Ubuntu: Basic Mail Server Configuration

We will try to install and configure a mail server solution in Ubuntu. We will use postfix as the SMTP server and Dovecot as the IMAP/POP3 server. We will also use Thunderbird as the client agent for testing purpose. Before you could install a mail server, you need to have a running DNS server with an MX record in it for your domain. Refer to this post on how to configure a DNS server.

First, download and install postfix with the command

sudo apt-get install postfix

When the installation is done, you will be asked by several questions, answer them with the following:

  • Internet Site
  • mail1.example.com (change this to what you want your mail server name and domain name to be)
  • administrator (change this to the administrator username)
  • mail.example.com, example.com, localhost.example.com, localhost (you can leave it as default or change this to suit your mail server and domain name)
  • No
  • [::ffff:]/104 [::1]/128 (you can leave it as default)
  • 0
  • +
  • all

Now, change the mailbox format used to Maildir with the command.

sudo postconf -e 'home_mailbox = Maildir/'
sudo postconf -e 'mailbox_command ='

The Maildir format will place each mail as a file in /home/username/Maildir. Now, verify postfix installation with the command (change mail1.example.com to your mailserver name or any other name just for testing purpose)

telnet localhost 25
ehlo example.com

If you are connected and you get responses, then postfix is running. Before we test sending an email, let's install Dovecot as the IMAP/POP3 server. Get and install Dovecot with the following command

sudo apt-get install dovecot-imapd dovecot-pop3d

Now, some configuration changes needs to be done to /etc/dovecot/dovecot.conf. Open the file using your favourite text editor. First, we will set the protocol that will be used supported by dovecot, search for the following protocols entry and change the value to support which protocol that you prefer.

protocols = pop3 pop3s imap imaps

configure dovecot to use Maildir format, search for the mail_location entry (it is commented by default) and set the value to the following

mail_location = maildir:~/Maildir

To allow login from remote machine, search for the "listen" parameter and set it to

listen = *

For Thunderbird specific configuration, search for the IMAP configuration part "protocol imap {", then search for "imap_client_workarounds" parameter before the closing bracket. Change it to

imap_client_workarounds = tb-extra-mailbox-sep

Then, add the Maildir skeleton directory to /etc/skel to automatically create the Maildirr directory structure for newly created users. Run the following command

sudo maildirmake.dovecot /etc/skel/Maildir
sudo maildirmake.dovecot /etc/skel/Maildir/.Drafts
sudo maildirmake.dovecot /etc/skel/Maildir/.Sent
sudo maildirmake.dovecot /etc/skel/Maildir/.Trash
sudo maildirmake.dovecot /etc/skel/Maildir/.Templates

Now, restart Dovecot and test if it is running. Run the following command

sudo /etc/init.d/dovecot start
telnet localhost pop3
telnet localhost imap2

If Dovecot is running correctly, you should get respond when you're connecting to the POP3 and IMAP server. Next, is to try to connect to the mailserver by using a client agent. We will use Thunderbird, if you haven't installed Thunderbird, get and install it. I believe it is not hard to be found. Before start configuring Thunderbird, let's add a localuser named "joe" at the mailserver for testing purpose with the command

sudo useradd -m -s /bin/bash joe
sudo passwd joe

Now, let's configure Thunderbird so that joe can check his mail on the mailserver. Once you've installed Thunderbird on the client, run it, then click the File > New > Mail Account. A dialog box will appear to configure a new mail account, fill in the your name, email address and password (the value depends on what you've set for joe's password) to match joe's account

Click Continue, Thunderbird will try to resolv the POP3/IMAP server and the SMTP server for the domain. If Thunderbird failed to resolv the mail server, Then click Manual Configuration. On the Server Name, fill in the mail server's hostname or IP address. Choose the port number 110 for POP3 or 143 for IMAP. Choose STARTTLS at the Connection security and Normal Password at the Authentication Method. Then click OK

You will notice that Thunderbird will list your newly created account on the left hand side. Right click on the account name, in this case "joe@example.com", then click Settings. Make sure the selected server for Outgoing Server (SMTP) is correct, if not, you can manually add your SMTP server into Thunderbird. On the left hand side of the dialog, look for the most bottom entry, there will be an Outgoing Server (SMTP) entry. Click Add, Then fill in the server name, which is the SMTP hostname or IP addres. Choose the connection security settings and Authentication Method used by your SMTP server, in this example, choose STARTTLS and No Autentication respectively. Click OK, then go back to the account setting, and select the correct SMTP server at the Outgoing Server (SMTP) entry.

