OSPF Question And Answers [CCNP]
Open shortest path first is an Open Standard Link State
routing protocol which works by using Dijkastra algorithm to initially
construct the shortest paths and follows that by populating the routing table
with resulting best paths.
What are the steps
required to change Neighborship into adjacency?
1.Two-way communication (using Hello Protocol).
2.Database Synchronization which means exchange of Database
Description (DD) packets, Link State Request (LSR) packets, Link State Update
(LSU) packets.
After Database synchronization is complete, the two routers
are considered adjacent.
Explain LSA
(Link-State Advertisement), LSU (Link State Update) and LSR (Link State
Request)?
The LSAs (Link-State Advertisements) are used by OSPF
routers to exchange routing and topology information. When two neighbors decide
to exchange routes, they send each other a list of all LSAs in their respective
topology database. Each router then checks its topology database and sends Link
State Request (LSR) message requesting all LSAs that was not found in its
topology table. Other router responds with the Link State Update (LSU) that
contains all LSAs requested by the neighbor.
Explain OSPF Router
ID?
Router Id is used to identify the Router. Highest IP address
of the router's loopback interfaces is chosen as the Router ID, If no loopback
is present than highest IP address of the router's physical interfaces will be
chosen as Router ID. OSPF prevents neighborships between routers with duplicate
RIDs. All OSPF RIDs in a domain should be unique. OSPF Router ID should not be
changed after the OSPF process is started and the OSPF neighborships are
established. If you change the OSPF router ID, we need to either reload the IOS
or use "clear ip ospf process" command (restart the OSPF process) for
changed RID to take effect.
To manually configure the router ID
R1(config)# router ospf 5
R1(config-router)# router-id 5.5.5.5
Can we use OSPF
without backbone area?
Yes, but than only intra-area communication is possible.
Inter-area communication is not possible without backbone area.
What is the
difference between an OPPF neighbor and an adjacent neighbor?
LSAs are exchanged only among adjacent routers not among
neighbor routers.
What are different
neighbour states in OSPF ?
OSPF routers need to go through several state before
establishing a neighbor relationship -
1. Down - No Hello packets have been received on the
interface.
2. Attempt - In Attempt state neighbors must be configured
manually. It applies only to nonbroadcast multi-access (NBMA) networks.
3. Init - Router has received a Hello message from the other
OSFP router.
4. 2way - the neighbor has received the Hello message and
replied with a Hello message of his own. Bidirectional Communication has been
established. In Broadcast network DR-BDR election can occur after this point.
5. Exstart - DR & BDR establish adjacencies with each
router in the network. Master-slave election will takes place (Master will send
its DBD first).
6. Exchange - Routing information is exchanged using DBD
(Database Descriptor) packets, Link-State Request (LSR) and Link-State Update
packets may also be sent.
7. Loading - LSRs (Link State Requests) are send to
neighbors for every network it doesn't know about. The Neighbor replies with
the LSUs (Link State Updates) which contain information about requested
networks. After all the requested information have been received, other
neighbor goes through the same process.
8. Full - All neighbor routers have the synchronized
database and adjacencies has been established.
What is an LSA? How does
an LSA differ from an OSPF Update packet?
A router originates a link state advertisement to describe
one or more destinations. An OSPF
Update packet transports LSAs from one neighbor to another.
Although LSAs are flooded
throughout an area or OSPF domain, Update packets never
leave a data link.
Explain different
OSPF
1. Router LSA (Type1) - Each router geLSA Types?nerates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost. LSA Type 1 is flooded only within an area.
1. Router LSA (Type1) - Each router geLSA Types?nerates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost. LSA Type 1 is flooded only within an area.
2. Network LSA (Type2) - Type2 LSA is sent out by the
designated router (DR) and lists all the routers on the segment it is adjacent
to. Type 2 LSA are flooded only within an area.
3. Summary LSA (Type3) - Type 3 LSAs are generated by Area
Border Routers (ABRs) to advertise networks from one area to the rest of the
areas in Autonomous System.
4. Summary ASBR LSA (Type4) - Generated by the ABR. It
contain routes to ASBRs.
5. External LSA (Type5) - External LSAs are generated by
ASBRs and contain routes to networks that are external to the current
Autonomous System.
6. Not-So-Stubby Area LSA (Type7) - Stub areas do not allow
Type 5 LSAs. A Not So Stubby Area (NSSA)
allows advertisement of Type 5 LSA as Type 7 LSAs. Type LSA is generated by an
ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into
the NSSA.
