Cisco CCNP ROUTE Implementing OSPF

 

Cisco CCNP ROUTE Implementing a Scalable Multiarea Network OSPF Based Solution

Cisco CCNP ROUTE Objectives

Cisco CCNP ROUTE Open Shortest Path First

If you remember, we discussed link state routing protocols and identified that they require more administration.  This design administration requires that all OSFP routers are configured in a hierarchical area design.

RIP & IGRP use the Bellman-Ford algorithm to find their best route, OSPF uses the Dijkstra algorithm to find the shortest path to remote networks.

OSPF uses protocol value 89 in the IP packet.

OSPF communicates through the use of multicast packets. 

All OSPF routers participate in multicast group 224.0.0.5

All OSPF designated routers (DR & BDR) participate in 224.0.0.5 as well as 224.0.0.6

Cisco CCNP ROUTE Why OSPF?

OSPF was developed to combat the limitation of RIP.

OSPF is a LINK STATE Routing protocol that has these following characteristics

Speed of convergence

Faster than RIP with minimum of traffic exchanged

VLSM

Supports summarization at area boarders

Reachability

No hop count limits, approx 40-50 routers per area

Bandwidth Utilization

Incremental updates, topology data kept within single area

Path selection

Metric based on bandwidth, equal cost load balancing over up to six paths

Cisco CCNP ROUTE OSPF Areas

• • Identified by a 32 bit field
• • Manually configured
• • If only one area exists, it’s ID can be ANY number
• • If more than one area is defined, area 0 must exist and is the backbone
• • Area 0 must be connected to all other areas 
• • There is only one area 0

Cisco CCNP ROUTE OSPF Terminology

Link – Interface on a router

Link-state – Status of a link between two routers

Includes a router’s interface and its relationship to its neighboring routers

Cost – Indication of the overhead required to send packets across an interface

High bandwidth =  low cost

Area – A contiguous grouping of networks and routers sharing the same Area ID and topology database

DR (Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routers

BDR (Backup Designated Router) – An elected router in Broadcast and Non-Broadcast environments that is adjacent to all other OSPF routers

 that will promote it self if the Designated Router is no longer available

Neighborship Database  – Listing of all the neighbors to which a router has established bi-directional communication 

Link State/Topological Database  – List of information about all other routers in the same area

Routers in the same area have identical link state databases  

Routing Table  – Generated when the SPF algorithm is run on the link-state database

Each router has a unique routing table

 

Cisco CCNP ROUTE OSPF Terminology

Neighbor

Two routers that have an interface on a common network

Usually discovered by hello’s but can also be configured administratively

Adjacency

Relationship formed between selected neighbors in which routing information is exchanged.  Not all neighbors are adjacent

Only Broadcast and Non-Broadcast network types have Designated and Backup Designated Routers!!!

Cisco CCNP ROUTE OSPF DR and BDR

• •Elected routers that coordinate the flow of route updates within a subnet

• • All routers on a sub-net form adjacencies with the DR and BDR

• • Clients send updates to the DR and BDR only

• • The DR informs all other clients of changes in topology

• • BDR assumes DR role if DR fails but performs no DR functions if DR remains available

• • It is generally not a good idea to select the same router to be designated router on many LANs simultaneously.

• • ONLY Broadcast and Non-Broadcast Network types have Designated and Backup Designated Routers.

Cisco CCNP ROUTE Electing the DR and BDR

The following outlines the process OSPF takes and rules that are followed when electing a Designated Router:

• • Routers elect a DR and BDR per network

• • All routers set by default to priority 1 (0-255)

• • Priority of zero (0) means router can not be elected as a DR

• • Router with highest priority wins BDR (1 – 255), if no other router has a higher priority the BDR will then become the DR

• • RouterID breaks tie, Router ID is either the Highest Loopback or Highest Configured IP address on any given active interface

• • If DR fails, BDR promoted to DR and a new BDR is elected

• • Existing DR will not be overthrown if “better” router is turned on after initial election

• • DRs and BDRs listen to multicast traffic on both multicast address 224.0.0.5 and 224.0.0.6  224.0.0.6 is exclusively listed to by DRs

 

Cisco CCNP ROUTE Router ID (RID)

Each router that is participating in OSPF needs to be uniquely identified. The method of identification that OSPF uses is Router IDs (RID).

32 bits that uniquely identifies an OSPF router

Highest IP address in router is RouterID

Overridden by Loopback interface if present

Even if Loopback address has lower value

Recommended to use loopback interface

Easier to manipulate this number

Always up

Interface loopback 0

Ip address 10.1.1.1 255.255.255.0

You can also Statically assign the Router ID in the OSPF router configuration mode:

(config)# router ospf 1

(config-router)# router-id

Do NOT use same loopback address on different routers

Cisco CCNP ROUTE  Modifying OSPF operation

Router(config-if)# ip ospf priority {0 – 255}

Change the priority of a router on an interface

0 means to not participate in election

1 is default, 255 is highest priority

Router(config-if)# ip ospf cost

Change the cost of an interface

Default cost is 100,000,000/bandwidth

10Mbps Ethernet = 10

100 Mbps Ethernet = 1

Can be used to ensure that two routers use the same cost to refer to the same interface. Ex. Nortel’s default cost for 10MBPS link is 1

r3(config-if)# ip ospf priority 10

r2# sho ip ospf neighbor detail

 Neighbor 10.1.30.1, interface address 10.1.23.3

    In the area 0 via interface Ethernet0

    Neighbor priority is 10, State is FULL, 6 state changes

    DR is 10.1.23.3 BDR is 10.1.23.2

    Options 2

    Dead timer due in 00:00:37

r2(config)# int s0

r2(config-if)# ip ospf cost 1000

r2# show ip route

     10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks

O       10.1.10.1/32