Now, you can add another user to the mail server local user for testing purpose, then try to send email from one to another. You will received the email in Thunderbird. For further enhancement, you will want to add authentication to postfix, therefore disabling spammers to use your SMTP server to send emails, Integrating Dovecot with LDAP for Single Sign-On, installing and configuring spam assasin.


Ubuntu: DNS Server

We will configure a DNS server on a Ubuntu machine. This server will act as the master DNS for the local domain in this example, which is example.com. First, get the bind9 and the utility package with the commands

sudo apt-get install bind9
sudo apt-get install dnsutils

Now, add the "zone" (domain) to /etc/bind/named.conf.local. Use your favourite text editor to edit that file. and add the following (you will need the super user privilege to edit the file).

zone "example.com" {
type master;
file "/etc/bind/db.example.com";

zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "/etc/bind/dbreverse.example.com";

From the above example, two zones are created. First is example.com zone. This zone will have entry of hosts stored in file /etc/bind/db.example.com, which will need to be created later. The second zone is 1.168.192.in-addr.arpa, the reverse zone which will holds the entry to resolv ip address to hostname. You need to change 1.168.192 with whatever private network address that you need in reverse order. If you use network, then the zone name must be 168.192.in-addr.arpa. Recognize that all of the db files are referenced with the absolute path, if relative path is given, bind9 will start finding the file from /var/cache/bind, like how it is configured in /etc/bind/named.conf.options.

Now, create the file, db.example.com in /etc/bind. Add the following to the file.

$TTL 604800
@ IN SOA ns.example.com. root.example.com. (
2011010101 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
@ IN NS ns.example.com.
@ IN MX 10 mail.example.com.

ns IN A
mail IN A
host1 IN A
gateway IN CNAME host1

In this file, the ";" sign mark the start of a comment, and whatever follows will be ignored by the DNS parser. The first line is the TTL value, this value tells how long does other DNS server can cache infomation queried from this server. The next line is the State of Authority record. The @ symbol is a shotcut for the zone name declared in /etc/bind/named.conf.local, IN specify that this DNS resource is the internet class. We will use this value most often. SOA should always be there. The next entry is the hostname of the DNS server that could provide DNS service for the domain. You can specify the FQDN of the nameserver, but remember to always put a dot at the end of the name server. The nxt entry is the email address of someone who is responsible of this zone, remember to always put a dot at the end of the entry if it is a FQDN. the next fields consists of several entries that are enclosed with a set of parenthesis. Those are

  • Serial. This number should always be incremented everytime a change has been made to the file. Most people will use the yyyymmddnn format, with the nn is the sequence number, giving you the feasible value of 00-99 for a day.
  • Refresh Interval. This is the value in seconds after which a slave DNS server will update its zone and reverse zone information from the master
  • Retry. This is the value in which if a slave DNS server failed to contact the master to update its zone and reverse information, should retry to contact the master after the amount of this value has elapsed. This value should be much smaller than the Refresh value.
  • Expiration. This is the amount of time which information in slave DNS server should be considered expired. If a slave DNS server failed to update its zone and reverse information and the amount of time in this entry has elapsed, it will stop responding queries asking information about this domain.
  • Negative Cache TTL. The amount of time that a negative response, such as a nonexistent domain response, will be cached by the DNS server.

The next part of the file is the entry that defines hostname to ip address. As can be seen, there is a nameserver, mail, host and alias entry. The nameserver record, marked with NS specify what is the name of the nameserver in this zone. The mx record, which is the mailserver record looks the same as the NS record except that it uses MX and there is a sequence number, in this case 10, specifying which mailserver will be preffered in the domain. Both of the records point to a hostname, therefore we need to specify the ip address of those hostname and that is done with the A record. The CNAME record specify an alias, in this example gateway is an alias for host1 and therefore, both will resolv to the same address. You can modify the value of this entries based on your requirement.

Next, create the reverse zone file information. Create the /etc/bind/dbreverse.example.com and fill the file with the following.