Can I use the distribute-list in/out command with OSPF to filter routes?
The distribute-list commands are supported in OSPF but work differently than distance-vector routing protocols such as Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering the OSPF database. Thedistribute-list in command only filters routes from entering the routing table; it does not prevent link-state packets from being propagated. Therefore, this command does not help conserve router memory, and it does not prohibit a router from propagating filtered routes to other routers.
Can I use the distribute-list in/out command with OSPF to filter routes?
The distribute-list commands are supported in OSPF but work differently than distance-vector routing protocols such as Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering the OSPF database. Thedistribute-list in command only filters routes from entering the routing table; it does not prevent link-state packets from being propagated. Therefore, this command does not help conserve router memory, and it does not prohibit a router from propagating filtered routes to other routers.
Caution: Use of the distribute-list
in command in OSPF may lead to routing loops in the network if not
implemented carefully.
The command distribute-list
out works only on the routes being redistributed by the Autonomous
System Boundary Routers (ASBRs) into OSPF. It can be applied to external type 2
and external type 1 routes, but not to intra-area and interarea routes.
Refer to configuration example of
distribute-list in OSPF,
According to section 11 of RFC 2328 
, the order of preference for OSPF routes is:
, the order of preference for OSPF routes is:- intra-area routes, O
- interarea routes, O IA
- external routes type 1, O E1
- external routes type 2, O E2
This rule of preference cannot be
changed. However, it applies only within a single OSPF process. If a router is
running more than one OSPF process, route comparison occurs. With route
comparison, the metrics and administrative distances (if they have been changed)
of the OSPF processes are compared. Route types are disregarded when routes
supplied by two different OSPF processes are compared.
Do I need
to manually set up adjacencies for routers on the Switched Multimegabit Data
Service (SMDS) cloud with the OSPF neighbor subcommand?
In Cisco IOS Software releases earlier than
Cisco IOS Software Release 10.0, the neighbor command was
required to establish adjacencies over nonbroadcast multiaccess (NBMA) networks
(such as Frame Relay, X.25, and SMDS). With Cisco IOS Software Release 10.0 and
later, you can use the ip ospf network broadcast command to
define the network as a broadcast network, eliminating the need for the neighbor command.
If you are not using a fully meshed SMDS cloud, you must use the ip ospf
network point-to-multipoint command.
When
routes are redistributed between OSPF processes, are all shortest path first
algorithm (SPF) metrics preserved, or is the default metric value used?
The SPF metrics are preserved. The
redistribution between them is like redistribution between any two IP routing
processes.
You can configure OSPF to understand whether
it should attempt to use multicast facilities on a multi-access interface.
Also, if multicast is available, OSPF uses it for its normal multicasts.
Cisco IOS Software Release 10.0
includes a feature called subinterfaces. You can use subinterfaces with Frame
Relay to tie together a set of virtual circuits (VCs) to form a virtual
interface, which acts as a single IP subnet. All systems within the subnet
should be fully meshed. With Cisco IOS Software Releases 10.3, 11.0 and later,
the ip ospf point-to-multipoint command is also available.
When an unnumbered interface is configured,
it references another interface on the router. When enabling OSPF on the
unnumbered interface, use the address-wild-mask pair of interfaces to which the
unnumbered interface is pointing.
No, OSPF does not work if you have one side
numbered and the other side unnumbered. This creates a discrepancy in the OSPF
database that prevents routes from being installed in the routing table.
OSPF picks up the highest IP address as a
router ID. If there are no interfaces in up/up mode with an IP address, it
returns this error message. To correct the problem, configure a loopback
interface.
Your software may not support OSPF. This
error message occurs most frequently with the Cisco 1600 series routers. If you
are using a 1600 router, you need a Plus image to run OSPF.
DR means designated
router. BDR means backup designated
router. DROTHER indicates a router that is neither the DR or the BDR.
The DR generates a Network Link-State Advertisement, which lists all the
routers on that network.
Why master slave
needs to be elected between two neighbour interface?
Master sends its DBD (Database Description) First.
What is the
requirement of doing summarization?
1. Reduces the amount of information stored in routing
tables.
2. Allocates an existing pool of addresses more
economically.
3. Lessens the load on router processor and memory
resources.
4. Less number of update messages.
5. Less bandwidth.
How routes are
selected in OSPF according to preference?
Intra-Area routes(0)> Inter-Area routes(0-IA)>
External-Type-1(E1)> External-Type-2(E2)> NSSA-1(N1)> NSSA-2(N2).
