CCNP Exam Prep Tips and Must Knows

Things You’ll Need to Know about OSPF:

OSPF is a link state routing protocol that requires a hierarchical network structure

• Transit Area – Area 0 (Also called Backbone Area)
• Normal areas – non-transit areas

Types of OSPF Routers:

• Backbone Router – A router with all interfaces in Area 0
• Internal Router – a router with all interfaces in a non-backbone area
• ABR – Area Border Router – A router that connects Area 0 to non-backbone area
• ASBR – connects any OSPF area to a different routing administration.  The ASBR is             where external routes can be redistributed into OSPF

OSPF uses cost as a metric.   Cost is calculated per link by the formula 10 to the 8^th / bandwidth           

            This is configurable with the auto-cost reference-bandwidth command (config-router)

OSPF Tables:

• Neighbor Table
• Link State Database
• Routing Table

OSPF Adjacencies:

• Routing updates and topology information are passed only between adjacent routers
• Forming OSPF adjacencies on LAN links is different from forming them on point to point links

Building the LinkState Database (LSDB)

• Hello protocol is used to define neighbors
• Adjacency is established
• Adjacent routers exchange Link State Advertisements (LSA)
• Each router builds a LSDB using LSAs

OSPF Packet Types

• Hello – discovers neighbors and builds adjacencies between them
• Database Description (DBD) – checks for DB synchronization between routers by sending a summary list of all routes in DB
• Links State Request (LSR) – requests specific link state records from another router
• Link State Update (LSU) – Sends specifically requested link state requests
• Link State Acknowledgement (LSAck) – Acknowledges the other packet types

OSPF Hello parameters that must match for neighbors to become adjacent:

• Area ID
• Hello and dead intervals
• DR IP Address
• MTU size
• Authentication (if authentication used)
• Stub area flag (if area is stubbed)

States an interface goes through before becoming adjacent to another router:

• Down – router has not exchanged information with any other router
• INIT – all directly connected routers that are running OSPF receive a hello packet and add each other to their list of neighbors
• Two-way – Each router can see its own Router ID in their neighbor’s hello packet
• Exstart – The DR and BDR establish adjacencies with each other and with each router in the network.  During this process a master-slave relationship is established between each router and its’ adjacent DR and BDR. 
• Exchange – the master and slave routers exchange one or more DBD packets
• Loading – When router receives a DBD packet, it acknowledges it.   It then compares the information it received with its own LSDB.  If it has a more recent link state entry, it sends an LSR to the other router.  The other router sends the complete information about the requested LSU packet.  When the other router receives the LSU, it sends an LSAck.
• Full – When all LSRs have been exchanged for a given router, the adjacent routers are considered synchronized

Adjacency Behavior in Point-to-Point Links

• OSPF packets sent using a destination address of 224.0.0.5

Adjacency Behavior in Broadcast Multiaccess Networks

• DR and BDR selection are required
• All neighbor routers form adjacencies with DR and BDR only
• Packets to the DR and BDR are addressed to 224.0.0.6
• Packets from DR to all other routers is 224.0.0.5

OSPF over Frame-Relay – Five Modes of OSPF Operation are Available

Broadcast – Cisco Extension

• Has one IP subnet 
• Uses multicast OSPF hello packets to discover neighbors
• Elects DR and BDR
• Requires a full-mesh or partial-mesh topology

Nonbroadcast (NBMA) – RFC 2328

• Has one IP subnet
• Requires neighbors to be manually configured
• Elects DR and BDR
• Requires that the DR and BR have full connectivity with all other routers
• Typically used in a full-mesh or partial-mesh topology

Point-to-Multipoint – RFC 2328

• Has one IP subnet
• Uses multicast OSPF hello packets to discover neighbors
• Does not require DR and BDR
• Typically used in a partial-mesh or star topology

Point-to-Multipoint Nonbroadcast – Cisco Extension

• Used in place of RFC compliant Point-to-Multipoint if multicast and broadcast are not enabled on the virtual circuit
• Requires neighbors to be manually configured
• Does not require DR and BDR

Point-to-Point – Cisco Extension

• Unique subnet on each subinterface
• Does not have DR and BDR
• Used when only two routers need to form an adjacency on a pair of interfaces
• Can be used with either LAN or WAN interfaces

LSA Types

• Type 1 – Router LSAs – generated by every router in an area and does not cross an ABR
• Type 2 – Network LSAs – Advertised by DR and does not cross an ABR
• Type 3 – Summary LSAs – Advertised by the ABR of the originating area
• Type 4 – Summary LSAs – Used to advertise a metric to the ASBR and advertised by the ABR of the originating area
• Type 5 – AS external LSAs – Used to advertise network from other Autonomous Systems and is advertised and owned by the originating ASBR (Need type 4 to find the ASBR)
• Type 6 – Multicast OSPF LSAs
• Type 7 – LSAs defined for not-so-stubby areas (NSSA) – used to advertise networks from other Autonomous Systems injected into an NSSA area and is advertised and owned by the originating ASBR.  Translated to type 5 by the NSSA ABR
• Type 8 – External attribute LSAs for BGP
• Types 9, 10 & 11 – Opaque LSAs

Types of OSPF Routes

• – OSPF intra-area routes (router LSA and network LSA)

• O IA – OSPF Interarea routes (summary LSA)
• O E1 – OSPF type 1 external routes (advertised by external LSA)
• E2 – OSPF type 2 external routes (advertised by external LSA) – this is the default for redistributed routes
• E1 – external routes calculate the cost by adding the external cost to the internal cost of each link that the packet crosses
• E2 – the external cost of O E2 packets routes is always the external cost only

OSPF Passive Interface

• The sending and receiving of routing updates is disabled
• The specified interface address appears as a stub network in the OSPF domain

Design Limitations of OSPF

• If more than one area is configured, one must be Area 0, the backbone area
• All areas must be connected to Area 0
• Area 0 must be contiguous

Virtual Links

• An extension of the backbone
• Carried by nonbackbone area
• Cannot be created across a stub or NSSA area
• Used to allow areas to connect to areas other than area 0
• Used to repair a discontiguous Area 0

Area Types

• Backbone Area – connects all other areas
• Normal Area – contains all internal and external routing information
• Stub area – contains internal and area routing  information, but not external routing information
• Totally stubby area – contains area routing information only.  Cisco proprietary
• NSSA – contains area and external routing information

OSPF Authentication

• Simple password authentication
• MD5 authentication
• Router generates and checks every OSPF packet
• The source of each routing update packet received is authenticated
• Each participating neighbor must have the same key (password) configured