$TTL 604800
@ IN SOA ns.example.com. root.example.com. (
2011010101 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
@ IN NS ns.example.com.
133 IN PTR ns.example.com.
140 IN PTR mail.example.com.
1 IN PTR host1.example.com.

Basically, in the reverse zone file you have to create a PTR record for each A record in the zone file. Now restart bind9 with the command

sudo /etc/init.d/bind9 restart

Next, add an entry of your newly configured nameserver in /etc/resolv.conf. add the following line to the beginning of file


Change to whatever your DNS server ip address is. Then, you can verify your configuration by using the dig command. Try the following command,

dig ns.example.com

If your configuration is working, it should give an output similar to this

; <<>> DiG 9.7.0-P1 <<>> ns.example.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47515
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0

;ns.example.com. IN A

ns.example.com. 604800 IN A

example.com. 604800 IN NS ns.example.com.

;; Query time: 1 msec
;; WHEN: Sat Oct 2 21:21:32 2010
;; MSG SIZE rcvd: 62


Ubuntu: OpenLDAP

We will install OpenLDAP in Ubunt server. Here, I user Ubuntu server 10.04. After that we will use OpenLDAP for authentication. First, download and install OpenLDAP by using apt-get.

sudo apt-get install slapd ldap-utils

Next, load some schemas to LDAP (LDAP schemas give structure/attributes to LDAP classes, the following schemas will be used for adding users later)

sudo ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/ldap/schema/cosine.ldif
sudo ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/ldap/schema/nis.ldif
sudo ldapadd -Y EXTERNAL -H ldapi:/// -f /etc/ldap/schema/inetorgperson.ldif

Then, load the backend configuration to LDAP. Copy the following configuration to a file, name it backend.ldif

# Load dynamic backend modules
dn: cn=module,cn=config
objectClass: olcModuleList
cn: module
olcModulepath: /usr/lib/ldap

olcModuleload: back_hdb

# Database settings
dn: olcDatabase=hdb,cn=config

objectClass: olcDatabaseConfig
objectClass: olcHdbConfig
olcDatabase: {1}hdb
olcSuffix: dc=example,dc=com
olcDbDirectory: /var/lib/ldap
olcRootDN: cn=admin,dc=example,dc=com
olcRootPW: adminpw
olcDbConfig: set_cachesize 0 2097152 0
olcDbConfig: set_lk_max_objects 1500
olcDbConfig: set_lk_max_locks 1500
olcDbConfig: set_lk_max_lockers 1500
olcDbIndex: objectClass eq
olcLastMod: TRUE
olcDbCheckpoint: 512 30
olcAccess: to attrs=userPassword by dn="cn=admin,dc=example,dc=com" write by anonymous auth by self write by * none
olcAccess: to attrs=shadowLastChange by self write by * read
olcAccess: to dn.base="" by * read
olcAccess: to * by dn="cn=admin,dc=example,dc=com" write by * read

Take a look at the olcSuffix, olcRootDN and the olcRootPW entry. The olcSuffix is the domain name, here we use example.com as the domain name. The olcRootDN is the DN that has the administrator privilege like. olcRootPW is the password for the root admin. You may want to change those value to meet your requirement. If everything is fine, load the configuration to LDAP with the command

sudo ldapadd -Y EXTERNAL -H ldapi:/// -f backend.ldif

Next, fill the frontend directory to LDAP. This is where we create our organization tree, the domain, ou, user, group, ect. Copy the following to a file named frontend.ldif

# Create top-level object in domain
dn: dc=example,dc=com
objectClass: top
objectClass: dcObject
objectclass: organization
o: Example Organization
dc: Example
description: LDAP Example