What is Route
Redistribution?
Route redistribution is the process of taking routes learned
via one routing protocol and injecting those routes into another routing
protocol domain.
For example two companies might merge, one company is using
Enhanced Interior Gateway Routing Protocol (EIGRP) and the other is using Open
Shortest Path First (OSPF). Route redistribution allows exchanging of routes
between the two routing domains with a minimal amount of configuration and with
little disruption to the existing networks.
Why are loopbacks advertised
as /32 host routes in OSPF?
Loopbacks are considered host routes in OSPF,
and they are advertised as /32. For more information, refer to section 9.1
of RFC 2328 
. In Cisco IOS Software Releases 11.3T and
12.0, if the ip ospf network point-to-point command is
configured under loopbacks, OSPF advertises the loopback subnet as the actual
subnet configured on loopbacks. ISDN dialer interface advertises /32 subnet
instead of its configured subnet mask. This is an expected behavior if ip
ospf network point-to-multipoint is configured.
. In Cisco IOS Software Releases 11.3T and
12.0, if the ip ospf network point-to-point command is
configured under loopbacks, OSPF advertises the loopback subnet as the actual
subnet configured on loopbacks. ISDN dialer interface advertises /32 subnet
instead of its configured subnet mask. This is an expected behavior if ip
ospf network point-to-multipoint is configured.
For example, consider two routers
(R1 and R2) connected via FastEthernet interface. R1 has the loopback
configured with the ip ospf network point-to-point command and
advertises the loopback in OSPF.
interface Loopback0
ip address 1.1.1.1 255.255.255.0
ip ospf network point-to-point
When checked in router R2 with
the show ip route ospf command, the route 1.1.1.1 is seen as:
!..output truncated
1.0.0.0/24 is subnetted, 1 subnets
O 1.1.1.0 [110/11] via 10.1.1.1, 00:00:02,
FastEthernet0/0
However, when the ip ospf
network point-to-point command is removed from R1 to 0 interface, the
route 1.1.1.1 on R2 is seen as:
1.0.0.0/32 is subnetted, 1
subnets
O 1.1.1.1 [110/11] via 10.1.1.1, 00:00:01,
FastEthernet0/0
What is the default
redistribution OSPF cost ?
Redistribution into OSPF uses the following defaults:-
1. When taking from BGP, use a default metric of 1.
2. When taking from another OSPF process, take the source
route’s metric.
3. When taking from all other sources, use a default metric
of 20.
What is the
difference between Type-1 (E1) & Type-2 (E2) redistribution?
Type-2 is the default route type for routes learned via
redistribution. The key with E2 routes is that the cost of these routes
reflects only the redistributed cost. E2 = only redistributed cost.
Type-1 redistributed routes reflects cost to reach ASBR +
redistributed cost. E1 = cost to reach ASBR + redistributed cost
Explain OSPF Virtual
Link?
OSPF requires the use of a backbone area (area 0) with each
area connecting to area 0 through an ABR. However in some cases, regular area
might not have a convenient point of connection to the backbone area. In this
case, OSPF uses virtual link to connect that regular area to backbone area
virtually. An OSPF virtual link allows two ABRs that connect to the same
non-backbone area to form a neighbor relationship through that non-backbone
area, even when separated by many other routers and subnets. This virtual link
acts like a virtual point-to-point connection between the two routers, with
that link inside area 0. The routers form a neighbor relationship, inside area
0, and flood LSAs over that link.
Explain OSPF Stub
Area and different types of Stub Areas?
Stub Area Sometimes we need to control the advertisement of external routes into an area. This area is called Stub area. Stub areas are not capable of importing routes external to ospf.Type 4 & Type 5 LSA are filtered from Stub areas and a default route is injected into that area by ABR in place of external routes.
- To make area stub we have to give # area 1 stub command on all routers of that area.
Three restrictions apply to OSPF stub areas
1.No virtual links are allowed in stub area.
2.Stub area cannot be a backbone area.
3.No Autonomous System Boundary Routers are allowed.
Totally Stubby Area
Like stub areas, totally stubby areas do not receive type 4
or 5 LSAs from their ABRs. However, they also do not receive type 3 LSAs. It
only allows advertisement of internal routes in that area.
- To make area totally stubby area we have to give # area 1 stub no-summary command on ABR.
