OSPF

#defs_ccna

OSPF

Open Shortest Path First (OSPF) is an Interior Gateway>Link State Routing Protocol. It uses "Cost" as the metric, which is automatically derived from interface bandwidth by default

5 OSPF network types (2 main, 3 odd)

  1. 2 Main
    1. Broadcast
      1. Enabled on Ethernet and FDDI links by default
      2. Routers elect DR/BDR to reduce overhead
      3. Default Hello interval is 10 seconds, default dead time 40 seconds
    2. Point-to-Point
      1. Enabled on PPP and HDLC interfaces by default
      2. Simplifies OSPF operation, no DR/BDR elections
      3. Default Hello interval is 10 seconds, default dead time 40 seconds
  2. 3 Odd (non-broadcast networks, like Frame Relay, X.25, and ATM (Asynchronous Transfer Mode)
    1. Non-broadcast multi-access (NBMA)
      1. Emulates a broadcast network (DR/BDR elections)
      2. Every router must be configured with the IP address of its neighbors
      3. Creates Permanent Virtual Circuits (PVCs) to be configured between endpoints
      4. Default Hello interval is 30 seconds
    2. Point-to-multipoint
      1. Emulates a point-to-point network
        1. Organizes PVCs into a collection of P2p networks
        2. Hello packets are replicated and transmitted individually to each neighbor
      2. Default Hello is 30 seconds
    3. Point-to-multipoint (non-broadcast)
      1. non-broadcast can be appended to P2m networks to force unicast packets instead of multicast packets
      2. Default Hello is 30 seconds

OSPF Process

  1. Discover neighbors
    1. Hello Packets
      1. When OSPF enabled, a router will send out and listen for Hello packets to form adjacencies with other OSPF routers
  2. Form Adjacencies
  3. Flood Link State Database (LSDB)
    1. First sends DBD (DataBase Description)
      1. Adjacent routers will tell each other their known networks
    2. If a router is missing a route, it will send a Link State Request (LSR)
    3. The supplying router will respond with a Link State Update (LSU), which contains one or more Link State Advertisements (LSA)
    4. After receiving an LSU, the receiver will send an LSAck (Link State Acknowledgement) back to the sender
      1. If the sender doesn't receive an LSAck, it will resend the LSU
  4. Compute Shortest Path
  5. Install best routes to routing table
  6. Respond to network changes

OSPF Areas and Route Types

Router Types

  1. Backbone Routers
    1. Routers which have all of their OSPF interfaces in Area 0
  2. Area Border Routers (ABRs)
    1. Routers that have interfaces in multiple areas
    2. Separates LSA flooding zones
    3. Is the primary point for area address summarization
    4. Maintains the LSDB for each area
    5. ABRs do not automatically summarize routes
    6. Routes to other areas appear as InterArea (IA) routes
  3. Internal Routers (also Normal Area Routers)
    1. Routers that have all their interfaces in a normal area (area 1+)
    2. Maintain a full LSDB of routers and links in their area
    3. Learn Inter-Area (IA) routes from their ABRs
  4. Autonomous System Boundary Routers (ASBR)
    1. Redistribute other areas into OSPF
    2. Routes that are redistributed into OSPF appear as External routes

OSPF Packets

  1. Hello packet (message type 1)
    1. When OSPF enabled, a router will send out and listen for Hello packets to form adjacencies with other OSPF routers
  2. DBD (DataBase Description) (message type 2)
    1. Adjacent routers will tell each other their known networks
  3. LSR (Link State Request) (message type 3)
    1. If a router is missing information, it will send a Link State Request
  4. LSU (Link State Update) (message type 4)
    1. LSU's are replies to LSR's
    2. Contains one or more LSA's (Link State Advertisements) which should be updated
        1. LSA (Link State Advertisement) is a routing update
    3. When an LSU is sent between routers, it floods the LSA information through the network
  5. LSAck (Link State Acknowledgement) (message type 5)
    1. Sent by receiving routers after getting the LSAs
    2. Notifies the source OSPF router that the LSA advertised by the LSU has been properly received
    3. The LSU will be re-sent if an LSAck is not received

