Networks & Protocols INF566 - X2010 - 2012 Lecture 3 - - - PowerPoint PPT Presentation

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Networks & Protocols INF566 - X2010 - 2012 Lecture 3 - - - PowerPoint PPT Presentation

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Networks & Protocols INF566 - X2010 - 2012 Lecture 3 - Enterprise Grade Routing - OSPF thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/ 1 Intro, kick-off, objectives and motivation (Thomas)

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Networks & Protocols

INF566 - X2010 - 2012 Lecture 3 - Enterprise Grade Routing - OSPF

1

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

1 9/1 Intro, kick-off, objectives and motivation (Thomas) “What Makes For A Successful Protocol?” (Mark) 2 16/1 “Bufferbloat & the Broken Internet” (Thomas & Mark) 3 23/1 “Carrier-Grade Routing” (Thomas) 4 30/1 “Peering” (Thomas & Mark) 5 6/2 RPKI (Mark) 6 13/2 "Indirection, Encapsulation, and Obfuscation" (Mark & Thomas) 7 20/2 SDOs: the ITU, IETF, ... - Guest Lecture by Elliot Lear 8 27/2 "Homenetworking and The Curse of the End-2-End Model" (Mark) 9 ??? Presentations by buddy-teams (Thomas + Mark)

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

1 9/1 Intro, kick-off, objectives and motivation (Thomas) “What Makes For A Successful Protocol?” (Mark) 2 16/1 “Bufferbloat & the Broken Internet” (Thomas & Mark) 3 23/1 “Carrier-Grade Routing” (Thomas) 4 30/1 “Peering” (Thomas & Mark) 5 6/2 RPKI (Mark) 6 13/2 "Indirection, Encapsulation, and Obfuscation" (Mark & Thomas) 7 20/2 SDOs: the ITU, IETF, ... - Guest Lecture by Elliot Lear 8 27/2 "Homenetworking and The Curse of the End-2-End Model" (Mark) 9 ??? Presentations by buddy-teams (Thomas + Mark)

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Objectives

  • The transition from “academic algorithm”

into “real-world deployment” of routing

  • Scalability, hierarchical routing
  • (Intentional) Obfuscation
  • Familiarize yourself with “the basics” of

OSPF - the (preferred) IGP on the Internet

3

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail

4

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SLIDE 9

thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford
  • Link-State Routing

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford
  • Link-State Routing
  • Global knowledge: state = network graph

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford
  • Link-State Routing
  • Global knowledge: state = network graph
  • Require all routers to have consistent

network graph

4

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SLIDE 13

thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford
  • Link-State Routing
  • Global knowledge: state = network graph
  • Require all routers to have consistent

network graph

  • Good local decisions when links, routers fail

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Distance

Vector Routing

  • Local knowledge only: state = routing table
  • Poor decisions when links, routers fail
  • Bellman-Ford
  • Link-State Routing
  • Global knowledge: state = network graph
  • Require all routers to have consistent

network graph

  • Good local decisions when links, routers fail
  • Dijkstra

4

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network
  • generate LSAs (Link ID, state, cost, neighbors)

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network
  • generate LSAs (Link ID, state, cost, neighbors)
  • 3. Learn complete network topology

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network
  • generate LSAs (Link ID, state, cost, neighbors)
  • 3. Learn complete network topology
  • Link State Database (LSDB)

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network
  • generate LSAs (Link ID, state, cost, neighbors)
  • 3. Learn complete network topology
  • Link State Database (LSDB)
  • 4. Run local path selection algorithm

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

  • Link State Principles
  • 1. Know links to neighbors
  • establish relationship “adjacency”
  • 2. Flood link-state through network
  • generate LSAs (Link ID, state, cost, neighbors)
  • 3. Learn complete network topology
  • Link State Database (LSDB)
  • 4. Run local path selection algorithm
  • Dijkstra

5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

Received LSAs IP Routing Table Dijkstra’s Algorithm Link State Database LSAs are flooded to other interfaces

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Background Recap from INF557

Initialization: R = {s} ∀v ∈ V |c(s,v) ≠ ∞ : d(v) ← c(s,v) ∀v ∈ V |c(s,v) = ∞ : d(v) ← ∞ Repeat: (v, w) ← (v ∈ R, w ∈ V\R)|c(v,w) is minimum p(w) ← v R = R ∪ {w} ∀u ∈ V |c(w,u) ≠ ∞ : d(u) ← min(d(u), d(w + c(w,u))) Until: ¬∃u ∈ R, v ∈ V \ R|c(u,v) ≠ ∞

Dijkstra’s Algorithm

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

OSPF Basics

Open Shortest-Path First

  • Most successful link-state protocol on the

Internet

  • As you know, successful doesn’t mean best....
  • Significant complexity; Mark has colleagues

spending a lifetime specializing in small parts

  • f OSPF..... [51 RFCs and counting....]
  • We’ll go over the basics.....