# Admin user.
dn: cn=admin,dc=example,dc=com
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: admin
description: LDAP administrator
userPassword: secret

dn: ou=people,dc=example,dc=com
objectClass: organizationalUnit
ou: people

dn: ou=groups,dc=example,dc=com
objectClass: organizationalUnit
ou: groups

dn: uid=john,ou=people,dc=example,dc=com
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount

uid: john
sn: Doe
givenName: John
cn: John Doe
displayName: John Doe
uidNumber: 1000
gidNumber: 10000
userPassword: password
gecos: John Doe
loginShell: /bin/bash
homeDirectory: /home/john
shadowExpire: -1
shadowFlag: 0
shadowWarning: 7
shadowMin: 8
shadowMax: 999999
shadowLastChange: 10877
mail: john.doe@example.com
postalCode: 31000
l: Toulouse
o: Example
mobile: +33 (0)6 xx xx xx xx
homePhone: +33 (0)5 xx xx xx xx
title: System Administrator
initials: JD

dn: cn=example,ou=groups,dc=example,dc=com
objectClass: posixGroup
cn: example
gidNumber: 10000

In the example above, we create a user with uid: john. under the people oum the uid and the userPassword attributes will be used for authentication later. A group named example is also created under the groups ou and john is a member of that group. Change the value of ou, user information to meet your requirement. What should be taken into consideration here is the uidNumber attribute of the user. This uidNumber should be unique, it should not be the same with other user, even with the local user. You can check if the uidNumber has been used by local user by checking the "/etc/passwd" file. In above example, john's uidNumber is 1000. To check if this uid number has been used by local user, enter the following command

egrep ":1000:" /etc/passwd

If there's any output, then it has been used. If everything has been set, then load the frontend directory to OpenLDAP with the following command

sudo ldapadd -x -D cn=admin,dc=example,dc=com -W -f frontend.ldif

We have finished populating LDAP directory, next we will configure to use LDAP user for authentication

LDAP Authentication

To configure LDAP for authentication, first we need to install the libnss-ldap package.

sudo apt-get install libnss-ldap

After finishing the installation, you wll be asked several questions. Assume that you use the example.com as your domain, for each question enter the following answer

  • ldapi:///example.com
  • dc=example,dc=com
  • 3
  • No
  • No

Then, enable auth-client-config LDAP profile

sudo auth-client-config -t nss -p lac_ldap

Now, enable PAM for LDAP by the command

sudo pam-auth-update

Choose LDAP and any other authentication mechanism if needed. Now, you should be able to login using your OpenLDAP user, in this example, as john. But something still has to be done. If we recall from the user entry in frontend.ldif file, we specify the home directory of john to be "/home/john", but this directory is not exist yet (not if you have created it before). The problem here is that, since we are adding user from LDAP, user's home directory is not automatically created. This is different from adding local user with the "useradd -m " command.

Creating User's Home Directory

Using your favourite text editor, edit the file "/etc/pam.d/common-session". Add the following entry if not exist

session required pam_unix.so
session required pam_mkhomedir.so skel=/etc/skel/
session optional pam_ldap.so
session optional pam_foreground.so

Now, if you logged using your LDAP user for the first time, user's home directory will be created.


Ubuntu: Interface IP Addressing

Interface addressing includes how to change the ip address of an interface, the default gateway and also the DNS address. You can do temporary or permanent changes. To temporarily change the ip address of an interface, use the following command

sudo ifconfig eth0 A.B.C.D netmask X.X.X.X

A.B.C.D: the new ip address
X.X.X.X: the netmask of the ip address

The example above will change the ip address and the netmask of the eth0 interface. Change eth0 from the command above to other interface based on your need. Verify the changes by using the command "sudo ifconfig eth0" If you change the ip address of an interface, chances are that you also want to configure the default gateway. To do that, use the command

sudo route add default default gw X.X.X.X eth0

X.X.X.X: the ip address of the default gateway

Verify that configuration by using the command "route". The output should display a route to your default gateway.

Next, to make a permanent changes there is a file that has to be edited. The network configuration is kept in "/etc/network/interfaces", open the file and you will see what is inside the file that would look something like this.

There are two interfaces in the example above, l0 and eth0. Now, from the example above, eth0 has been configured to use dhcp. If you want to edit an interface to use dhcp configuration, follow the configuration

auto eth0
iface eth0 inet dhcp

That are the what should be entried in the "etc/network/interfaces" file. After that, you can use the command "sudo ifup eth0" to refresh and initiate the DHCP process. But, if we want to configure the ip address and default gateway statically, edit at the appropriate interface part and add these lines

auto eth0
iface eth0 inet static

Change the ip addressing to suit your requirement.