OSPF LSA types

LSA Name Generated By Function Flooding Map
Type 1 Router Normal Area Routers Advertising router's interface and status to neighbors Intra-area (area of origin)
Type 2 Network DR Advertising DRs direct-connected neighbors Intra-area (area of origin)
Type 3 Summary ABR Advertising ABRs' areas summary Inter-Area (multiple areas)
Type 4 Summary ASBR ABR Advertising the presence of ASBRs Inter-Area (multiple areas)
Type 5 AS External ASBR Advertising external routes (to internet) Inter-Area (multiple areas)
Type 7 Not-So-Stubby Area ASBR Advertising external routes to NSSA areas Inter-Area (multiple areas)

OSPF Configuration

  1. Process ID
    1. A Process ID is the ID of the OSPF process to which to the interface belongs, and it is local to the router
      1. Process ID's don't need to match between routers
    2. It can be used to create two or more overlapping OSPF regions that don't exchange routing information
    3. Typically only 1 process ID is used
    4. Each process ID can be configured
  2. Network
    1. The Network command means "Look for interfaces with an IP address that falls within a range and enable OSPF on those ports with a specific area"
      1. It uses a Wildcard mask to identify relevant ports, add them to an area, and then advertise the networks configured on those ports
  3. Passive-Interface
    1. Passive interfaces do not form adjacencies and do not provide internal information
    2. Should be configured on Loopback interface
    3. "default" sets all interfaces to be passive by default
  4. IP Route
    1. Static route injection
  5. Auto-cost Reference-bandwidth
    1. Change the reference bandwidth when calculating cost; default is 100, should be set to 1000000 or higher
  6. Router ID
    1. Formed as an IP address, typically the highest IP address of the loopback or highest IP on the router if loopback is not configured
    2. Can be set manually, but best practice is to use the loopback
    3. Router ID updates when the OSPF process restarts (read: router reboots)

OSPF DR (Designated Router) and BDR (Backup Designated Router)

  1. OSPF on multiaccess segments
    1. On point-to-point links, OSPF router pairs form a FULL adjacency
    2. On multiaccess segments (such as Ethernet), where there can be multiple routers, it is inefficient for all routers to form a FULL OSPF adjacency with each other
      1. Rather than sharing full information, just share information with the Designated Router (DR) to fully manage routing
        1. If the DR goes down, there could be a big problem
      2. Backup Designated Router (BDR) adds redundancy
  2. DR, BDR, and DROTHER
    1. DR (designated router) and BDR (backup designated router) are elected based on priority and router ID
      1. The router with the highest priority becomes the DR, and the router with the second-highest priority is BDR
        1. Default priority is 1, the highest is 255
        2. 0 says "this router will never be the DR"
      2. The highest Router ID is used in case of a tie
      3. All other routers on the network establish FULL neighbor state with the DR and BDR routers
        1. They will appear as FULL/DR and FULL/BDR
    2. DRother routers are NOT a DR or BDR
      1. DRothers only form full adjacencies with the DR/BDR
        1. On the DR/BDR, the adjacency will look like FULL/DROTHER
      2. DRothers remain the 2WAY state with other DRother routers
        1. They will appear as 2WAY/DROTHER

Hello packets

  1. Sent to the OSPF Multicast address of 224.0.0.5 ('all OSPF routers')
  2. Hello Packet contents
    1. Router ID
      1. 32-bit number that uniquely identifies each OSPF router
    2. Hello Interval
      1. How often router sends Hello packets
    3. Dead interval
      1. How long a router waits to hear from a neighbor before declaring it out of service
        1. Default is 4x Hello Interval
    4. Neighbors
      1. A list of adjacent OSPF routers that this router has received a Hello packet from
    5. Area ID
      1. The area configured for that interface
    6. Router priority
      1. An 8-bit number used to select DR (designated router) and BDR (backup designated router)
    7. DR and BDR IPv4 address (if known)
    8. Authentication Flag
      1. Authentication details if configured
      2. This is important; stops someone from accidentally or maliciously joining a router to your network
    9. Stub area flag
      1. If the area is a stub area
        1. Stub areas have a default route to their ABR rather than learning routs outside the area
! Configure a specific OSPF ID
Config# router ospf (process ID)
!
! Add interfaces to a specific area
Config-router# network (network address) (wildcard mask) area (area number)
!
! Set a passive interface
Config-router# passive-interface ("default"/interface name)
!
! Configure a static route (typically to default gateway)
Config-router# ip route (ip address) (network mask) (IP address of gateway to network)
Config-router# ip route 0.0.0.0 0.0.0.0 (default gateway IP address)