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

OSPF

Open Shortest-Path First

  • History:

1989 - RFC 1131 - OSPF version 1 1991 - RFC 1247 - OSPF version 2 1994 - RFC 1583 - OSPF version 2 (revision 1) 1997 - RFC 2178 - OSPF version 2 (revision 2) 1998 - RFC 2328 - OSPF version 2 (revision 3) 1999 - RFC 2740 - OSPF version 3 (for IPv6)

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses 10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses 10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 Links

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses 10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric”

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric” Metric in range [0 , 216] (Metric can be asymmetric)

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric” Metric in range [0 , 216] (Metric can be asymmetric) Interfaces

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric” Metric in range [0 , 216] (Metric can be asymmetric) Interfaces Address

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric” Metric in range [0 , 216] (Metric can be asymmetric) Interfaces Address From the “link prefix”

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Some Taxonomy

Router IDs selected independent of interface addresses

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24 Links Address Cost - called “Metric” Metric in range [0 , 216] (Metric can be asymmetric) Interfaces Address From the “link prefix” In this case, all .X are the same

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

Advertising Router:

10.10.10.1 = Router ID

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

Advertising Router:

10.10.10.1 = Router ID

Number of links:

3 = 2 links plus router itself

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

Advertising Router:

10.10.10.1 = Router ID

Number of links:

3 = 2 links plus router itself

Description of Link 1: Link ID = 10.1.1.1, Metric = 4 Description of Link 2: Link ID = 10.1.2.1, Metric = 3 Description of Link 3: Link ID = 10.10.10.1, Metric = 0

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

Advertising Router:

10.10.10.1 = Router ID

Number of links:

3 = 2 links plus router itself

Description of Link 1: Link ID = 10.1.1.1, Metric = 4 Description of Link 2: Link ID = 10.1.2.1, Metric = 3 Description of Link 3: Link ID = 10.10.10.1, Metric = 0 Other stuff: checksum, sequence number, ....

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Link State Advertisements

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

  • LSA of router 10.10.10.1:

Link State ID:

10.10.10.1 = Router ID

Advertising Router:

10.10.10.1 = Router ID

Number of links:

3 = 2 links plus router itself

Description of Link 1: Link ID = 10.1.1.1, Metric = 4 Description of Link 2: Link ID = 10.1.2.1, Metric = 3 Description of Link 3: Link ID = 10.10.10.1, Metric = 0 Other stuff: checksum, sequence number, ....

Each router sends its LSA to all routers in the network (using a method called reliable flooding)

13

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

3 4 2 5 2

10.10.10.1 10.10.10.2 10.10.10.3 10.1.1/24 10.1.4/ 10.1.5/ 10.1.2/24 10.1.3/24

.1 .2 .2 .1 .3 .3 .3 .2

LSDB

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

LSDB

3 4 2 5 1 1 3 2

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 10.1.1/24 10.1.4/24 10.1.7/24 10.1.5/24 10.1.8/24 10.1.2/24 10.1.3/24 10.1.6/24

.1 .2 .2 .4 .4 .6 .1 .3 .3 .3 .2 .4 .6 .5 .5 .5

Each router has a database which contains the LSAs from all other routers

15

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

LSDB

  • The collection of all LSAs is called the link-state

database

  • Each router has and identical link-state database
  • Useful for debugging: complete network description
  • When neighboring routers discover each other for the

first time, they will exchange their link-state databases

  • The link-state databases are synchronized using reliable

flooding

16

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

OSPF Packet Format

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) TTL: set to 1 (in most cases) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89

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OSPF Header Format

18

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OSPF Header Format

2: current version is OSPF V2

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OSPF Header Format

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

OSPF Header Format

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated

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OSPF Header Format

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken

  • ver entire packet

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OSPF Header Format

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken

  • ver entire packet

0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet)

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OSPF Header Format

2: current version is OSPF V2

Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken

  • ver entire packet

0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

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OSPF LSA Format

LSA Header Link 1 Link 2

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Know Thy Neighbors

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  • Routers multicasts OSPF Hello packets on all OSPF-

enabled interfaces.

Know Thy Neighbors

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  • Routers multicasts OSPF Hello packets on all OSPF-

enabled interfaces.

