CCNP (Pre 2020) Cisco Routing

OSPF Route Summarisation

Scalability of the routing protocol is an important factor for large networks such as service providers. By splitting up an OSPF network into multiple OSPF areas, it can reduce the size of the LSDB for that area. The number of routers and networks remain the same but Type 1 and Type 2 LSAs are exchanged for summarised Type 3 LSAs.

Type 3 LSAs give an opportunity for further summarisation with larger network prefixes; allowing SPF calculations to run faster and provide easier processing for the CPU and memory. It can do this easily simply by hiding the multiple smaller prefixes behind a larger prefix that is advertised to the rest of the OSPF network.

Interarea Summarisation

Interarea summarisation reduces the number of type 3 LSAs that the area border router (ABR) advertises into an area when it receives type 1 LSAs.

When the Type 1 LSA within the summarisation range reaches the area border router from the source area, the area border router creates a Type 3 LSA for the summarised network range with the more specific type 3 LSAs being suppressed.

The default metric for a summarised LSA is the lowest metric associated with it. For example if there were three more specific prefixes in the summarised prefix consisting of 15, 73, 109, 15 would be the chosen metric used in the summary route.

Configuring Interarea Summarisation

Under the OSPF process configuration mode, enter the command area <area number> range <network> <subnet-mask>

There are optional keywords to the command, advertise or not-advertise. Advertise is implicitly included so does not need to be specified explicitly. Adding a metric on the end with the keyword metric <value> allows for a specific metric to be set.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Discontiguous Networks

If an OSPF topology is created, where traffic will need to cross a non-backbone area to reach a destination, it is known as a discontiguous network.

The fix for a discontigious network is to ensure that Area 0 is contiguous.

There are workarounds, such as GRE tunnels or a OSPF virtual link.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF LSA Type 3: Summary Link

A Type 3 LSA represents an advertisement of a network from another area.

Area Border Routers (ABRs) do not forward Type 1 or 2 LSAs between areas.

If a ABR receives a Type 1 LSA from a area, it creates a Type 3 LSA referencing the network from the Type 1 LSA. The Type 2 LSA is used to determine the network mask of any multi-access networks. The ABR advertises the resulting Type 3 LSA into other areas.

If an ABR receives a Type 3 LSA from area 0, it recreates a new Type 3 LSA for the other area, listing itself as the advertising router with an additional metric cost.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF LSA Type 2: Network Link

The Type 2 LSA represents a multi-access network segment (such as Ethernet) that utilises a DR/BDR.

The DR always advertises the Type 2 LSA, which identifies all routers attached to the multi-access network segment.

If a DR is not yet elected in the segment, there are no Type 2 LSAs in the LSDB. If a new DR is elected or changed, a new Type 2 LSA is created.

Type 2 LSAs are not flooded outside of an area.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF LSA Type 1: Router Link

Every router in an OSPF network advertises a Type 1 LSA.

The Type 1 LSA is a essential part within the link state database.

An entry exists as a Type 1 LSA for every OSPF enabled link including its interface, and attached networks.

Type 1 LSAs are not advertised outside of an area.

The packet fields on a Type 1 LSA give an indication of the Router ID for the advertising router, LSA age, LSA sequence number, link count and link ID.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Link State Advertisements (LSAs)

When OSPF routers become neighbours, they synchronise their Link State Databases between them. If a OSPF router adds or removes a directly connected route from its database, it floods a Link State Advertisment (LSA) out of the OSPF enabled interfaces. There are six types of LSAs that are used in an OSPF network:

TypeLSA NameDescription
1RouterThe LSAs that are within an area
2NetworkA multi-access network attached to a designated router
3SummaryA network prefix that had originated from a different area
4ASBRA summary LSA for a autonomous system boundry router (ASBR)
5AS externalA LSA for a route that had been redistributed into OSPF
7NSSARedistribute routes in not so stubby areas
The six types of link state advertisement

LSA Sequence Numbers

LSAs have sequence numbers in them to help prevent any problems where an LSA may become delayed in network transmission.

The sequence number is a 32-bit value that is incremented each time a LSA is sent out.