Configuring BGP

BGP configuration is quite different from other routing protocols configuration. There are two kinds of BGP, which are IBGP and EBGP. EBGP is when BGP is configured between routers in a different Autonomous System (AS), IBGP is when BGP is configure between routers within the same AS. EBGP behaves differently from IBGP. We will see what other differences are as we configure BGP in the following topology.

In BGP we have to specify each of our neighbor manually and the network command work differently as with other routing protocol. We will start by configuring every interface and start BGP in each router.


interface FastEthernet1/0
ip address
no shutdown
interface Serial2/0
ip address
no shutdown
interface loopback0
ip address
router bgp 3500


interface FastEthernet1/0
ip address
no shutdown
interface FastEthernet1/1
ip address
no shutdown


interface FastEthernet1/0
ip address
no shutdown
interface Serial2/0
ip address
no shutdown
interface loopback0
ip address
router bgp 3500


interface loopback0
ip address
no shutdown
interface Serial1/0
ip address
no shutdown
router bgp 2500


interface Serial1/0
ip address
no shutdown
router bgp 4500

At this point, you can run the "show processes" or only "show processes | include BGP" (the BGP should be in upper case) command to see that BGP processes is running. You can also use the "show processes cpu | include BGP" to see the cpu usage instead of the memory usage.

Now, all interfaces have been configured and BGP processes are running. We can now make them BGP neighbor. There could be two type of BGP neighbor, IBGP or EBGP neighbor. EBGP neighbor is a neighbor of which AS number is different from the AS number of the configured router. First we will configure EBGP peers between R1-R4 and R3-R5. Neighbor in BGP should be configured manually, here is the configuration (enter the configuration at each BGP )


router bgp 3500
neighbor remote-as 2500


router bgp 2500
neighbor remote-as 3500


router bgp 3500
neighbor remote-as 4500


router bgp 4500
neighbor remote-as 3500

At this point, you can verify the BGP neighbor status with "show ip bgp neighbors" or "show bgp summary ". Here's the example of running "show bgp summary " in R1

Now, we will configure IBGP peer between R1-R3. Notice, that I create a loopback interface on R1 and R3, so instead of using a physical interface to make a peer between R1-R3, we will use that loopback interfaces. But, we won't be able to do that before telling each other how to get to peer loopback interface. We can do this by using a static route or a routing protocol. In this case, we will use OSPF. So, let's configure OSPF


router ospf 1
network area 0
network area 0


router ospf 1
network area 0
network area 0


router ospf 1
network area 0
network area 0

R1 and R3 should now have known the route to reach each other's loopback interface. We can verify this issuing the "show ip route" command

Now, we can configure IBGP peer for R1-R3, the configuration is quite the same as EBGP neighbor configuration


router bgp 3500
neighbor remote-as 3500
neighbor update-source loopback 0


router bgp 3500
neighbor remote-as 3500
neighbor update-source loopback 0

If we are using loopback interfaces for BGP neighbor, remember the "update-source" and "ebgp-multihop" command. Because loopback interface is not a directly connected interface, we need the "ebgp-multihop" command to make the EBGP neighbor works. For IBGP peer, the "update-source" command is used instead.

Now, the BGP routers have become neighbors but no network is exchanged between them. We will try to advertise a network that is directly connected to R1 to R3 via BGP. For this purpose, we create another loopback interface on R1, and


interface loopback 1
ip address

Then we use the BGP network command to advertise this network. In BGP, the network command should match exactly the network address and the subnet mask that is to be advertised.


router bgp 3500
network mask

At this point, the network won't be advertised not only to R3 but also to R4. You can verify this network being learned by both routers by issuing the command "show bgp".

To avoid advertising this network to R4, we can use a route-map. Here's the configuration on R1


access-list 1 deny
access-list 1 permit any
route-map filter_r4 permit 10
match ip address 1
router bgp 3500
neighbor route-map filter_r4 out

The idea is to first create an access-list that will deny the route that we do not want to advertise, that is the network. Then we will create a route-map that match the access-list, which will deny the unwanted routes. Finally, we apply the route-map to the R4's BGP neighbor command. The out parameter means we want filter outgoing networks to R4. If there's no changes after applying those commands, try to clear bgp process on R1 and R4 by the command "clear ip bgp *".