  • If two routers share a link, they can become neighbors,

and establish an adjacency

Know Thy Neighbors

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 Scenario: Router 10.1.10.2 restarts

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  • Routers multicasts OSPF Hello packets on all OSPF-

enabled interfaces.

  • If two routers share a link, they can become neighbors,

and establish an adjacency

Know Thy Neighbors

  • After becoming adjacent, routers exchange their link

state databases

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.1.10.2 restarts

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.1.10.2 restarts

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

Discovery of adjacency After neighbors are discovered the nodes exchange their databases

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

Discovery of adjacency After neighbors are discovered the nodes exchange their databases

Database Description: Sequence = X Send empty Database Description

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

Discovery of adjacency After neighbors are discovered the nodes exchange their databases

Database Description: Sequence = X Send empty Database Description Database Description: Sequence = X, 5 LSA Headers = { Router-LSA, 10.10.10.1, 0x80000006 Router-LSA, 10.10.10.3, 0x80000007 Router-LSA, 10.10.10.4, 0x8000003a Router-LSA, 10.10.10.5, 0x80000038 Router-LSA, 10.10.10.6, 0x80000005 } Sends database description. (description only contains LSA headers)

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

Discovery of adjacency After neighbors are discovered the nodes exchange their databases

Database Description: Sequence = X Send empty Database Description Database Description: Sequence = X, 5 LSA Headers = { Router-LSA, 10.10.10.1, 0x80000006 Router-LSA, 10.10.10.3, 0x80000007 Router-LSA, 10.10.10.4, 0x8000003a Router-LSA, 10.10.10.5, 0x80000038 Router-LSA, 10.10.10.6, 0x80000005 } Database Description: Sequence = X+1 1 LSA Headers = { Router-LSA, 10.10.10.2, 0x80000005 } Sends database description. (description only contains LSA headers) Database Description of 10.10.10.2

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Database Exchange

10.10.10.2 10.10.10.1 OSPF HELLO OSPF HELLO: I heard 10.10.10.2 DBx Scenario: Router 10.10.10.2 restarts

Discovery of adjacency After neighbors are discovered the nodes exchange their databases

Database Description: Sequence = X Send empty Database Description Database Description: Sequence = X, 5 LSA Headers = { Router-LSA, 10.10.10.1, 0x80000006 Router-LSA, 10.10.10.3, 0x80000007 Router-LSA, 10.10.10.4, 0x8000003a Router-LSA, 10.10.10.5, 0x80000038 Router-LSA, 10.10.10.6, 0x80000005 } Database Description: Sequence = X+1 1 LSA Headers = { Router-LSA, 10.10.10.2, 0x80000005 } Database Description: Sequence = X+1 Sends database description. (description only contains LSA headers) Database Description of 10.10.10.2 Acknowledges receipt of description

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Regular LSA Exchange

10.10.10.2 10.10.10.1

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Regular LSA Exchange

10.10.10.2 10.10.10.1

Link State Reequest packets, LSAs = { Router-LSA, 10.10.10.1, Router-LSA, 10.10.10.3, Router-LSA, 10.10.10.4, Router-LSA, 10.10.10.5, Router-LSA, 10.10.10.6, } 10.10.10.2 explicitly requests each LSA from 10.10.10.1

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Regular LSA Exchange

10.10.10.2 10.10.10.1

Link State Reequest packets, LSAs = { Router-LSA, 10.10.10.1, Router-LSA, 10.10.10.3, Router-LSA, 10.10.10.4, Router-LSA, 10.10.10.5, Router-LSA, 10.10.10.6, } Link State Update Packet, LSAs = { Router-LSA, 10.10.10.1, 0x80000006, LSA... Router-LSA, 10.10.10.3, 0x80000007, LSA... Router-LSA, 10.10.10.4, 0x8000003a, LSA... Router-LSA, 10.10.10.5, 0x80000038, LSA... Router-LSA, 10.10.10.6, 0x80000005, LSA.... } 10.10.10.1 sends requested LSAs 10.10.10.2 explicitly requests each LSA from 10.10.10.1

