Routing Notes 07 OSPF Design

BASICS

1. All areas must be connected to AREA 0 (backbone)
2. All routers in an area have the same topology table
3. Objective is to localize the updates within the area
4. Requires a hierarchical design
5. Runs on DIJKSTRA SPF algorithm

OSPF Router Types:
1. Area Border (ABR) - Summarize Routes
2. ASBR (Autonomous System Boundary Router) - Bridges outside world

Virtual Links - OSPF VPN Tunnel / acts as directly connected to AREA 0

OSPF Tables
1. Neighbor - provides the list of all routers forms relationship
2. Topology - provides the best route, second best route, roadmap of the network
3. Routing - consolidates the routes

OSPF NEIGHBOR RELATIONSHIPS

1. Determine the Router ID (loopback address is the highest)

2. Add interface to the LS database (dictated by network command)
    - Hello
    - Adverstise the network to other routers

3. Sends Hello messages on chosen interfaces
    - 10 seconds on broadcast
    - 30 seconds on nbma networks
    - contains all information
       Router ID
       Hello and Dead Timers*
       Network Mask*
       Area ID*
       Neighbors
       Router Priority
       DR / BDR IP Address
       Authentication Password*
       * - all must match to have neighbor relationship

4. Recives Hello
    - Check Hello / Dead Interval
    - Check Netmasks
    - Check Area ID
    - Check Authentication Passwords

5. Send Reply Hello
    - Check if new neighbor or old
    - Old - will reset the dead timers

6. Master - Slave Relationship Relationship
    - Determined by Priority Router-ID Breaks Tie
    - Higher Router ID becomes the master
    - Master sends description DBD packet
    - Slave sends its DBD packet

7. DBD are acknowledged and reviewed
    - Slave request details - LSR
    - Master sends updates - LSU
    - Master request details - LSR
    - Slave sends updates - LSU

8. Neighbors are synchronized
    - Full State


1. ------HELLO----]       Init
2. [----HELLO------       Init
3. --------DBD------]      Exstart
4. [------DBD-------       Exchange
5. --------LSU-------]      Loading
6. [------LSR--------       Loading
7.    FULL STATE

OSPF NEIGHBOR STATES

1. Down - No hello packets
2. Attempt - used for manual configured neighbors
3. Init - hello packet recived
4. 2 way state - identifies compatible neighbors
5. Exstart/Exchange - master slave relationship and DBDs exchange
6. Loading - Beautiful dance of LSR / LSA / LSU
7. Full - OSPF

DR -  provides update to the whole topology
BDR - provides update to the whole topology
Both manage neighbor relationship per conneciton
Must be elected to make sure all updates are reach in the topology

224.0.0.6 - Multicast Message for updates/connections
224.0.0.5 - Multicast Message by DR

OSPF METRIC

Cost = 100 / BW-IN-MBPS

Common Cost
56K = 1785
64K = 1562
T1 (1.544) = 65
E1 (2.048) = 48
Ethernet = 10
Fast Ethernet = 1

router ospf
auto-cost reference-bandwidth

SUMMARIZATION ON ABR AND ASBR

ABR Configuration

ROUTER 2
router ospf 1
router-id 2.2.2.2
network 172.16.1.0 0.0.0.255 area 0
network 172.16.2.0 0.0.0.255 area 1

show ip ospf neighbors

area 0 range 10.1.0.0 255.255.248.0
route summarization
area 1 range 10.2.0.0 255.255.248.0
route summarization

show ip route

ASBR Configuration

ROUTER 1
router rip
version 2
no auto summary
network 10.0.0.0
network 172.16.0.0

ROUTER 2
router rip
version 2
no auto summary
network 172.16.0.0

router ospf 1
redistribute rip metric 20 subnets
summary-address 10.2.0.0 255.255.248.0


OSPF SPECIAL AREAS

Backbone Area
LSA Type 1: Router LSA

Standard Area
LSA Type 2: Network LSA (DR Generated)

Stub Area
LSA Type 3: Summary LSA (ABR Summary Route)
- doesnt accept external network

Stub Network 
conf t
router ospf 1
area 23 stub

ABR
conf t
router ospf 1
area 23 stub

Totally Stubby Area
LSA Type 4: Summary LSA (ASBR Location)
- doesnt accept internal network
- doesnt accept external network

Stub Router
conf t
router ospf 1
area 45 stub

ABR
conf t
router ospf 1
area 45 no summary
Not-so-Stubby Area

LSA Type 5
External LSA (ASBR Summary Route)

OSPF VIRTUAL LINKS

Router with 2 Endpoints - Transit Area

[Router 1] -----(transit area)-------[Router 2]-----[Router3]

Router 2
router ospf 1
area 67 virtual-link 6.6.6.6

Router 1
router ospf1
area 67 virtual-link 7.7.7.7

show ip ospf virtual-links
Runs on Demand Circuit

Router 3
show ip route

OSPFv3