Next, We will try to advertise network from AS 2500 to AS 4500. To achieve this, we must concern the BGP synchronization rule. First, we enter the network command on R4.


network mask

At this point, the network should have been advertised to R1 and R3. But in R3, the route won't be installed to the routing table. And also the network won't be advertised to R5 because of the synchronization rule. First we will make the route installed on R3. If we issue the command "show bgp" on R3, there's no best mark for network

This is because the next hop address is still pointing to the address of R4's Serial1/0 interface. In BGP, for a route to be chosen as the best route, the receiving router should know how to reach the address of the next hop. And by nature, a route learned via EBGP, won't have the next hop address changed as it is being advertised to another IBGP. To solve this, we add the "" command on R1


router bgp 3500
neighbor next-hop-self

Now, the route should have been chosen as the best route on R3. Next, we will make the network to be advertised to R5. To achieve this, we can either turn off synchronization rule or redistribute the to a running IGP, that is OSPF. We will try to redistribute the network to OSPF, so that the BGP network comply the synchronization rule.


router ospf 1
redistribute bgp 3500 subnets

Now, if we issue the command "show ip route" on R5, we will see that there's a route to network and is learned via BGP.

We also see the network advertised to R5, because no route-map is applied to filter this route. Now, the BGP is working and routes have been advertised each other.


Border Gateway Protocol Concepts

Border Gateway Protocol (BGP) is quite different from other routing protocol. BGP is the Internet routing protocol and it is a slow routing protocol which is good for an Internet routing protocol. There are two kinds of BGP, which are Interior BGP and Exterior BGP. IBGP is when BGP is configured between routers within same Autonomous System (AS), while EBGP is when BGP is configured between routers in a different AS. EBGP behaves differently with IBGP. As we know that other Interior Gateway Routing Protocol such as RIP, OSPF or EIGRP will update the next hop address when advertising routes to its neighbor. IBGP doesn't do this.

Neighbor in BGP should be configured manually. Another different concept in BGP is that neighbor in BGP shouldn't always be direct connected. In this topology bellow, R1 and R3 could be a neighbor without having a direct connection to each other.

Instead of having a metric to determine the best path to a network, BGP uses a list of attribute. These attributes have an order of precedence, each attribute of the feasible routes will be evaluated in order and the best path will be chosen when there is an attribute that is better than the others.

There are also two rules in IBGP, which are the synchronization rule and the split-horizon rule.

Synchronization Rule

The synchronization rule says that BGP won't advertised a route to other EBGP peers, if the route has not been learned by an IGP. Consider the following scenario

R4 advertises the network to R1, which will then advertise the network to R3, since they are an IBGP peer. But R3 won't advertise the network to R5 if the IGP running in its AS has not learned the network. This rule is used to avoid the "blackhole" problem. If R3 ever advertises the network to R5, then R5 will send packets destined to network to R3. Then R3 will send the packets to R2, but R2 does not know about that network since it is not running BGP. The packets will be dropped by R2. However, this synchronization rule can be turned off. You can safely turn this off if

  • You are not plannig to be a transit AS, that is having traffic from one AS to another AS over your AS.

  • All of your router is running BGP.

Split Horizon Rule

The split-horizon rule says that routes learned via IBPG won't be sent ot other IBGP peers. This rule is used to prevent loop in an AS. Consider the following scenario

R1 receives the network from R4, which then will advertise the network to R2 and R3. What if then R2 sends the network to R3 and vice-versa. In IGP such as EIGRP and OSPF, the routers could determine the best path to the network, but BGP couldn't. This is because BGP doesn't use the metric like bandwidth or delay to determine the best path, instead it uses some attributes which will not change over IBGP routers in the same AS. So, if R2 and R3 ever send the network to each other, they will end up having two routes with the same attribute, and therefore could potentially create a loop in the network. In Cisco routers, you could turn off this rule if you're sure that no potential loop will be created.


Configuring Multicast: Sparse-Dense Mode

There are two modes that we can use in multicast routing. They are sparse mode and dense mode. The dense mode uses the source trees multicast routing technique which is more to broadcast and prune multicast traffic. The sparse mode uses the sharde trees technique, which uses a rendezvous-point to get multicast traffic. But there's another mode which is the sparse-dense mode which quite uses the combination of both modes. We will try to configure multicast routing in sparse-dense mode but we won't get into the detail of the theory of each mode.

