Maintaining Network Routers In a link-state
routing environment, it is very important that the LSDBs of all
routers stay synchronized. When there is a change in a
link-state, the routers use a flooding process to notify the
other routers in the network of the change. LSUs provide the
mechanism for flooding LSAs. In general, the flooding process
in a broadcast multiaccess network is as follows:
Note
Although it is not shown in Figure , all LSUs
are acknowledged.
Step 1 On broadcast networks,
routers form adjacencies only with the DR and BDR. Therefore,
when a router notices a change in a link-state, it multicasts
an LSU packet (which includes the updated LSA entry) to all
OSPF DRs and BDRs. DRs and BDRs have their own multicast
address, which is 224.0.0.6. An LSU packet may contain several
distinct LSAs. Step 2 The DR acknowledges the receipt
of the change and floods the LSU to others on the network using
the OSPF multicast address 224.0.0.5. After receiving the LSU,
each router responds to the DR with an LSAck. To make the
flooding procedure reliable, each LSA must be acknowledged
separately. Step 3 If a router is connected to other
networks, it floods the LSU to those other networks by
forwarding the LSU to the DR of the other network (or to the
adjacent router if it is in a point-to-point network). That DR,
in turn, would multicast the LSU to the routers on the other
network. Step 4 The router updates its LSDB using the
LSU that includes the changed LSA. It then recomputes the SPF
algorithm against the updated database after a short delay (the
SPF delay) and updates the routing table as necessary. The same
DR/BDR functionality exists on NBMA networks, except that the
LSAs are unicast from the routers to the DR and BDR. In return,
the DR unicasts a copy of the LSA to all adjacent neighbors.
OSPF simplifies synchronization by requiring only adjacent
routers to remain synchronized. Summaries of individual
link-state entries, not the complete link-state entries, are
sent every 30 minutes to ensure LSDB synchronization. Each
link-state entry has a timer to determine when the LSA refresh
update must be sent. Each link-state entry also has a maximum
age of 60 minutes. If a link-state entry has not been refreshed
within 60 minutes, it is removed from the LSDB.
Note
On a Cisco router, if a route already exists,
the routing table is used at the same time the SPF algorithm is
calculating. However, if the SPF is calculating a new route,
the new route is used only after the calculation is complete.
Interactive Media Activity Drag and Drop: OSPF
Operation Upon completion of this activity, the student
will be able to list the steps of OSPF operation.
Content
3.2 Review of OSPF Operation 3.2.7
Maintaining Link-State Sequence Numbers A combination of
the maximum age (maxage) and refresh timers, as well as
link-state sequence numbers, helps OSPF maintain a database of
only the most recent link-state records. The link-state
sequence number field in an LSA header is 32 bits. Beginning
with the leftmost bit set, the first legal sequence number is
0x80000001. It is used to detect old or redundant LSAs; the
larger the number, the more recent the LSA. To ensure an
accurate database, a router running OSPF floods each LSA every
30 minutes. Each time a record is flooded, the sequence number
is incremented by one. An LSA record resets its maximum age
when it receives a new LSA update. An LSA never remains in the
database longer than the maximum age, one hour, without a
refresh. It is possible for an LSA to remain in the database
for long periods of time, being refreshed every 30 minutes. At
some point, the sequence number needs to wrap back to the
starting sequence number. When this process occurs, the
existing LSA is prematurely aged out (the maxage timer is
immediately set to one hour) and flushed. The LSA then begins
its sequencing at 0x80000001 again. The output of the show
ip ospf database command shown in Figure provides an
example of how the link-state age and sequence numbers are kept
in the database. Every OSPF router announces a router LSA for
those interfaces that it owns in that area. The link ID is the
ID of the router that created the router LSA. The advertising
router (shown as ADV Router in the output) is the router ID of
the OSPF router that announced the router LSA. Generally, the
link ID and advertising router are the same. The first router
LSA entry in the OSPF database indicates that the router LSA
with link ID 192.168.1.67 has been updated eight times (because
the sequence number is 0x80000008) and that the last update
occurred 48 seconds ago.
Content 3.2 Review of
OSPF Operation 3.2.8 Verifying Packet Flow
The debug ip ospf packet command is used in
troubleshooting and to verify that OSPF packets are flowing
properly between two routers. The output of the debug
command is shown in Figure . Notice that the output shows the
fields in the OSPF header, but they are not described in any
detail. Figure provides a summary of the various OSPF packet
fields.
Content 3.3 Implementing
and Verifying OSPF 3.3.1 Configuring Basic
Single-Area and Multiarea OSPF To configure basic
single-area and multiarea OSPF, complete the following steps:
Step 1 Enable OSPF on the router using the router
ospf process-id command as shown in Figure .
Note
Unlike the process ID in EIGRP, the OSPF
process ID is not an autonomous system number. The
process-id an be any positive integer and only has
significance to the local router. Step 2 Identify which
interfaces on the router are part of the OSPF process, using
the network area command, as shown in Figure . This
command also identifies the OSPF area to which the network
belongs. Figure describes the parameters of this command.
Starting with Cisco IOS Software Release 12.3(11)T (and some
specific versions of earlier releases), OSPF can be enabled
directly on the interface using the ip ospf area
command, which simplifies the configuration of unnumbered
interfaces. Since the command is configured explicitly on the
interface, it takes precedence over the network area
command. The command and its parameters are shown in Figure and
. Web Links Configuring OSPF
http://www.cisco.com/en/US/products/sw/iosswrel/
ps1828/products_configuration_guide_
chapter09186a00800ca56c.html
Content
3.3 Implementing and Verifying
OSPF 3.3.2 Configuring Basic Single-Area OSPF
Example Figure shows an OSPF configuration for Fast
Ethernet broadcast networks and serial point-to-point links,
the two most commonly used methods. All three routers in the
figure are assigned to area 0 and configured for network
10.0.0.0. Router A uses a general network 10.0.0.0
0.255.255.255 statement. This technique assigns all
interfaces defined in the 10.0.0.0 network to OSPF process 1.
Router B uses a specific host address technique. The wildcard
mask of 0.0.0.0 requires a match on all four octets of the
address. This technique allows the operator to define which
specific interfaces will run OSPF. OSPF can also be configured
using other methods. For example, a range of subnets could be
specified with one network statement and appropriate wildcard
mask. Note
The network statement and wildcard
mask are not used for route summarization purposes. The
network statement is used strictly to turn OSPF on for
an interface or for multiple interfaces.
Content
3.3 Implementing and Verifying
OSPF 3.3.3 Configuring Basic Multiarea OSPF
Example Figure shows an example of a multiarea OSPF
configuration. Router A is in area 0, router C is in area 1,
and router B is the ABR between the two areas. The
configuration for router A is the same as in the previous
example. Router B has a network statement for area 0. The