Question

Which two OSPFv3 LSAs advertise address prefix information? (Choose two.)

Which is a difference between OSPFv2 and OSPFv3

In the planning stages for an OSPF deployment, which step can be taken to reduce router overhead by limiting the size of the link state databases, and LSA Type 1 and Type 2 propagation?

Which type of LSAs are reduced through interarea summarization?

Answer 1

Q.1: Which two OSPFv3 LSAs advertise address prefix information?

Answer: OSPFv3 is specified in RFC 2740. There are some high-level similarities between the relationships of RIPng to RIPv2 and OSPFv3 to OSPFv2. Most important, OSPFv3 uses the same fundamental mechanisms as OSPFv2—the SPF algorithm, flooding, DR election, areas, and so on. Constants and variables such as timers and metrics are also the same.

Another similarity to the relationship of RIPng to RIPv2 is that OSPFv3 is not backward-compatible with OSPFv2. So if you want to use OSPF to route both IPv4 and IPv6, you must run both OSPFv2 and OSPFv3. As this chapter is being written, there is discussion of adding IPv4 support to OSPFv3, but no specifications have yet been completed. I, for one, hope this support is added, because OSPFv3 is a significantly improved protocol.

It is assumed that you have read the previous chapter and understand the operation of OSPFv2. This section does not repeat that information, but presents the significant differences—primarily operational and LSA formats—of OSPFv3.

OSPFv3 routers use their link-local addresses as the source of hello packets. No IPv6 prefix information is contained in hello packets. Multiple IPv6 addresses can be configured on a link. None of the addresses are defined as secondary addresses, as is done with IPv4 to configure multiple addresses on a single link. Two routers will become adjacent even if no IPv6 prefix is common between the neighbors except the link-local address. This is different from OSPFv2 for IPv4. OSPFv2 neighbors will only become adjacent if the neighbors belong to the same IP subnet, and the common subnet is configured as the primary IP address on the neighboring interfaces. In Figure 9-15, Hedwig is configured with both 2001:db8:0:4::/64 and 2001:db8:0:5::/64 on its Ethernet interface. Pigwidgeon is configured with 2001:db8:0:5::/64 and Crookshanks is configured with 2001:db8:0:4::/64 on their Ethernet interfaces.

OSPFv3 LSA Types

Each LSA begins with a standard 20-byte LSA header. Each LSA describes a piece of OSPF routing domain. All LSAs are flooded throughout the OSPF routing domain. The flooding is reliable, ensuring all routers have the same collection of LSAs. This collection of LSAs is called link-state database (LSDB). From the LSDB, each router constructs the shortest-path tree with itself as the root. This yields a routing table.

LSA Header:

This header contains enough information to uniquely identify each LSA. The LS Type, Link State ID and the Advertising Router field are used to uniquely identify an LSA.

Different instances of the same LSA could be present. The most recent instance could be identified using LS Age, LS Sequence number and LS Checksum fields present in the LSA Header.

LS Age: Time in seconds since the LSA was originated.
LS Type: Indicates the function performed by the LSA.
Link State ID: Together with LS Type and Advertising Router, uniquely identifies the LSA in the LSDB
Advertising Router: The Router ID of the router that originated the LSA
LS Sequence Number: detects old or duplicate LSA
LS Checksum: Complete checkcum of the LSA including the LSA Header but excluding the LS Age field
length: The length in bytes of the LSA including 20-bytes for LSA Header

Router LSA:

Each OSPF router originates Router LSAs indicating the state and cost of the router's interfaces to the area. Router LSAs are flooded throughout the single area only.

A router may originate one or more Router LSAs, distinguished by their Link State IDs. The receiving router concatenates the Router LSAs if it receives more than one Router LSA from a single router.

The Router LSA indicates if the router is an ASBR or an ABR or if it is one end-point of a virtual link. These LSAs have no address information.

Network LSA:

Network LSAs are originated by the DR for a broadcast or NBMA network in the area which supports two or more routers. The LSA describes all routers connected to the link, including the DR. The LSA's Link State ID field is set to the Interface ID that the DR has been using in Hello packets. No address information is carried in the Network LSA.

