interface goes down, the OSPF process has no
router ID and therefore ceases to function until the interface
comes up again. To ensure OSPF stability there should be an
active interface for the OSPF process at all times. A loopback
interface, which is a logical interface, can be configured for
this purpose. When a loopback interface is configured, OSPF
uses this address as the router ID, regardless of the value. On
a router that has more than one loopback interface, OSPF takes
the highest loopback IP address as its router ID. To create and
assign an IP address to a loopback interface use the following
commands: Router(config)#interface loopback
number
Router(config-if)#ip address
ip-address subnet-mask It is considered good practice to
use loopback interfaces for all routers running OSPF. This
loopback interface should be configured with an address using
a 32-bit subnet mask of 255.255.255.255. A 32-bit subnet mask
is called a host mask because the subnet mask specifies a
network of one host. When OSPF is requested to advertise a
loopback network, OSPF always advertises the loopback as a host
route with a 32-bit mask. In broadcast multi-access networks
there may be more than two routers. OSPF elects a designated
router (DR) to be the focal point of all link-state updates and
link-state advertisements. Because the DR role is critical, a
backup designated router (BDR) is elected to take over if the
DR fails. If the network type of an interface is broadcast, the
default OSPF priority is 1. When OSPF priorities are the same,
the OSPF election for DR is decided on the router ID. The
highest router ID is selected. The election result can be
determined by ensuring that the ballots, the hello packets,
contain a priority for that router interface. The interface
reporting the highest priority for a router will ensure that it
becomes the DR. The priorities can be set to any value from 0
to 255. A value of 0 prevents that router from being elected. A
router with the highest OSPF priority will be selected as the
DR. A router with the second highest priority will be the BDR.
After the election process, the DR and BDR retain their roles
even if routers are added to the network with higher OSPF
priority values. Modify the OSPF priority by entering global
interface configuration ip ospf priority command on an
interface that is participating in OSPF. The command show ip
ospf interface will display the interface priority value as
well as other key information. Router(config-if)#ip
ospf priority number
Router#show ip
ospf interface type number Lab Activity Lab
Exercise: Configuring OSPF with Loopback Addresses This lab is
to configure routers with a Class C IP addressing scheme.
Lab Activity e-Lab Activity: Configuring OSPF with Loopback
Addresses In this lab, the student will observe the election
process for designated routers, DR, and BDR. Web Links
Configuring a Loopback Interface
http://www.cisco.com/en/US/products/ sw/iosswrel/ ps1835/
products_ configuration_guide_ chapter09186a0080087093.html#1012547
Content 2.3 Single Area OSPF Configuration
2.3.3 Modifying OSPF cost metric OSPF uses cost
as the metric for determining the best route. Cost is
calculated using the formula 108/bandwidth, where bandwidth is
expressed in bps. The Cisco IOS automatically determines cost
based on the bandwidth of the interface. It is essential for
proper OSPF operation that the correct interface bandwidth is
set. Router(config)#interface serial 0/0
Router(config-if)#bandwidth 64 The default bandwidth
for Cisco serial interfaces is 1.544 Mbps, or 1544 kbps. Cost
can be changed to influence the outcome of the OSPF cost
calculation. A common situation requiring a cost change is in a
multi-vendor routing environment. A cost change would ensure
that one vendor’s cost value would match another vendor’s cost
value. Another situation is when Gigabit Ethernet is being
used. The default cost assigns the lowest cost value of 1 to a
100 Mbps link. In a 100-Mbps and Gigabit Ethernet situation,
the default cost values could cause routing to take a less
desirable path unless they are adjusted. The cost number can be
between 1 and 65,535. Use the following interface configuration
command to set the link cost: Router(config-if)#ip ospf
cost number Lab Activity Lab Exercise:
Modifying OSPF Cost Metric This lab is to setup an Open
Shortest Path First (OSPF) area. Lab Activity e-Lab
Activity: Modifying OSPF Cost Metric In this lab, the student
will modify the OSPF cost metric. Web Links ip ospf
cost http://www.cisco.com/en/US/products/sw/
iosswrel/ps1835/products_command_
reference_chapter09186a00800917e6.
html#1018073
Content 2.3 Single Area OSPF Configuration
2.3.4 Configuring OSPF authentication By default, a router
trusts that routing information is coming from a router that
should be sending the information. A router also trusts that
the information has not been tampered with along the route. To
guarantee this trust, routers in a specific area can be
configured to authenticate each other. Each OSPF interface can
present an authentication key for use by routers sending OSPF
information to other routers on the segment. The authentication
key, known as a password, is a shared secret between the
routers. This key is used to generate the authentication data
in the OSPF packet header. The password can be up to eight
characters. Use the following command syntax to configure OSPF
authentication: Router(config-if)#ip ospf
authentication-key password After the password is
configured, authentication must be enabled:
Router(config-router)#area area-number
authentication With simple authentication, the password
is sent as plain text. This means that it can be easily decoded
if a packet sniffer captures an OSPF packet. It is recommended
that authentication information be encrypted. To send encrypted
authentication information and to ensure greater security, the
message-digest keyword is used. The MD5 keyword specifies the
type of message-digest hashing algorithm to use, and the
encryption type field refers to the type of encryption, where 0
means none and 7 means proprietary. Use the interface
configuration command mode syntax: Router(config-if)#ip ospf
message-digest-key key-id md5
encryption-type key The key-id is an identifier and takes
the value in the range of 1 through 255. The key is an
alphanumeric password up to sixteen characters. Neighbor
routers must use the same key identifier with the same key
value. The following is configured in router configuration
mode: Router(config-router)#area area-id
authentication message-digest MD5 authentication
creates a message digest. A message digest is scrambled data
that is based on the password and the packet contents. The
receiving router uses the shared password and the packet to
re-calculate the digest. If the digests match, the router
believes that the source and contents of the packet have not
been tampered with. The authentication type identifies which
authentication, if any, is being used. In the case of
message-digest authentication, the authentication data field
contains the key-id and the length of the message digest that
is appended to the packet. The message digest is like a
watermark that cannot be counterfeited. Lab Activity