> AS2 [label=<<font color="red">$</font>>, color=red];
  AS1 -> AS4 [dir=none,label=<<font color="blue">=</font>>, color=blue];
  AS4 -> AS2 [dir=none,label=<<font color="blue">=</font>>, color=blue];
  AS5 -> AS4 [dir=none,label=<<font color="blue">=</font>>, color=blue];
  1. AS3 advertises prefix 2001:db8:3/48. Compute the BGP routing tables of all ASes towards this prefix.
  2. After a few months, AS3 decides to become a customer of AS4. Explain which BGP messages are exchanged when the link between AS3 and AS4 becomes active.
  3. After a few months, AS3 decides to become a customer of AS5. Explain which BGP messages are exchanged when the link between AS3 and AS5 becomes active.
  4. AS3 is satisfied with its connectivity via AS4 and AS5 and decides to stop the BGP session with AS2. Explain the BGP messages that are exchanged when this link is disabled.

Netkit BGP lab

This lab, /netkit/, allows you to experiment with the BGP operation. The simulated internetwork consists of 8 BGP routers, each one corresponding to a different Autonomous System (AS). They have the peering relations shown in the figure:


To run BGP, these routers uses daemons called zebra and bgpd .

You can launch the lab using lstart in the lab folder. For monitoring traffic, you can use tcpdump and wireshark.

In this lab, router r9 announces via BGP an IPv6 prefix throughout the network. You can observe in the routing tables of the other routers that there are entries for the local prefixes and for the prefix announced by r9, in which the nexthop is indicated. But routers have no addional information about prefixes from other routers.

You can have more advanced informations about how BGP runs on the routers by accessing to the bgpd daemon via telnet:

telnet localhost bgpd

The password is zebra, as usual.

In the telnet terminal you can use:

show ipv6 bgp summary
show ip bgp neighbors
show ip community-list

to get more infos. (Note that neighbors and community list queries use the ip command instead of the ipv6 command). See the quagga manual for bgpd for a more complete description of available commands.

You can find the configuration files of the running daemons in the routers’ folders. For instance, consider router r1. You can find 3 configuration files in lab/r1/etc/quagga:

  • The first one is daemons. This file contains informations about which daemon should be started on our router.

  • The second one is zebra.conf. This file contains the password that we use to connect to the zebra daemon when we are on the router. (The password asked when accessing telnet localhost zebra)

  • The third one is bgpd.conf. This is the configuration file of our bgpd daemon. The following picture details the meaning of Let’s see what all these lines means.


With this in mind, you are able to play with the topology and even create new routers that use BGP. Try some different configurations, try to change how the filters work and observe what happens. On the original lab, for instance, you can cause a failure on the AS9-AS1 link (with the command ifconfig ... down). Observe which BGP messages are exchanged and how the state of router r7 changes. What are your expectations? Are your observations consistent with what you expected ?