Sunday, January 11, 2015

Cisco CCNP - 300-101 - OSPF Basics

So it's that time again for me to renew my Cisco Certifications. As a result, this post is based on my preparation for the CCNP Route Exam (300-101).

In this post I will be focusing on OSPF basics

Open Shortest Path First (OSPF)
    - Uses Link-state (LS)
    - Database is called Link-state database
    - Information held in Link State Database include router IDs, interface IP address, mask and subnet, list of reachable routers
    - Each router interface exists in a single area
    - Uses Multicast Hello Messages
    - Process ID does not need to match for host to become neighbors
    - Uses Finite State Machine (FSM)
    - Uses 8 states to determine neighbors status
    - Reducing hello and dead timers can speed up convergence

Area Border Routers
    - Uses Area Border Routers (ABR)
    - ABRs are responsible for sharing information between different areas
    - ABRs doe not advertise detailed information. They instead advertise subnets prefix and length
    - Backbone (Area 0) is connected to all other areas
    - All communications between non backbone devices must pass through at lease 1 backbone
    - ABR keeps a copy of the Link State Database for each area attached to it
    - First network number matching an interface shown in the running config determines the interface area
    - Does not forward Type 1 or Type 2 LSA into another area

OSPF Key Terms
    - Link-state Database (LSDB) - stores topology database   
    - Shortest Path First (SPF) - The algorithym used by the protocol
    - Link-state update (LSU) - Contains detail topology information
    - Link State Advertisement (LSA) - Holds topology information. Gets sent out via LSU
    - Area - Contigious group of routers
    - Area Border Router (ABR) - Has interfaces connected to two or more areas
    - Backbone Router - Connects with other router
    - Internal Routers - Connected to only one area
    - Designated Router (DR) - Elected to perform special functions
    - Backup Designated Router (BDR) - Secondary DR Router
    - Transport - Uses TCP/UDP 89
    - Metric - cost of all outgoing interface in a route-id
    - Hello Interval - notify neighbors (10 seconds default on LAN)
    - Dead Interval - detect failed neghbors (40 seconds default on LAN)
    - Update destination address - sent to broadcast and
    - Updates (Full or partial)
    - Authentication - MD5 and clear text
    - VLSM/classless - masks gets included in each route
    - route tags - tag for redistributed routes
    - next-hop field - manually specify a next hop router other than the one advertised
    - Manual Route Summarization - summary routes
Steps for choosing router id
    - router-id command takes precedence
    - if no router-id configured, highest IP on loopback interface is used
    - if no route-id or loopback interface, it uses the highest IP address on any non-loopback interface
Common OSPF configuration verification command   
    - show ip ospf interface brief - Shows OSPF enabled interface
    - show ip protocols - shows OSPF network configurations
    - show ip ospf neighbors - shows known neighbors
    - show ip ospf database - LSA for connected areas
    - show ip route-id - shows the routing table
Two classes of neighborship
    - Neighbors
    - Fully Adjacent Neighbors

The following must match for neighborship to be completed
    - router id
    - stub area flag
    - hello interval
    - dead interval
    - subnet mask
    - list of neighbors reachable on the interface
    - area id
    - router priority
    - designated router (DR) IP address
    - Backup DR IP address
    - Authentication digest

OSPF Network Types
    - Broadcast
    - Point-to-point
        - Normally serial links
        - DRs are not typically elected
    - Loopback
    - Nonbroadcast (NBMA)
    - Point-to-multipoint
    - Point-to-multipoint nonbroadcast

OSPF Virtual Links
    - two ABRs connecting to the same nonbackbone area can form neighborship
    - acts like a virtual point-to-point connection between two routers
    - ABR sends messages as unicast
    - Do Not Age (DNA) Bit is set LSAs - other routers do not expect flooded messages every 30 minutes
    - uses area virtual-link command
    - There is no interface to use for virtual links
CCNP official Cert Guide - CCNP Routing and Switching - Route 300-101

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