Not-So-Stubby Areas
The motivation behind NSSA is to allow OSPF stub areas to
carry external routes. External routes are imported into OSPF NSSA as Type 7
LSA by ASBR. Type 7 LSA cannot go into area 0 so it is converted back into Type
5 LSA by ABR and injected into area 0.
- To make area Not-So-Stubby Area we have to give # area 1 NSSA command on all routers of that area.
Totally NSSA
Along with Type 4 & Type 5 LSA, Type 3 LSA will also be
filtered in Totally NSSA.
- To make area Totally Not-So-Stubby Area we have to give # area 1 nssa no-summary command on ABR of that area.
How do I change the
reference bandwidth in OSPF?
We can change the reference bandwidth using the ospf
auto-cost reference-bandwidth command under router ospf. By default, reference
bandwidth is 100 Mbps.
How does OSPF
calculate its metric or cost?
OSPF uses Cost as its metric. The formula to calculate the
OSPF cost is reference bandwidth divided by interface bandwidth. For example,
in the case of Ethernet, it is 100 Mbps / 10 Mbps = 10.
If # ip ospf cost _ command is used on the interface, it
overrides this formulated cost.
What algorithm is used by OSPF if
equal cost routes exist?
If equal cost routes exist, OSPF uses CEF load
balancing. For more information, refer to Troubleshooting Load Balancing
Over Parallel Links Using Cisco Express Forwarding.
Explain OSPF
Authentication?
These are the three different types of authentication
supported by OSPF to secure routing updates.
1.Null Authentication - also called Type 0. It means no
authentication information is included in the packet header. It is the default.
2.Plain Text Authentication - also called Type 1. It uses
simple clear-text passwords.
3.MD5 Authentication - also called Type 2. It uses MD5
cryptographic passwords.
Plain Text Authentication
Step1 - To configure plain text authentication, first we
have to enable authentication. Authentication can be enabled either under area
or for specific interface.
To enable authentication for area
Router(config)#
router ospf 100
Router(config-router)#
network 192.168.1.0 0.0.0.255 area 0
Router(config-router)#
area 0 authentication
This will enable authentication for all the interfaces of
the router in area 0.
OR
If we dont want to enable authentication for an area, we can
enable it for the specific interface. This is useful if different interfaces
that belong to the same area need to use different authentication methods..
Router(config)#
interface fa0/1
Router(config-if)# ip
ospf authentication
Step2 - Next, We have to configure authentication key on the
interface
Router(config)#
interface fa0/1
Router(config-if)# ip
ospf authentication-key Cisco123
Here Cisco123 is the password value.
MD5 Authentication
Step1 - To configure MD5 authentication, first we have to
enable authentication.
Router(config)#
router ospf 1
Router(config-router)#
network 192.168.1.0 0.0.0.255 area 0
Router(config-router)#
area 0 authentication message-digest
OR
Router(config)# interface fa0/1
Router(config-router)# ip ospf authentication message-digest
Step2 - Next, We have to configure authentication key on the
interface
Router(config)# interface fa0/1
Router(config-router)# ip ospf message-digest-key 10 md5
Cisco123
Here Cisco123 is the password value and 10 is the Key ID
(number). It doesn’t matter which key ID you choose but it has to be the same
on both ends.
Authentication passwords do not have to be the same
throughout an area. However, they must be same between neighbors.
How do I change the reference
bandwidth in OSPF?
You can change the reference bandwidth in
Cisco IOS Software Release 11.2 and later using the ospf
auto-cost reference-bandwidth command under router
ospf. By default, reference bandwidth is 100 Mbps. The ospf link-cost is a
16-bit number. Therefore, the maximum value supported is 65,535.
OSPF uses a reference bandwidth of 100 Mbps
for cost calculation. The formula to calculate the cost is reference bandwidth
divided by interface bandwidth. For example, in the case of Ethernet, it is 100
Mbps / 10 Mbps = 10.
Note: If ip ospf
cost cost is used on the interface, it
overrides this formulated cost. For more information, refer to OSPF Cost.
Which command enables
OSPF for IPv6 on a router?
# ipv6 router ospf process-id
What is the
link-state retransmit interval, and what is the command to set it?
OSPF must send acknowledgment of each newly received
link-state advertisement (LSA). LSAs are retransmitted until they are
acknowledged. The link-state retransmit interval defines the time between
retransmissions. We can use the command ip ospf retransmit-interval to set the
retransmit interval. The default value is 5 seconds.
When routes are
redistributed between OSPF processes, are all shortest path first algorithm
(SPF) metrics preserved or is the default metric value used?
The SPF metrics are preserved. The redistribution between
them is like redistribution between any two IP routing processes.