If a router receives an LSA greater than the one in the LSDB, it processes it. If the LS is less than that in the LSDB, it is discarded due to it being observed as outdated.

LSA Aging

Each OSPF LSA entered into the Link State Database has an age that increments by 1 every second.

Once the age reaches 30 minutes (1800 seconds) for the networks, the originating router advertises a new LSA with the age set back to 0. Each router will forward this LSA, and it is incremented with a calculated delay that reflects the link.

If the LSA age reaches 3600, the LSA is classified as invalid and is dropped from the link state database.

This ensures the OSPF LSDB has fresh routes, and not anything stale.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Route Types

Intra-Area Routes

A network route learned from another router within the same area is known as a intra-area route.

In show ip route ospf , any routes learned this way are displayed with the key symbol O

These routes are advertised using a Type 1 LSA, and are preferred over Type 3 LSAs.

If multiple intra-area routes exist, the path with the lowest total metric is preferred and installed in the routing table. If there are multiple routes with the same lowest metric, they are installed into the routing table up to a total number of four by default

Inter-Area Routes

Network routes learned from other OSPF routers in a different area through an Area Border Router (ABR) are known as an inter-area route.

show ip route ospf shows any routes learned this way with the key O IA

Interarea routes are similar that the path with the lowest metric is installed into the routing table. It is the same case that if multiple equal lowest metric paths exist, they are installed into the routing table up to a default value of four.

Equal Cost Multipathing

The default number of equal cost metric paths in the routing table can be changed from the default of four. The command maximum-paths <number> in the routing process configuration mode will do this.

OSPF Path Type Selection Priority

Inra area routes are preferred first, followed by Interarea, then External Routes.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Areas

An OSPF area is a logical grouping of OSPF enabled interfaces across multiple routers.

Area IDs

The area is identified by a number or dotted decimal that is assigned at an interface level. The interface can only belong to one area.

The area number is included in OSPF Hello packets that are sent by the router.

The field in the Hello Packet is a 32-bit field, that can be formatted with a number (0 to 4294967295) or dotted decimal ( –

A router with a number on one side and a dotted decimal on the other can still form an adjacency.

Single Area vs Multi Area

All routers within the area maintain an identical link-state database (LSDB)

Using a single area in an OSPF topology can keep things simplified but there are disadvantages:

  • The SPF algorithm will need to run when a link flaps in the area
  • The LSDB will continue to increase in size, consuming more memory and taking long to re-compute
  • There is no summarisation of routes

Splitting up an OSPF domain into multiple areas will keep the size of the link state database manageable.

A router will multiple interfaces in multiple areas will have multiple link state bases, one for each area. The internal topology of one area will not be visible in the other areas database. If a link state change occurs on one area, the other area will be unaware of that internal change and will not need to perform any recalculation of routes.

Area 0

The first area on a router, area 0, is a special type of area. It is known as the backbone area. In an OSPF design, all areas must connect to Area 0. There is an expectation in OSPFs design that all areas will advertise their routing information into Area 0, and Area 0 will advertise all routing information to other areas.

Area Border Routers (ABR)

An Area Border Router is an OSPF enabled router that as an interface in Area 0 and another interface in another OSPF area.

An Area Border Router is responsible for taking routers from one area and and advertising them into the other area.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Network Type: Loopback Networks

The OSPF network type loopback is enabled by default for loopback interfaces, it can also only be used on loopback interfaces.

The network type of loopback will always advertise the host address of the loopback into the network, and not the network itself, even if not configured with a subnet.

Changing the loopback to a point-to-point network type will allow the entire subnet to be advertised into OSPF.

CCNP Enterprise Core (350-401) Cisco Routing

OSPF Network Type: Point to Point

The network circuit that only has two devices on it that can communicate is considered a point to point circuit. Point to point circuits do not require the use of ARP or broadcast traffic.

OSPF sets the network type of point to point by default for serial, GRE, and point-to-point frame relay interfaces. The Hello timer is set to 10 seconds.

As point to point networks do not use a DR/BDR, it has a ‘-‘ in the state field when running the command show ip ospf interface