OSPF in-depth

    Router Types

    DR

    All routers communicate with the DR as well as the BDR over the multicast address 224.0.0.6. Then 224.0.0.5 is used by the DR to share those routes.

    BDR

    Lets explore LSA types

    NumberNamewhoWhat
    1RouterEveryone within an areaall links and cost
    2NetworkDR (BDR)Broadcast segment information
    3SummaryABRSends summary routes into area 0 and receives them
    4ASBRABR/ASBRThis pushes a route in which other ABR can reach it
    5External NetASBRThis carries prefixes as well as the external types (1 or 2)
    6Multicast OSPF---replaces by PIM
    7NSSA SummaryASBRABR/ASBR
    8Externall AttrABRThis allows ospf to carry attributes for protocols

    VEB or Vbit signifies that a router can do more than others and is part of recognizing an ASBR. V - Virtual link (0x4) E - External bit (0x2) B - Border Router (0x1)

    OSPF LSA packet types

    Something I overlooked the first time around is the different type of packets in ospf.

    Hello

    This is sent by routers to maintain connectivity, announce that they are present but probabaly the most important thing they do is help elect DR. "The Hello protocol determines the designated router (DR). The DR is adjacent to all routers on the network, and its role is to generate and flood the LSAs, on behalf of the network. In a broadcast network, such as Ethernet, having a DR reduces the amount of router protocol traffic that is generated." Multicast Address: 224.0.0.5

    Database description

    Once the hello packets are exchanged and two way communication is established, These are the packets that carry the meat and the potatoes synchornizing databases between routers. These are sent using the exchange protocol.

    Until the routers are i nthe full state, they are constantly sending and requesting updates using both the link-state request and link-state updates. Routers establish a primary and secondary in these udpates The primary router sends its database description, one at a time. The secondary router then sends an acknowledgement and also includes that ackowledgement in its link-state database . The sequence number helps to determine if the record is newer or older.

    after all descriptions are received, the neighbors sed out database requests for more complete information about hte records that were requested. These requests are followed with a floding of link-state updates containing the requested information. Each update is ack'd, and finally once all databases are synchronized, then we have adjacenies. Now would be a good place to list out the various adjacency states. Soon to follow would be problems which can occur at each;

    • Down
    • Init
    • Attempt
    • 2-way
    • Exstart
    • Exchange
    • Loading
    • Full

    Each newely received LSA must be acknolwedged, so here we have this!

    LSA types

    So lets explore these LSA types with some detail.

    Type 1: Router

    • Flooded by everyrouter within an area
    • This contains information about connected links
    • Identification is done by the originating router ID
    • Does NOT cross area boundries

    Type 2: Network

    • Only generated by the GR on an ethernet segment
    • It includes the network ID, subnet mask and also the list of attahed routers in the transit
    • We only have one of these for each NBMA network
    • Does NOT cross area boundries

    Type 3: Summary (v3: Inter-Area Prefix)

    • Used to share the network information with other areas. Inter-Area (IA)
    • These are generated by the ABR
    • These are represented with "OIA" in the routing table
    • These do and can cross area boundries

    Type 4: Summary ASBR (v3 Inter-Area Router)

    • A type of summary LSA used to advertise routes to an ASBR.
    • Its generated by the ABR of the originating area
    • They are flooded throughout the entire AS
    • These will contain the router ID of the ASBR only

    Type 5: External

    • Used to advertise external routes from other Autonomous systems
    • These are flooded throughout the entire AS
    • The advertising router is not changed across the network
    • Routes are not summarized by defualt
    • These contain informatio on either type 1 external routes or type 2, more on this later

    Type 6: Multicast OSPF

    • used for ospf multicast ...
    • These are not supported by Cisco Routers

    Type 7: External

    • So remember that external LSA are not allowed in subby areas, so in NSSA, we have these guys to operate as a Type 5 within an NSSA
    • When it reached an ABR, its converted to a type 5

    Type 8: External attributes LSA for BGP

    • Used to work with BGP

    Type 9: Intra-area Prefix LSA

    Type 11: Grace LSA

    ref: https://ipwithease.com/ospf-lsa-types/

    Timers

    I found this really helpful guide to os-pf timers. I think this is interesting with each protocol because this can show you how fast a routing protocol can operate which obviously helps dictate which routing protocols you should use and why.

    There are 3 timers you can influnce in ospf; Hello, Dead, and Retransmit Intervals. This timers are configured withint the interface configuration for those interfaces participating in ospf.

    Hello

    ospf hello packets are used to establish and maintain neighbor relationships. This timer or interval is the time a router will wait before sending another packet out of each of its active ospf interfaces. This must be the same amount of time on all devices within a network.

    10 seconds (broadcast, and point-to-point) 30 seconds (nonbroadcast multiple access)

    to specify the hello timers in a cisco IOS device we enter the interface and: ip ospf hello-interval 20

    Dead Interval

    With an ospf device, as much as it sends hello packets it also expects to receive hello packets from its neighbors. If it fails to receive this informatio then it considers the link dead. Seeing as this is a link state protocol, this is a significant event. The default for this times is and should be 4 times the hello timer: 40 seconds (broadcast, and point-to-point) 120 seconds (non broadcast multiple access)

    to set these times you do so with the ip ospf dead-interval command ip ospf dead-interval 80

    Retransmit Intervals

    One of the main mechanisms of ospf which allows it to send state information is the LSDB. LSA's are used to send this information across the network. When a router sends out this information it expects to receive an acknowledgement of these messages. Routers will wait for this interval before they retransmit the LSA update. This timer is specified using ip ospf retransmit-interval. The default for this is 5 seconds.

    5 seconds

    and the configuration for this is: ip ospf retransmit-interval 8

    Network Types

    There are 5 different network types in ospf; Broadcast, Non-Broadcast, Point-to-point, Point-to-Multipoint Non-Broadcast, and Point-to-Multipoint. Now that we have listed them out, lets get into the weeds on these.

    Point-to-Point

    Broadcast

    Just as it sounds, its for networks that support broadcasts. On these networks neighbors are discovered dynamically using the multicast address 224.0.0.5. There is always a DR and BDR elected on each of these networks. Other routers on the same broadcast networks can only form full neighbor relationships with either the DR or the BDR.

    Non-Broadcast

    These are multiaccess networks that do not support broadcast packets. Frame relay was an example of this back in the day. There are two sub-groups of this: Non-broadcast multi-access (NBMA), or point-to-multipoint.

    NBMA

    This is a network of devices that cannot broadcast to eachother. One of the main limitations is that there is a full mesh between the devices.

    Point-to-Multipoint

    In this mode, ospf treats each link as a point-to-point connection. Because of that there are no BR/BDR election but neighbors may be automatically discovered.

    Also worth noting the virtual links here as well as it is tied directly to these network types. It allows for an area that doesnt touch the backbone to exist. This is really just for a stop gap until it can be properly integrated into the network.

    # Area Types
Stub - Stops type 4 and type 5 LSA
 Totally Stubby - this takes the next step to block 3 as well with the exception of the default route.
  Not so stubby - Still no 4 or 5 but creates a type 7 to carry external routes and the abr translates to type 4 or 5

   

   # Multi-area Single Adjacency
   So this is kind of crazy, simply by adding a secondary flag to the interface configuration within an area, you can add an interface that has an existing adjacency to a second area as a secondary adjacency. `