Inter-Area Prefix LSA:

These LSAs are IPv6 equivalent of IPv4's Type-3 Summary LSAs. These LSAs are originated by the ABR to specify IPv6 prefixes that belong to other areas. A separate LSA is originated for each address prefix.

For Stub areas, the Inter-area Prefix LSA is used to describe a default route. The prefix length of the default route is set to 0.

Inter-Area Router LSA:

These LSAs are IPv6 equivalent of IPv4's Type-4 Summary LSAs. Originated by the ABR, the Inter-Area Router LSA describes the route to the ASBR. Each LSA describes a route to a single router.

Intra-Area Prefix LSA:

A router uses Intra-Area Prefix LSA to advertise IPv6 prefixes that are associated with
a) the router itself (in IPv4, this was carried in Router LSA)
b) an attached stub network segment (in IPv4, this was carried in Router LSA)
c) an attached transit network segment (in IPv4, this was carried in Network LSA)

A router can originate multiple Intra-Area Prefix LSAs for each router or transit network; each LSA is distinguished by its Link State ID.

Options field:

The 24-bit Options field is included in Hello and DBD packets, and Router, Network and Inter-area Router LSAs. It enables OSPF routers to support optional capabilities, and to communicate their capabilities to other OSPF routers in the network.

Q.2: Which is a difference between OSPFv2 and OSPFv3

Answer: OSPFv2 stands for Open Shortest Path First version 2 and OSPFv3 stands for Open Shortest Path First version 3. OSPFv2 is the IPv4’s OSPF version, whereas OSPFv3 is the IPv6’s OSPF version. In OSPFv2, many OSPF instances per interface are not supported, whereas in OSPFv3, many OSPF instances per interface are supported.

There are few similarities present between OSPFv3 and OSPFv2, which are:

1. Packet Type
2. Interface Type
3. Neighbor Discovery Pattern
4. LSA flooding & aging

Besides these similarities, there are some differences which are given below:

OSPFV2	OSPFV3
OSPFv2 is the IPv4’s OSPF version.	While OSPFv3 is the IPv6’s OSPF version.
The header size of OSPFv2 is 24 bytes.	While the header size of OSPFv2 is 16 bytes.
OSPFv2 have seven link-state advertisement.	OSPFv3 have nine link-state advertisement.
There is only one instance per link, in OSPFv2.	While there are many instances per link, in OSPFv3.
In OSPFv2, many OSPF instances per interface are not supported.	Whereas in OSPFv3, many OSPF instances per interface are supported.
There is no flooding space in OSPFV2.	While there is present flooding space in OSPFv3.
OSPFv2 runs on subnets rather than links.	While OSPFv3 runs on links rather than subnets.
Need network mask for adjacency formation.	Doesn’t need network mask for adjacency formation.
MD5 Hashing is used for Authentication.	IPSec is used for Authentication.
Nodes from different subnets can’t communicate.	Nodes from different subnets can communicate.

Q.3: In the planning stages for an OSPF deployment, which step can be taken to reduce router overhead by limiting the size of the link state databases, and LSA Type 1 and Type 2 propagation?

Answer: The Open Shortest Path First (OSPF) protocol, is an Interior Gateway Protocol used to distribute routing information within a single Autonomous System.

OSPF protocol was developed due to a need in the internet community to introduce a high functionality non-proprietary Internal Gateway Protocol (IGP) for the TCP/IP protocol family. The discussion of the creation of a common interoperable IGP for the Internet started in 1988 and did not get formalized until 1991. At that time the OSPF Working Group requested that OSPF be considered for advancement to Draft Internet Standard.

The OSPF protocol is based on link-state technology, which is a departure from the Bellman-Ford vector based algorithms used in traditional Internet routing protocols such as RIP. OSPF has introduced new concepts such as authentication of routing updates, Variable Length Subnet Masks (VLSM), route summarization, and so forth.

Link-States

OSPF is a link-state protocol. We could think of a link as being an interface on the router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on. The collection of all these link-states would form a link-state database.

Link-State Database Synchronization

OSPF Design Tips

The OSPF RFC (1583) did not specify any guidelines for the number of routers in an area or number the of neighbors per segment or what is the best way to architect a network. Different people have different approaches to designing OSPF networks. The important thing to remember is that any protocol can fail under pressure. The idea is not to challenge the protocol but rather to work with it in order to get the best behavior. The following are a list of things to consider.

