11. What is ospf router id and how is it elected? Router id is 32 bit number similar to ip address. It is elected as highest ip address of loopback interfaces or highest ip of the active physical interfaces if loopback is not present. It can also be set manually but it will take effect when ospf process is reset completely or device is reloaded. There should be only one router-id per device running ospf to avoid database issues
12. What are lsa types?
Lsa type 1 (router lsa)
Router link state advertisements type 1 are generated by every router for each link that belongs to an area. They are flooded only inside of area to which they belong. Link id of this lsa is the router id of the router that generated it.
Lsa type 2 (network lsa)
Generated by designated router (dr) for multiaccess networks and describe the routers that are connected to that segment. They are sent inside the area to which the network segment belong. The link id is the interface ip address of the designated router which describe that particular segment.
Lsa type 3 (summary lsa)
Type 3 lsas are generated by area border routers (abrs). In type 3 lsas are advertised networks from an area to the rest of the areas in as. Advertised networks in type 4 lsa can be summarized or not. The link-state id used by this lsa is the network number advertised.
Lsa type 4 (summary lsa)
Generated by abrs, this type os lsa contain routes to asbrs. Link id used is router id of the asbr described. Are not flooded in stub areas.
Lsa type 5 (external lsa)
Autonomous system external lsas are generated by asbrs and contain routes to networks that are external to current as. Link-state id is network number advertised in lsa. Type 5 lsas are not flooded inside any stub areas.
Lsa type 6 (multicast lsa)
This type of lsa is used for multicast applications.
Lsa type 7 (nssa external lsa)
Type 7 lsa allow injection of external routes throug not-so-stubby-areas (nssa). Generally external routes are advertised by type 5 lsa but they are not allowed inside any stub area. That’s why type 7 lsa is used, to trick ospf. Type 7 lsa is generated by nssa asbr and is translated into type 5 lsa as it leaves the area by nssa abr, which is then propagated throughout the network as type 5 lsa.
Lsa type 8 (external attributes lsa for border gateway protocol (bgp))
Used to work with bgp.
Lsa type 9,10,11 (opaque lsas)
For future use.
13. What are area types? Why there are different area concepts?
Standard areas can contain lsas of type 1, 2, 3, 4, and 5, and may contain an asbr. The backbone is considered a standard area.
Stub areas can contain type 1, 2, and 3 lsas. A default route is substituted for external routes. Totally stubby areas can only contain type 1 and 2 lsas, and a single type 3 lsa. The type 3 lsa
describes a default route, substituted for all external and inter-area routes.
Not-so-stubby areas implement stub or totally stubby functionality yet contain an asbr. Type 7 lsas generated by the asbr are converted to type 5 by abrs to be flooded to the rest of the ospf domain.
14. What is the main importance of stub network? Why it is been developed in ospf?
A stub network is a somewhat casual term describing a computer network, or part of an internetwork, with no knowledge of other networks, that will typically send much or all of its non-local traffic out via a single path, with the network aware only of a default route to non-local destinations
15. What will happen if we redistribute between different processes?
There can be several reasons for redistribution between multiple processes. These are a few examples:
Filter an ospf route from part of the domain Separate different ospf domains
Migrate between separate domains
16. What is the adjacency type? Is adjacency is formed in between all the ospf enabled routers or not? Explain
Down
This is the first ospf neighbor state. It means that no information (hellos) has been received from this neighbor, but hello packets can still be sent to the neighbor in this state.
During the fully adjacent neighbor state, if a router doesn't receive hello packet from a neighbor within the router dead interval time (routerdead interval = 4*hello interval by default) or if the manually configured neighbor is being removed from the configuration, then the neighbor state changes from full to down.
Attempt
This state is only valid for manually configured neighbors in an nbma environment. In attempt state, the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not been received within the dead interval.
Init
This state specifies that the router has received a hello packet from its neighbor, but the receiving router's id was not included in the hello packet. When a router receives a hello packet from a neighbor, it should list the sender's router id in its hello packet as an acknowledgment that it received a valid hello packet.
2-way
This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router has seen the other's hello packet. This state is attained when the router receiving the hello packet sees its own router id within the received hello packet's neighbor field. At this state, a router decides whether to become adjacent with this neighbor. On broadcast media and non-broadcast multiaccess networks, a router becomes full only with the designated router (dr) and the backup designated router (bdr); it stays in the 2-way state with all other neighbors. On point-to-point and point-to-multipoint networks, a router becomes full with all connected routers.
