Understanding Multi-Area OSPFv3
Before we look at Multi-area OSPF, it is important that you understand OSPF single area concepts and configuration.
OSPF is more efficient than distant vector routing protocols, any routing changes trigger OSPF routing updates which is propagated quickly to neighbors.
How Does OSPF Work?
• OSPF is a link-state routing protocol, which is a routing protocol that makes its routing decisions based on the state of the links that connect source and destination devices.
• The interface information includes the IPv6 prefix on the interface, the type of network it is connected to, and the routers connected to that network.
• OSPF routers generate routing updates only when a change occurs in the network topology.
• When a link changes state, the device that detects the change creates an LSA and forwards it to the DR using FF02::6 multicast address who informs all devices within an area using FF02::5 multicast address. Each device then updates its Link State Database.
OSPF Single Area Characteristics:
* Its a Link State Routing Protocol.
* Faster Convergence.
* Cost Metric (Cisco – Bandwidth).
* It maintains three Identical Link-State Databases (Adjacency database, Forwarding database and LSDBs).
* SPF – Dijkstra’s Algorithm.
* Determine Neighbors on Directly-connected links.
* Use Link-State Packets (LSP) for each directly-connected link.
* Flood LSPs to neighbors.
OSPF Packet Types:
OSPF uses five packet types in exchanging messages about up to date routing information to maintain accurate information about the network with neigbors.
These packets are:
Hello packet: used to discover and build adjacencies with neigbors.
Database description packet: used for checking database synchronization with other OSPF routers.
Link-state request packet: used to request specific link-state information from other routers.
Link-state update packet: used for sending specific requested link-state information to neighbors.
Link-state acknowledgment packet: used to acknowledge other packet types.
Trouble with OSPF Configuration in a Large Network.
• Large routing table : route summarization in OSPF is disabled by default. If the routes are not summarized, the routing table can become very large, depending on the size of the network.
• Large link-state database (LSDB) : Because the LSDB covers the topology of the whole network, each router must maintain an entry for every network in the area, even if not every route is selected for the routing table.
• Frequent SPF algorithm calculations: when there is a change in a large network, network routers spend many CPU cycles recalculating the SPF algorithm and updating the routing table.
The need for multiarea ospf.
An OSPF area is a logical collection of OSPF networks or group of routers that share the same link-state information in their link-state databases.
If you are managing a large network, to make OSPF more efficient and scalable, the network can be divided into multiple OSPF areas.
Multiarea OSPF requires a hierarchical network design and the main area is called the backbone area (area 0) and all other areas must connect to the backbone area.
Advantages of MultiArea OSPF .
To divide your large OSPF network into area has the following advantages:
• Reduced frequency of SPF calculations: Detailed route information exists within each area, link-state changes not flooded to other areas.
• Smaller routing tables: Instead of advertising these explicit routes outside the area, routers can be configured to summarize the routes into one or more summary addresses.
Reduced LSU overhead: Rather than send an LSU about each network within an area, a router can advertise a single summarized route or small number of routes between areas.
OSPF Two-Layer Area Hierarchy.
Each OSPF configured network that is divided into areas must follow these rules:
Backbone area :
Area whose primary function is the fast and efficient movement of IP packets.
• Interconnect with other OSPF area types
• Known as OSPF area 0 which all other areas must directly connect to.
Regular area (Non-backbone) :
• Connects users and resources.
• A regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must cross through area 0.