Number of Routers per Area
The maximum number of routers per area depends on several factors, including the following:

• What kind of area do you have?
• What kind of CPU power do you have in that area?
• What kind of media?
• Will you be running OSPF in NBMA mode?
• Is your NBMA network meshed?
• Do you have a lot of external LSAs in the network?
• Are other areas well summarized?
• For this reason, it's difficult to specify a maximum number of routers per area. Consult your local sales or system engineer for specific network design help.

OSPF uses different types of LSAs (Link States Advertisements) to build a LSDB (Link State Database), which is like a map of the OSPF network topology.
These are the most common LSAs:

Type 1 – Router LSA: The Router LSA is generated by each router for each area it is located. In the link-state ID you will find the originating router’s ID.
Type 2 – Network LSA: Network LSAs are generated by the DR. The link-state ID will be the interface IP address of the DR.
Type 3 – Summary LSA: The summary LSA is created by the ABR and flooded into other areas.
Type 4 – Summary ASBR LSA: Other routers need to know where to find the ASBR. This is why the ABR will generate a summary ASBR LSA which will include the router ID of the ASBR in the link-state ID field.
Type 5 – External LSA: also known as autonomous system external LSA: The external LSAs are generated by the ASBR.
Type 6 – Multicast LSA: Not supported and not used.
Type 7 – External LSA: also known as not-so-stubby-area (NSSA) LSA: As you can see area 2 is a NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type 5). To overcome this issue we are generating type 7 LSAs instead.

Q.4: Which type of LSAs are reduced through interarea summarization?

Answer: Route summarization helps to convert multiple routes into a single route which make routing tables small. After converting a large routing table into a small routing table it is propagated into the backbone area. We have already discussed the type of LSAs in the previous article.

The LSA type1 and type2 are generated inside each area, and then translated into type 3 LSAs, and sent to other areas. Suppose area 1 had 20 networks to advertise, and then 20 types 3 LSAs would be forwarded into the backbone. With summarization, the ABR combine the 20 networks into one of two advertisements.

OSPF doesn’t support automatic summarization and also we cannot summarize routes on every router in an EIGRP network. OSFP can summarize routes only on ABRs and ASBRs routers. Route summarization helps minimizes OSPF traffic and reduces route computation.

In Figure 1, R2 combines all of the network advertisements into one summary LSA. Instead of forwarding individual LSAs for each route in area 10, R2 forwards a summary LSA to area 0. The R3 forward the summary LSA all respected routers in area 20, in this case to R4.

Route Summarization increaase the network's stability because it reduces gratuitous LSA flodding. It also reduces the extra overhead onthe bandwidth, CPU, memory resourcse and routing table process. With route summarization, every specific-link is not propagated into the OSPF backbone and further than, which causing unnecessary network traffics and router overhead.

Fig 2 illustrate that if the network link on R1 fails. R1 sends an LSA to R2 (ABR). But, R2 (ABR) does not propagate the update to the backbone, because it has a summary route configured. Specific-link LSA floddind outside the area does not occur

Interarea and External Route Summarization

As I already discussed earlier in this lesson that summarization in OSPF can only be configured on ABRs or ASBRs. The ABR and ASBR routers advertise only a summary route. ABR routers summarize type 3 LSAs and ASBR routers summarize type 5 LSAs. By default the type 3 and type 5 LSAs do not contain summarized routes; because, by default, summary LSAs are not summarized. Route summarization can be configured as follows:

Inter-area route summarization

OSPF interarea route summarization enables an ABR to summarize neighbouring networks into a single network and advertise the network to other areas. The summarization does not apply to external routes join the OSPF via redistribution. For better route summarization it is important to plan network addresses closely so that these addresses can be summarized into least number of summary addresses.

External route summarization

This is the OSPF route summarization of injected route via redistribution. It is important to make sure the continuity of the external address ranges that are being summarized. The ASBRs can summarize the external routes. We can configure the external route summarization on ASBRs using the summary-address address mask router configuration mode command.