At the end of this stage, the dr and bdr for broadcast and non-broadcast multiacess networks are elected. For more information on the dr election process, refer to dr election.
Note: receiving a database descriptor (dbd) packet from a neighbor in the init state will also a cause a transition to 2-way state.
Exstart
Once the dr and bdr are elected, the actual process of exchanging link state information can start between the routers and their dr and bdr.
In this state, the routers and their dr and bdr establish a master-slave relationship and choose the initial sequence number for adjacency formation. The router with the higher router id becomes the master and starts the exchange, and as such, is the only router that can increment the sequence number. Note that one would logically conclude that the dr/bdr with the highest router id will become the master during this process of master-slave relation. Remember that the dr/bdr election might be purely by virtue of a higher priority configured on the router instead of highest router id. Thus, it is possible that a dr plays the role of slave. And also note that master/slave election is on a per-neighbor basis.
Exchange
In the exchange state, ospf routers exchange database descriptor (dbd) packets. Database descriptors contain link-state advertisement (lsa) headers only and describe the contents of the entire link-state database. Each dbd packet has a sequence number which can be incremented only by master which is explicitly acknowledged by slave. Routers also send link-state request packets and link-state update packets (which contain the entire lsa) in this state. The contents of the dbd received are compared to the information contained in the routers link-state database to check if new or more current link-state information is available with the neighbor.
Loading
In this state, the actual exchange of link state information occurs. Based on the information provided by the dbds, routers send link-state request packets. The neighbor then provides the requested link-state information in link-state update packets. During the adjacency, if a router receives an outdated or missing lsa, it requests that lsa by sending a link-state request packet. All link-state update packets are acknowledged.
Full
In this state, routers are fully adjacent with each other. All the router and network lsas are exchanged and the routers' databases are fully synchronized.
Full is the normal state for an ospf router. If a router is stuck in another state, it is an indication that there are problems in forming adjacencies. The only exception to this is the 2-way state, which is normal in a broadcast network. Routers achieve the full state with their dr and bdr in nbma/broadcast media and full state with every neighbor in the remaining media such as point-to-point and point-to-multipoint.
Note: the dr and bdr that achieve full state with every router on the segment will display full/drother when you enter the show ip ospf neighbor command on either a dr or bdr. This simply means that the neighbor is not a dr or bdr, but since the router on which the command was entered is either a dr or bdr, this shows the neighbor as full/drother.
17. If router id is same between ospf enabled routers will they form adjacency or not?
Yes
18. What is lsa 7 and how it can be used?
Type 7 lsa allows injection of external routes throug not-so-stubby-areas (nssa). Generally external routes are advertised by type 5 lsa but they are not allowed inside any stub area. That’s why type 7 lsa is used, to trick ospf. Type 7 lsa is generated by nssa asbr and is translated into type 5 lsa as it leaves the area by nssa abr, which is then propagated throughout the network as type 5 lsa.
19. What is ospf metric? How is it identified in routing table? Does it support redistribution?
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.
20. What are the states used in ospf when forming neighbor ship?
2-way. The 2-way state indicates that the local router has received a hello packet with its own router id in the neighbor field. Thus, bidirectional communication has been established and the peers are now ospf neighbors
21. What are the packet types does ospf use? Explain hello, dead timers, lsa, lsu, lsr packets.
Types of ospf packets:
1. The hello packet:
the hello packets are sent over a period of time on all interfaces for the purpose of establishing and maintaining neighbor relationships. Hello packets are multicast on the networks having multicast capability, which enables discovery of neighboring routers dynamically. The inhabitance of differences among hello packets can form neighbor relationships by agreeing certain parameters.
2. The database description packet:
at the time of adjacency is being initialized, these packets are exchanged. These packets describe topological database contents. The database may be described by using multiple packets. A poll-response procedure is used for the description of multiple packets usage. Among the routers, one is designated to be master, and the other a slave. The database description packets are sent by the slave after sending the database description packets by the master.
3. The link state request packet:
a router may find the parts of its topological database are out of date, after database description package exchange with a neighboring router. The link state request packet is utilized for requesting the pieces of the neighbor’s database which are more up to date. There may be a need to utilize multiple link state request packets.
4. The link state update packets:
the flooding of link state advertisements is implemented by these packets. A collection of link state advertisements are carried by each link statement update packet, one hop further from its origin. A packed may be included by several link state advertisements.
5. The link state acknowledge packets:
the reliability of flooding link state advertisement is made by explicitly acknowledging flooded advertisements. The accomplishment of this acknowledgement is done through the sending and receiving of link sate acknowledgement packets. A single link state acknowledgement packet is used to acknowledge the multiple link state advertisements.
