** Understanding OSPF Network types**

Broadcast, Multi-Access Networks
–Example: Ethernet, Token Ring
–Single Operation Mode
–DR/BDR election, 10 sec. Hellos, DUAL Multicast add.
Point-to-Point Networks
–Example: T1 cas, ISDN BRI/PRI
–Single operation mode
–No DR/BDR, 10 sec. Hellos, Single Multicast add.
Non-Broadcast Multi-Access (NBMA) Networks
–Example: Frame-Relay, ATM
–Five Modes of Operation
NBMA Topologies
–Hub-and-Spoke (STAR) Topology. cheapest way to impliment.
–Partial Mesh
–Full Mesh (MPLS)
OSPF over NBMA: Five Modes
1–Non-Broadcast (NBMA) – RFC standard
——default mode for x.25, FR, ATM
——Neighbors Statically configured
——Must be one subnet
——Acts like a Lan Environment
——DR/BDR Elected (must have full connectivity)
2–Point-to-Multipoint – RFC standard
——Fixes Issues with NBMA Networks
——Requires Single Subnet
——No DR/BDR Elected
——Neighbors Automatically Form
3–Point-to-Point – Cisco Proprietary
——Uses separate sub-interfaces
——Requires different subnets
——No DR/BDR elected
——Neighbors automatically form
4–Broadcast – Cisco Proprietary
——Acts as an ethernet LAN over a WAN
5–Point-to-Multipoint, Non-Broadcast – Cisco Proprietary
——Fixes Issues with NBMA Networks
——Requires Single Subnet
——No DR/BDR Elected

** Implementing OSPF over Multiple Network types**

During route redistribution –When OSPF brings in external routes into the network, it will be one of two types:
– Type E1 will increment the metric
– Type E2 will not (Default)
R1(config)#router ospf 1
R1(config-router)#redistribute static subnets metric 200 metric-type 2

**OSPF’s COST**

Cost = 100 / BW-IN-MBPS
Common Costs:
–56k = 1785
–64k = 1562
–T1 (1.544) = 65
–E1 (2.048) = 48
Ethernet = 10
Fast Ethernet (100mega-bits-per-second)= 1

**UNDERSTANDING THE DR AND BDR**

–The way they operate is as a control for a shared network segment.
–One Router on the Network will be elected as the DR (Designated Router) and another will be elected as the BDR(Backup Designated Router).
–When a link goes down in the network, the update is sent to the DR and the BDR, Then the DR and the BDR send that update to the rest of the routers in the network.
–There is one DR and one BDR for every shared network segment. This is not true for point-to-point network.
–The router with the higher Router-ID becomes the DR.

**THE FLURRY OF OSPF PACKET TYPES**

–Hello
–Database Description (DBD)
–Link-State Request (LSR)
–Link-State Advertisement (LSA) – Link state advertisements are updates about individual … Read More »

**LINK STATE ROUTING PROTOCOL CONCEPTS**

Two Types of link state routing protocol
–IS-IS
–OSPF
Link State Routing Protocol Maintains Three Tables
–Neighbor Table
–Topology Table
–Routing Table
OSPF is considered a routing by rumor protocol. Distance Vector only knows what their neighbors know. They do have a road-map of their area.
Use Dijkstra’s shortest path first (SPF) algorithm
Send triggered updates to announce network changes
–They only send updates when there are changes to the network.
Send periodic updates (LS Regresh) on long intervals
–Once every thirty minutes they check the network to make sure everything is good.

**OSPF AREA DESIGN & TERMINOLOGY**

–Area 0 (Zero) is considered the backbone.
–All other Areas must be connected to Area 0.
–Area Border Routers (ABR) connects the two Areas.
–Make sure to keep ip addressing scheme together in each area. Make sure each area can be summarized. That is the point of Areas.
–In OSPF, the only two router types that can do route summarization are the Area … Read More »