Transcript Discovery_Routing_Switching_Chapter6

Routing With a
Link-State Protocol
Introducing Routing and Switching in the Enterprise –
Chapter 6
Version 4.0
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
1
Objectives

Describe and plan a network using Open Shortest
Path First (OSPF) .

Design and configure a network using single-area
OSPF.

Work with multi-protocol environments.
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
2
Describe and Plan a Network Using
OSPF
 Link-state routing protocol
 Open standard developed by IETF
 Classless interior gateway protocol
 Divides the network into areas for scalability
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
3
Link State Advertisements (LSA)
 Link-state information must be
synchronized between routers
 LSAs are reliable; there is a
method for acknowledging the
delivery of LSAs.
 LSAs are flooded throughout the
area (or throughout the domain if
there is only one area).
 LSAs have a sequence number
and a set lifetime so that each
router recognizes that it has the
most up-to-date version of the
LSA.
 LSAs are periodically refreshed to
confirm topology information
before the information ages out of
the link-state database.
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
4
Cost calculations in OSPF
 Bandwidth determines the cost of a link
 Higher bandwidth = lower cost = desirable route
 Cost = 100,000,000 / bandwidth of link in bps
 Manual cost configuration at an interface: ip ospf cost
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
5
Describe and Plan a Network Using OSPF
 SPF algorithm (Diijkstra’s Algorithm)
 Each router has a topological tree
 SPF tree information stored in topology database
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
6
Describe and Plan a Network Using
OSPF
 Adjacencies:
neighbor
relationships
 Hello protocol –
each router sends
a profile to other
routers
 Router ID – The
highest IP address
on a Physical
Interface
 Role of loopback
interfaces – Trump
card
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
Hello intervals and dead timers
must be the same - between
neighbors.
7
Describe and Plan a Network Using OSPF
 Designated router (DR)
 Backup designated router (BDR)
 DROther
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
8
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
9
Describe and Plan a Network Using
OSPF
 Area 0
 Roles of routers in an OSPF Autonomous System:
Area Border Router (ABR)
Autonomous System Border Router (ASBR)
A router has a
separate LSDB for
each area to which
it connects
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
10
All areas must connect to the Transit
Area
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
11
Design and Configure a Network Using
Single-Area OSPF
OSPF configuration steps:
 Enable OSPF + process ID
 Advertise networks + wildcard mask and area ID
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
12
Design and Configure a Network Using
Single-Area OSPF
 Authentication protects integrity of routing information
 Simple password authentication
 Message Digest 5 (MD5)
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
13
Design and Configure a Network Using
Single-Area OSPF
Tune OSPF parameters:
 Interface priority
 Router ID
 Loopback and interface addresses
 Bandwidth (cost )
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
14
Design and Configure a Network Using
Single-Area OSPF
Verification and troubleshooting commands:
 show ip ospf neighbor
 show ip protocols, show ip route
 show ip ospf, show ip ospf interface
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
15
Work with Multi-Protocol Environments
 Default route configured on ASBR
 ASBR distributes the route into the OSPF network
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
16
Work with Multi-Protocol Environments
Benefits of OSPF summarization:
 Reduces number of networks advertised
 Reduces memory requirements
 Reduces number of entries in router updates
 Isolates flapping and other problems to their location
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
17
Work with Multi-Protocol Environments
OSPF limitations:
 Increased router memory and processing demands
 Strict design requirements
 Knowledgable administrator required
 Initial discovery process takes up network bandwidth
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
18
Work with Multi-Protocol Environments
 Administrative distance (AD) determines routes that
appear in routing table
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
19
Summary
 OSPF is a classless interior link-state routing protocol
used in enterprise networks
 OSPF uses bandwidth to generate the cost metric
 OSPF routers elect a DR and BDR on multi-access
networks
 Router ID or router priority can be used to dictate the
selection of DR and BDR
 The OSPF network command uses a wildcard mask
 Default route distribution and inter-area route
summarization are used in OSPF networks
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
20
More Labs
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
21
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
22