How do I stop
individual interfaces from developing adjacencies in an OSPF network?
To stop routers from becoming OSPF neighbors on a particular
interface, issue the passive-interface command at the interface.
When I have two type
5 link-state advertisements (LSAs) for the same external network in the OSPF
database, which path should be installed in the routing table?
When you have two type 5 LSAs for the same external network
in the OSPF database, prefer the external LSA that has the shortest path to the
Autonomous System Boundary Router (ASBR) and install that into the IP routing
table. Use the show ip ospf border-routers command to check the cost to the
ASBR.
Should I use the same
process number while configuring OSPF on multiple routers within the same
network?
OSPF, unlike Border Gateway Protocol (BGP) or Enhanced
Interior Gateway Routing Protocol (EIGRP) does not check the process number (or
autonomous system number) when adjacencies are formed between neighboring
routers and routing information is exchanged.
Can we have OSPF run
over a GRE tunnel?
Yes we can have OSPF run over a GRE tunnel.
What is an OSPF
adjacency?
An OSPF adjacency is
a conceptual link to a neighbor over which LSAs can be sent.
What are the five OSPF packet types? What is the purpose of each type?
The five OSPF packet
types, and their purposes, are:
Hellos - which
are used to discover neighbors, and to establish and maintain adjacencies
Updates - which are used to send LSAs between
neighbors
Database Description packets - which a router
uses to describe its link state database to a
neighbor during database synchronization
Link State Requests - which a router uses to
request one or more LSAs from a neighbor's
link state database
Link State Acknowledgments - used to ensure
reliable delivery of LSAs
What is a link state database? What is link
state database synchronization?
The link state
database is where a router stores all the OSPF LSAs it knows of, including its
own. Database synchronization is the process of ensuring that all routers
within an area have identical link state databases.
What is the default
HelloInterval?
The default OSPF HelloInterval is 10 seconds.
What is the default
Router Dead Interval?
The default Router DeadInterval
is four times the HelloInterval.
What is a Router ID? How is a Router ID
determined?
A Router ID is an
address by which an OSPF router identifies itself. It is either the numerically
highest IP address of all the router's loopback interfaces, or if no loopback
interfaces are configured, it is the numerically highest IP address of all the
router's LAN interfaces.
What is an area?
An area is an OSPF
sub-domain, within which all routers have an identical link state database.
What is the significance of area 0?
Area 0 is the
backbone area. All other areas must send their inter-area traffic through the
backbone.
What is MaxAge?
MaxAge, 1 hour, is
the age at which an LSA is considered to be obsolete.
Are OSPF routing protocol
exchanges authenticated?
Yes, OSPF can authenticate all packets
exchanged between neighbors. Authentication may be through simple passwords or
through MD5 cryptographic checksums. To configure simple password
authentication for an area, use the command ip ospf
authentication-key to assign a password of up to eight
octets to each interface attached to the area. Then, issue the area x authentication command
to the OSPF router configuration to enable authentication. (In the
command, x is the area number.)
Cisco IOS Software Release 12.x
also supports the enabling of authentication on a per-interface basis. If you
want to enable authentication on some interfaces only, or if you want different
authentication methods on different interfaces that belong to the same area,
use the ip ospf authenticationinterface mode command.
What is
the link-state retransmit interval, and what is the command to set it?
OSPF must send acknowledgment of each newly
received link-state advertisement (LSA). It does this by sending LSA packets.
LSAs are retransmitted until they are acknowledged. The link-state retransmit
interval defines the time between retransmissions. You can use the
command ip ospf retransmit-interval to set the
retransmit interval. The default value is 5 seconds.
What are the four
OSPF router types?
The four OSPF router
types are:
Internal Routers = whose OSPF interfaces all belong to the same
area
Backbone Routers = which are Internal Routers in Area 0
Area Border Routers = which have OSPF interfaces in more
than one area
Autonomous System Boundary Routers = which advertise external routes into the
OSPF Domain
What are the four OSPF path types?
The four OSPF path
types are:
Intra-area paths
Inter-area paths
Type 1 external paths
Type 2 external paths
What is the purpose
of the subnets keyword when redistributing OSPF?
Without the Subnets keyword,
only major network addresses that are not directly connected to the router will
be redistributed.
What are the five
OSPF network types?
The four OSPF network
types are:
Point-to-point networks
Broadcast networks
Non-broadcast
Non-broadcast multi-access (NBMA) networks
Point-to-multipoint networks
Note- To change network
type we use “Router(config-if)# ip ospf network
point-to-multipoint [non-broadcast]” command
What is a Designated Router?