22. What will be hello/dead timers in network having link capacity of >t1 and <=t1?
For link above t1 i.e. 1.544mbps hello = 10 seconds, dead is 40 seconds
For link = or below hello = 30 seconds, dead = 120 seconds.
Dead interval is exact four times than hello and its value can’t be set manually.
23. What is the dr/bdr in ospf? How these are elected and in what scenario?
Dr/bdr election happens in shared multi access network to avoid full mesh scenario. In shared environment, every ospf router will try to form neighbor ship with other router, so dr/bdr comes in to action and act as single point of contact. Neighborship will only form with dr and backup dr. Dr router will be the highest priority router and bdr will be second highest priority. If there is a tie, then highest router-id will be used to make the decision. Router id is elected as the highest numerical loopback ip or the highest physical active ip address or the interface ip which comes up first.
24. What is the multicast ips used by ospf? What multicast ip does dr/bdr router and non-dr use?
Ospf routers use 224.0.0.5 multicast ip address. However, in dr/bdr it uses 224.0.0.5 and 224.0.0.6. Dr/bdr will use 224.0.0.6 while replying to ospf routers in broadcast multi access network.
25. 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
26. How to generate default route in ospf?
There are two ways to inject a default route into a normal area. If the asbr already has the default route in its routing table, you can advertise the existing 0.0.0.0/0 into the ospf domain with the default-information originate router configuration command
27. How external routes are advertised generally (e1 or e2)? How to change from e2 to e1?
When we redistribute a protocol in ospf, by default itaccept those route as e2 in ospf domain. E1 & e2 is external type 1 and rxternal type 2. In cisco external route are e2 by nature. Basically difference between e1 and e2 is : e1 includes internal cost to asbr added to external cost, e2 does not include internal cost. It is same as external cost.E1 is prefered if route is equal cost path exist. Example: r1-r2-r3 is connected and r3 is a asbr. And suppose a cost of between r1 to r2 is 10 and r2 to r3 is 10. Cost of external router on r3(asbr) is 1800. Now if you thik about e1 cost to external route from r1 is (10+10+1800) 1820 but in case of e2 it is 1800.. E1 is usefull when there are multiple asbr to same destination and e2 is usefull when there is only one asbr in to reach destinaltion
28. What is ospf super backbone area?
The idea is that this super backbone can override the default behavior of ospf and provide connectivity between customers without the need for crazy virtual-links or tunneling (can’t use vls across area 0 anyway).
29. What is the concept of auto-cost in ospf?
The default reference bandwidth for OSPF is 10^8 bps or 100Mbit. Increasing the reference-bandwidth allows a more granular OSPF design. If changed it should be changed on all routers in the OSPF domain.The reason you would change the reference-bandwidth is that you may have link faster than 100M in your network. If you have Gigabit networks but are using the default reference-bandwidth, then Gigabit links are equal in cost to fast Ethernet.
device# configure
device(config)# router ospf
device(config-ospf-router)# auto-cost reference-bandwidth 500
30. Is there any dr and bdr concept in /31 networks?
No
31. What are the different network types in ospf?
Ospf network types
Having worked almost exclusively with ethernet transport my whole career, it took me a while to really grasp the concept of non-broadcast networks. Dynamic routing protocols, particularly ospf, demand familiarity with all sorts of layer two topologies, so i knew i had to better educate myself on the matter. Fortunately, working withdynamips and virtual frame relay networks provided the experience i needed to feel comfortable implementing all the different ospf network types.
Ospf addresses three classes of network (as listed in section 1.2 of rfc 2328): point-to-point, broadcast, and non-broadcast.
Point-to-point
This is by far the simplest network type, and serves as a convenient anchor from which to advance the discussion. A point-to-point network is, as its name aptly describes, a link between exactly two points (or routers). A packet sent from on of the routers will always have exactly one recipient on the local link.
Broadcast
Obviously, point-to-point links don't scale well. A much more efficient manner of connecting a large number of devices is to implement a multiaccess segment; that is, a segment which can be accessed by multiple end points. An ethernet segment is an example of such a network.
Ethernet networks support broadcasts; a single packet transmitted by a device can be multiplied by the medium (in this case an ethernet switch) so that every other end point receives a copy. This is advantageous not only in bandwidth savings, but also in facilitating automatic neighbor discovery.