Let's say we have this topology.

First of all, we have to turn on multicast routing on all routers. Enter the following command in global configuration level

ip multicast-routing

Then we have to which interfaces will participate in the multicast routing. Go to the interface configuration level and enter the following command (enter the command on all interfaces listed in the above topology):

ip pim sparse-dense-mode

To verify that every interface has been multicast-enabled, enter the following command:

show ip pim interface

At the output of the command, you can see which interfaces are participating and in which mode.

At this point, you should already have multicast connectivity among your routers. But, your routers will be connected in dense mode. We still have to configure the sparse mode. To verify multicast connectivity, we will simulate a multicast server on R1, go to loopback0 interface of R1 and enter the following command:

ip igmp join-group A.B.C.D

A.B.C.D: ip address of the multicast network. You can use one the private multicast address range, which is -

Then in any other router, try to ping the multicast address. Here, I use as the multicast address that I joined in. If the router could reach the multicast address, you will see something like "Reply to request 0 from, 168 ms", but if the ping failed, you will see a ".".

Next, as in sparse mode, we have to specify at least a rendezvous-point. There are two ways to specify the rendezvous-point, static or auto rendezvous-point. If you configure your rendezvous-point statically, you have to configure it in every router in your network where you want multicast to be supported, that could be tedious if you have many routers. Another way is by using the auto-rp (auto rendezvous-point). This way, you go to the rp router and tells it that it has to announce itself as a rendezvous-point, so that the rest multicast-enabled routers will know it. To configure rp statically, we use the following command:

ip pim rp-address A.B.C.D

A.B.C.D: the ip address of the rp router.

There are some other options to this command. Try putting the question mark and hit enter will list other options such as applying access list so that a specific rp only serves for a group of mutlicast address only.

But, we will use the auto-rp technique. Here, we will configure the rp router to announce itself as a rp for the network. Go to the rp router and in the global configuration level, type in the following command:

ip pim send-rp-announce INTERFACE-TYPE INTERFACE-NUMBER scope NUMBER

INTERFACE-TYPE INTERFACE-NUMBER: the type and number of the interface of which address will be used as the rp address. ex, serial0/1, lo1.

NUMBER: the number of hops the announcement should traverse the network.

It is a good idea to use loopback interface as the address of the rp since a physical interface could be down and it mark the rp as unreachable. So, use a loopback interface and advertise that loopback address by using your routing protocol, so the rest router in the network know how to reach it and do not forget to enable ip pim in the loopback interface. In this example, we will make R3 as the rp. Go to R3 and enter the following command "ip pim send-rp-announce loopback 0 scope 15".

Then configure the mapping agent for your rp router. The mapping agent concept is similar to DR in OSPF. So, your rp will send the announcement to the mapping agent, and then it is the task of the mapping agent to send the mapping to the rest multicast-enabled router in the network. Configure a mapping agent by entering the following command:

ip pim send-rp-discovery scope NUMBER

NUMBER: the number of hops the announcement should traverse the network.

In this example, I make R3 as the mapping agent, so I configure R3 as the mapping agent by entering the command "ip pim send-rp-discovery scope 15" at the global configuration level. Next, we will have to configure other multicast-enabled router to accept auto-rp announcement. To configure this, go to R1, R2 and R4, at the global configuration level enter the following command:

ip pim accept-rp auto-rp

Then, you can also see the multicast routing table created in your router by using the command "show ip mroute". The output of the command would look something similar like this

The next thing to configure is to make your switch support multicasting. This is only one command, but without this command your switches will treat multicast frames just like a broadcast frame. Type in the following command in global configuration level of your switches:

ip igmp snooping

Another optimization that could be done is that if you want to limit your multicast packets from entering a part of your network. Let's say that you don't want to let multicast packet to go off your specific interface of your router. Enter the following command in your interface configuration level:

ip multicast ttl-threshold NUMBER

NUMBER: the number you want to reduce the multicast packet TTL.

Every multicast packet has an TTL. With this command, you can reduce the TTL of multicast packets so it will not go any further beyond the interface where multicast packet is received. To be sure that you reduce the TTL to 0 (or even bellow it), just give the NUMBER parameter a big value such as 255.