A Designated Router
is a router that represents a multiaccess network, and the routers connected to
the network, to the rest of the OSFP domain.
How does a Cisco
router calculate the outgoing cost of an interface?
Cisco IOS calculates the outgoing cost of an interface as
100/BW, where BW is the configured bandwidth of the interface.
What is the purpose of the
variable IP-OSPF-Transmit-Delay?
This variable adds a specified time to the
age field of an update. If the delay is not added before transmission over a
link, the time in which the link-state advertisement (LSA) propagates over the
link is not considered. The default value is 1 second. This parameter has more
significance on very low-speed links.
What is a partitioned
area?
An area is
partitioned if one or more of its routers cannot send a packet to the area's
other routers without sending the packet out of the area.
What is a virtual link?
A virtual link is a
tunnel that extends an OSPF backbone connection through a non-backbone area.
What is the difference between OSPF network
entries and OSPF router entries?
OSPF network entries
are entries in the route table, describing IP destinations. OSPF router entries
are entries in a separate route table that record only routes to ABRs and
ASBRs.
Which three fields in
the LSA header distinguish different LSAs? Which three fields in the LSA header
distinguish different instances of the same LSA?
The three fields in the LSA header that distinguish
different LSAs are the Type, Advertising
Router, and the Link State ID fields. The three fields in
the LSA header that distinguish different
instances of the same LSA are the Sequence Number, Age, and
Checksum fields.
Is it true that only the static
option of the virtual link in OSPF allows discontiguous networks, regardless of
the mask propagation properties?
No, virtual links in OSPF maintain
connectivity to the backbone from nonbackbone areas, but they are unnecessary
for discontiguous addressing. OSPF provides support for discontiguous networks
because every area has a collection of networks, and OSPF attaches a mask to
each advertisement.
What does the clear ip ospf
redistribution command do?
The clear ip ospf
redistribution command flushes all the type 5 and type 7 link-state
advertisements (LSAs) and scans the routing table for the redistributed routes.
This causes a partial shortest path first algorithm (SPF) in all the routers on
the network that receive the flushed/renewed LSAs. When the expected
redistributed route is not in OSPF, this command may help to renew the LSA and
get the route into OSPF.
The only time that OSPF forms adjacencies
between neighbors that are not on the same subnet is when the neighbors are
connected through point-to-point links. This may be desired when using
the ip unnumbered command, but in all other cases, the
neighbors must be on the same subnet.
OSPF sends out its self-originated LSAs when
the LSA age reaches the link-state refresh time, which is 1800 seconds. For
more information, refer toLink-State Advertisements.
To stop routers from becoming OSPF neighbors
on a particular interface, issue the passive-interface command
at the interface.
In Internet service provider
(ISP) and large enterprise networks, many of the distribution routers have more
than 200 interfaces. Configuring passive-interface on each of
the 200 interfaces can be difficult. The solution in such situations is to
configure all the interfaces as passive by default using a single passive-interface
default command. Then, configure individual interfaces where
adjacencies are desired using the no passive-interfacecommand. For
more information, refer to Default Passive Interface
Feature.
There are some known problems
with the passive-interface default command. Workarounds are
listed in Cisco bug ID CSCdr09263 ( registeredcustomers only) .
When I
have two type 5 link-state advertisements (LSAs) for the same external network
in the OSPF database, which path should be installed in the IP routing table?
When you have two type 5 LSAs for the same external
network in the OSPF database, prefer the external LSA that has the shortest
path to the Autonomous System Boundary Router (ASBR) and install that into the
IP routing table. Use the show ip ospf border-routers command
to check the cost to the ASBR.
Why is it
that my Cisco 1600 router does not recognize the OSPF protocol?
Cisco 1600 routers require the Plus feature
set image of Cisco IOS Software to run OSPF. Refer to Table 3: Cisco 1600 Series
Routers Feature Sets in the Release Notes for Cisco IOS
Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for
more information.
Why is it
that my Cisco 800 router does not run OSPF?
Cisco 800 routers do not support OSPF.
However, they do support Routing Information Protocol (RIP) and Enhanced
Interior Gateway Routing Protocol (EIGRP). You can use the Software Advisor ( registered customers only) tool for
more information on feature support.
Should I
use the same process number while configuring OSPF on multiple routers within
the same network?