In the example pictured above, r1 can multicast (a broadcast intended only for certain recipients) an ospf hello message to the link, knowing that all other ospf routers connected to the link will receive it and reply with their own multicast message. Consequently, neighbors can quickly identify each other and form adjacencies without knowing addresses beforehand. Isn't that convenient?
Ospf routers on a multiaccess segment will elect a designated router (dr) and backup designated router (bdr) with which all non-designated routers will form an adjacency. This is to ensure that the number of adjacencies maintained does not grow too large; a segment containing ten routers would require 45 adjacencies to form a mesh, but only 17 when a dr and bdr are in place.
Non-broadcast
Unfortunately, not all multi access technologies support broadcast transmissions. Frame relay and atm are probably the most common examples of non-broadcast transport, requiring individual permanent virtual circuits (pvcs) to be configured between end points.
Notice in the frame relay topology pictured above, r1 must craft and transmit an individual packet for every destination he wants to reach. Aside from being horribly inefficient with regard to bandwidth, this limitation requires the router to know the addresses of his neighbors before he can communicate to them.
Ospf can operate in one of two modes across a non-broadcast network: non-broadcast multi-access (nbma) or point-to-multipoint. Each of these topologies tackles the absence of broadcast capability from a different direction.
Non-broadcast multi-access (nbma)
An nbma segment emulates the function of a broadcast network. Every router on the segment must be configured with the ip address of each of its neighbors. Ospf hello packets are then individually transmitted as unicast packets to each adjacent neighbor.
As in a true broadcast network, a dr and bdr are elected to limit the number of adjacencies formed.
Point-to-multipoint
A point-to-multipoint configuration approaches the non-broadcast limitation in a different manner. Rather than trying to emulate broadcast capability, it seeks to organize the pvcs into a collection of point-to-point networks. Hello packets must still be replicated and transmitted individually to each neighbor, but the multipoint approach offers two distinct advantages: no dr/bdr is needed, and the emulated point-to-point links can occupy a common subnet.
All routers attached to a non-broadcast network must be manually configured to recognize it as a point-to-multipoint segment:
Router(config-if)# ip ospf network point-to-multipoint [non-broadcast]
The non-broadcast parameter can be appended to the ospf network type to force unicasting of packets rather than relying on multicast. This might be necessary when dynamic circuits are in use.
32. How does ospf calculate it’s metric or cost?
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.
33. What algorithm is used by ospf if equal cost routes exist?
If equal cost routes exist, ospf uses cef load balancing.
34. 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.)
35. 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.
36. 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.
37. 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 originates always.
38. 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. The distribute-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.
39. How can i give preference to ospf inter area routes over intra-area routes?
According to section 11 of rfc 2328 , 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.
40. 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.
41. When routes are redistributed between ospf processes, are all shortest path first algorithm (spf) metrics preserved, or are the default metric value used?
The spf metrics are preserved. The redistribution between them is like redistribution between any two ip routing processes.
42. How does cisco accommodate ospf routing on partial-mesh frame relay networks?
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.
43. Which address-wild-mask pair should i use for assigning an unnumbered interface to an area?
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.
44. Can i have one numbered side and leave the other side unnumbered in ospf?
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.
45. Why do i receive the “cannot allocate router id” error message when i configure router ospf one?
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.
46. Why do i receive the “unknown routing protocol” error message when i configure router ospf one?
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.
47. Is it true that only the static option of the virtual link in ospf allows discontinuous networks, regardless of the mask propagation properties?
No, virtual links in ospf maintain connectivity to the backbone from non backbone 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.
48. Are the multicast ip addresses mapped to mac-level multicast addresses?
No, virtual links in ospf maintain connectivity to the backbone from non backbone 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.
49. Does the cisco ospf implementation support ip tos-based routing?
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.
50. Does the offset-list subcommand work for ospf?
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.
51. Do i need any special commands to run ospf over bri/pri links?
In addition to the normal ospf configuration commands, you should use the dialer map command. When using the dialer map command, use the broadcast keyword to indicate that broadcasts should be forwarded to the protocol address.
52. Which cisco ios software release began support for per-interface authentication type in ospf?
Per-interface authentication type, as described in rfc 2178 , was added in cisco ios software release 12.0(8).
53. How can you detect the topological changes rapidly?
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 using bidirectional forwarding detection (bfd) instead of tuning the routing protocol timers. Bfd also gives sub-second convergence. Refer to ospf support for bfd over ipv4 for more information.
54. How does ospf use two multilink paths to transfer packets?
Ospf uses the metric a cost, which is related to the bandwidth. If there are equal cost paths (the same bandwidth on both multilink), 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.