Requirements to form an Adjacency

  1. Area number must match
  2. Interfaces must be on the same subnet
  3. OSPF process must not be shutdown
  4. Unique OSPF Router IDs
  5. Matching Hello and Dead timers
  6. Authentication settings must match
    7. IP MTU (Maximum Transmission Unit) settings must Match
    8. OSPF Network Type must match

NOTE: A mismatch on 7 and 8 will allow routers to become OSPF neighbors, but OSPF will not function properly.

OSPF Neighbor States

  1. Establishing neighbor states
    1. Down - no connection
      1. R1 sends a multicast Hello packet to 224.0.0.5
        1. I am 172.16.1.1, and I have no neighbors
        2. If no response received, link is perceived down after 40 seconds (default)
    2. INIT - Initiate 2-way Communication
    3. 2-Way - Response hello-packet received
      1. R2 replies with a unicast Hello packet to 10.0.0.1
        1. I am 172.16.2.1, and I see 172.16.1.1
          1. If it had other neighbors, it would include them here
      2. R1 send a unicast hello packet to 10.0.0.2
        1. I am 172.16.1.1 and I see 172.16.2.1
      3. Routers are now in a 2-way communication, but haven't exchanged any routes
      4. DR and BDR is elected in 2-Way state
    4. ExStart - Routers negotiate who is going to start the exchange, all traffic in unicast
      1. R1 sends a DBD (database description) packet
        1. I will start exchange with my Router ID 172.16.1.1
      2. R2 replies with a DBD packet
        1. No, I will start exchange because I have a higher Router ID 172.16.2.1
    5. Exchange - Routers exchange DBD information, and respond with LSAck
      1. R2 sends a DBD packet with the its LSDB (link state database) summary
      2. R1 replies with an LSAck
      3. R1 sends a DBD packet its LSDB summary
      4. R2 replies with an LSAck
    6. Loading - Exchange LSRs and LSUs
      1. R2 sends a LSR (link state request) to R1
        1. I need full info on 172.16.1.0/24
      2. R1 sends an LSU
      3. R1 sends an LSR for 172.16.2.0/24
      4. R2 replies with an LSU
    7. Full - Neighbors are fully adjacent
      1. R1 sends an LSAck acknowledging all info
      2. R2 sends an LSAck acknowledging all info
  2. Neighbor State Stages Summary
    1. Down: no active neighbor detected
    2. INIT: Hello packet is received from a neighbor
    3. 2-way: Neighbor responds, includes both router IDs in Hello
    4. Exstart: Primary and Secondary roles determined
    5. Exchange: Database description packets sent
    6. Loading: Exchange of LSRs and LSUs
    7. Full: Neighbors fully adjacent

Various Commands

OSPF Mnemonics

OSPF Network Types

Breathe Fiddy, Elect 10
P2P HD, Tyrant 10
Weird nets 30 secs

Broadcast: Ethernet, FDDI, Elects DR/BDR, Hello 10 seconds
Point-to-Point and HDLC, no elections, Hello 10 seconds
Everything else, Hello 30 seconds
(some of them elect DR, some don't, didn't fit the mnemonic)

OSPF Neighbor States

Dinit 2-Wexstart Exchange LoFull

  1. Down
  2. Init
  3. 2-Way
  4. ExStart
  5. Exchange
  6. Loading
  7. Full

Metadata

OSI or TCP/IP Layer

CCNA Exam Topic

#extop-3-4

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