OSPF, unlike Border Gateway Protocol (BGP) or
Enhanced Interior Gateway Routing Protocol (EIGRP), does not check the process
number (or autonomous system number) when adjacencies are formed between
neighboring routers and routing information is exchanged. The only case in
which the OSPF process number is taken into account is when OSPF is used as the
routing protocol on a Provider Edge to Customer Edge (PE-CE) link in a
Multiprotocol Label Switching (MPLS) VPN. PE routers mark OSPF routes with the
domain attribute derived from the OSPF process number to indicate whether the
route originated within the same OSPF domain or from outside it. If the OSPF
process numbering is inconsistent on PE routers in the MPLS VPN, the domain-id OSPF
mode command should be used to mark that the OSPF processes with different
numbers belong to the same OSPF domain.
This means that, in many
practical cases, you can use different autonomous system numbers for the same
OSPF domain in your network. However, it is best to use consistent OSPF-process
numbering as much as possible. This consistency simplifies network maintenance
and complies with the network designer intention to keep routers in the same
OSPF domain.
I have a
router that runs Cisco Express Forwarding (CEF) and OSPF, who does
load-balancing when there are multiple links to a destination?
CEF works by performing the switching of the
packet based on the routing table which is populated by the routing protocols
such as OSPF. CEF does the load-balancing once the routing protocol table has
been calculated. For more details on load balancing, refer to How does load-balancing work?
OSPF uses the metric aCost, which is related
to the bandwidth. If there are equal cost paths (the same bandwidth on both
multilinks), OSPF installs both routes in the routing table. The routing table
tries to use both links equally, regardless of the interface utilization. If
one of the links in the first multilink fails, OSPF does not send all the
traffic down the second multilink. If the first multilink peaks 100%, OSPF does
not send any traffic down the second multilink because OSPF tries to use both
links equally, regardless of the interface utilization. The second is used
fully only when the first multilink goes down.
In order to have a rapid fault detection of
topology changes, the hello timer value needs to be set to 1 second. The hold
timer value, which is is four times that of the hello timer, also needs to be
configured. There is a possibility of more routing traffic if the hello and
hold timer values are reduced from their default values.
Note: Tuning OSPF Timers might result in network as
well device resource overhead. Cisco recommends to use Bidirectional Forwarding
Detection (BFD) instead of tuning the routing protocol timers. BFD also gives
sub-second convergence. Refer toOSPF Support for BFD over IPv4 for
more information.
Yes, the 3800 Series Router that runs
Advanced IPServices image supports the OSPF Stub feature.
What does
the error message %OSPF-4-FLOOD_WAR: Process process-id re-originates LSA
ID ip address type-2 adv-rtr ip address in
area area id means?
The error message is due to the some router
that is flushing the network LSA because the network LSA received by the router
whose LSA ID conflicts with the IP address of one of the router's interfaces
and flushes the LSA out of the network. For OSPF to function correctly the IP
addresses of transit networks must be unique. If it is not unique the
conflicting routers reports this error message. In the error message the router
with the OSPF router ID reported as adv-rtr reports this message.
Yes, refer to Configuring a GRE Tunnel over
IPSec with OSPF.
Is there
a way to manipulate and prefer the Type 3 LSAs to originate from two different
areas sent to the non-backbone area?
Type 3 LSA is originated by the Area Border
Router (ABR) as a summary route. Manipulating the summary route is not possible
in an ABR router.
Is there
a drop/flap of an OSPF neighborship when changing an OSPF area type from nssa
no-summary to nssa?
When the NSSA ABR is configured to move from
nssa no-summary to nssa, the OSPF neighborship does not flap.
In
the %OSPF-5-ADJCHG: Process ID, Nbr [ip-address] on Port-channel31 from
FULL to EXSTART, SeqNumberMismatcherror message, what
does SeqNumberMismatch signify?
The OSPF neighbor was changed state from FULL
to EXSTART because of the receipt of a Database Description (DBD) packet from
the neighbor with an unexpected sequence number.
SeqNumberMismatch means that
a DBD packet during OSPF neighborship negotiation has been received that
either:
- has an unexpected DBD sequence number
- unexpectedly has the Init bit set
- has an Options field differing from the
last Options field received in a Database Description packet.
Cisco IOS has a limit of 32 routing
processes. Two of these are saved for static and directly connected routes. The
Cisco 7600 router supports 28 OSPF processes per VRF.
How does ISPF impact or improve the OSPF network?.
Incremental SPF is more efficient than the
full SPF algorithm, thereby allowing OSPF to converge faster on a new routing
topology in reaction to a network event. The incremental SPF is designed in
such a way that it only updates the affected nodes without rebuilding the whole
tree. This results in a faster convergence and saves CPU cycles because the
unaffected nodes do not need to be processed. Concerning the best practice ISPF
would make more of a difference for a large OSPF domain.
Incremental SPF provides greater
improvements in convergence time for networks with a high number of nodes and
links. Incremental SPF also provides a significant advantage when the changes
in the network topology are further away from the root of the SPT; for example,
the larger the network the more significant the impact. A segment of 400-1000
nodes should see improvements. However, it might be hard to verify in a
deployed production network without some kind of facility or tool to measure
the end-to-end delay. For more information, refer to OSPF Incremental SPF.
Yes, refer to Cisco NX-OS/IOS OSPF
Comparison.
The OSPF Shortest Path First Throttling
feature makes it possible to configure SPF scheduling in millisecond intervals
and to potentially delay SPF calculations during network instability. SPF is
scheduled to calculate the Shortest Path Tree (SPT) when there is a change in
topology.
Syntax of the command under OSPF:
timers throttle spf [spf-start]
[spf-hold] [spf-max-wait]
Where:
- spf-start—Initial delay to schedule an SPF
calculation after a change, in milliseconds. Range is from 1 to 600000.
- spf-hold—Minimum hold time between two
consecutive SPF calculations, in milliseconds. Range is from 1 to 600000.
- spf-max-wait—Maximum wait time between two
consecutive SPF calculations, in milliseconds. Range is 1 to 600000.
For more information on the OSPF
Throttling feature, refer to OSPF Shortest Path First
Throttling.
What
does BADSEQNUM in the %OSPF-5-NBRSTATE: ospf-101 [5330] Process
101, Nbr 10.253.5.108 on Vlan7 02 from FULL to EXSTART, BADSEQNUM OSPF log
message mean?
This message is related to the DBD exchange
process, which uses a sequence number for the synchronization of the database.
For some reason a bad sequence number was reported in the DBD packet. This
might occur because of transient conditions, which includes packet loss or
packet corruption.
OSPF sends all advertisements using multicast
addressing. Except for Token Ring, the multicast IP addresses are mapped to
MAC-level multicast addresses. Cisco maps Token Ring to MAC-level broadcast
addresses.
Cisco OSPF only supports TOS 0. This means
that routers route all packets on the TOS 0 path, eliminating the need to calculate
nonzero TOS paths.
The offset-list command does
not work for OSPF. It is used for distance vector protocols such as Interior
Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and RIP
version 2.
Can an
OSPF default be originated into the system based on external information on a
router that does not itself have a default?
OSPF generates a default only if it is
configured using the command default-information originate and
if there is a default network in the box from a different process. The default
route in OSPF is 0.0.0.0. If you want an OSPF-enabled router to generate a
default route even if it does not have a default route itself, use the
command default-information originate always.
When I issue the show ip
ospf neighbor command, why do I only
see FULL/DR and FULL/BDR, with all other neighbors
showing 2-WAY/DROTHER?
To reduce the amount of flooding on broadcast
media, such as Ethernet, FDDI, and Token Ring, the router becomes full with
only designated router (DR) and backup designated router (BDR), and it
shows 2-WAY for all other routers.
This is normal. On point-to-point and
point-to-multipoint networks, there are no designated routers (DRs) or backup
designated routers (BDRs).
In addition to the normal OSPF configuration
commands, you should use the dialer map command. When using
the dialer map command, use thebroadcast keyword
to indicate that broadcasts should be forwarded to the protocol address.
A. In addition to the normal OSPF configuration
commands, you should use the async default routing command on
the asynchronous interface. This command enables the router to pass routing
updates to other routers over the asynchronous interface. Also, when using
the dialer map command, use the broadcast keyword
to indicate that broadcasts should be forwarded to the protocol address.
Per-interface authentication type, as
described in RFC 2178 , was added in Cisco IOS Software
Release 12.0(8).
, was added in Cisco IOS Software
Release 12.0(8).
When external routing information is imported
into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has
only area flooding scope. To further distribute the external information, type
7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the
type 7 LSA Options field indicates whether the type 7 LSA should be translated.
Only those LSAs with the P-bit set are translated. When you redistribute
information into the NSSA, the P-bit is automatically set. A possible
workaround applies when the Autonomous System Boundary Router (ASBR) is also an
Area Border Router (ABR). The NSSA ASBR can then summarize with the not-advertise keyword,
which results in not advertising the translated type 7 LSAs.
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