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Regular LSA Exchange

10.10.10.2 10.10.10.1

Link State Reequest packets, LSAs = { Router-LSA, 10.10.10.1, Router-LSA, 10.10.10.3, Router-LSA, 10.10.10.4, Router-LSA, 10.10.10.5, Router-LSA, 10.10.10.6, } Link State Update Packet, LSAs = { Router-LSA, 10.10.10.1, 0x80000006, LSA... Router-LSA, 10.10.10.3, 0x80000007, LSA... Router-LSA, 10.10.10.4, 0x8000003a, LSA... Router-LSA, 10.10.10.5, 0x80000038, LSA... Router-LSA, 10.10.10.6, 0x80000005, LSA.... } Link State Update Packet, LSAs = { Router-LSA, 10.10.10.6, 0x80000006, LSA.... } 10.10.10.1 sends requested LSAs 10.10.10.2 explicitly requests each LSA from 10.10.10.1 10.10.10.2 has more recent value for 10.10.1.6 and sends it to 10.10.10.1 (with higher sequence number)

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 Update database

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Routing Data Distribution

10.10.10.1 10.10.10.2 10.10.10.3 10.10.10.4 10.10.10.5 10.10.10.6 Update database

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

LSA Updates

  • Triggered by local topology or link cost changes
  • A received LSA that contains no new information

is suppressed

  • Exception: very infrequent (every 30 min)

flooding

  • Acknowledgement:
  • explicit ACK
  • implicit via reception of LSA Update

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Question

  • What is the role of the Database Description

packets?

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

OSPF Autonomous System

  • A region of the Internet administered by a single

entity.

  • Examples of autonomous regions are:
  • X’s campus network
  • Frees backbone network
  • Routing is done differently within an autonomous

system (intradomain routing) and between autonomous system (interdomain routing).

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Large OSPF nets

  • Large link-state table
  • Each router maintains a LSDB for all links
  • The LSDB requires the use of memory
  • Frequent SPF calculations
  • Topology changes; re-run Dijkstra
  • Flapping links
  • Large routing table
  • Solution: Divide the network into multiple areas

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Multi-Area OSPF

  • A flapping link will affect only an area
  • Dijkstra only when changes within that area
  • Complete topology only for that area

➡Separate large network into smaller ➡Hierarchical routing ➡Area == Isolation of “problems”

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS Internal Routers

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS Internal Routers Backbone Routers

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS Internal Routers Backbone Routers Area Border Router

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS Internal Routers Backbone Routers Area Border Router AS Border Router

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Area 0 / Backbone Area Area 1 Area 2 Ext. AS Internal Routers Backbone Routers Area Border Router AS Border Router

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Internal Routers Backbone Routers Area Border Router AS Border Router

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Internal Routers Backbone Routers Area Border Router AS Border Router Routers with all their interfaces within the same area Routers with at least one interface connected to area 0 Routers that have at least one interface connected to an another autonomous system Routers with interfaces attached to multiple areas within the same autonomous system

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Internal Routers Backbone Routers Area Border Router AS Border Router Routers with at least one interface connected to area 0 Routers that have at least one interface connected to an another autonomous system Routers with interfaces attached to multiple areas within the same autonomous system

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Internal Routers Backbone Routers Area Border Router AS Border Router Routers that have at least one interface connected to an another autonomous system Routers with interfaces attached to multiple areas within the same autonomous system

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thomas@thomasclausen.org http://www.thomasclausen.org mark@townsley.net http://www.townsley.net/

Internal Routers Backbone Routers Area Border Router AS Border Router Routers with interfaces attached to multiple areas within the same autonomous system

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Internal Routers Backbone Routers Area Border Router AS Border Router

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Internal Routers Backbone Routers Area Border Router AS Border Router

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Internal Routers Backbone Routers Area Border Router AS Border Router

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Internal Routers Backbone Routers Area Border Router AS Border Router LSA Type 1 - Router LSA

Generated by each router for each area it belongs to. Describes the states of the routers links to that area Flooded only within this particular area

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Internal Routers Backbone Routers Area Border Router AS Border Router LSA Type 1 - Router LSA

Generated by each router for each area it belongs to. Describes the states of the routers links to that area Flooded only within this particular area

LSA Type 2 - Network LSA

Generated by Designated Router in multiaccess link Describe et of routers attached to that link Flooded only within the area containing that link

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LSA Type 3/4 - Summary LSA

Generated by Area Border Router Describe links between Area Border Router and internal routers of local area Flooded through backbone area

Internal Routers Backbone Routers Area Border Router AS Border Router LSA Type 1 - Router LSA

Generated by each router for each area it belongs to. Describes the states of the routers links to that area Flooded only within this particular area

LSA Type 2 - Network LSA

Generated by Designated Router in multiaccess link Describe et of routers attached to that link Flooded only within the area containing that link

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LSA Type 5 - AS External LSA

Generated by AS Border Router Describe routes to destinations external to AS Flooded through an OSPF AS (Except for stub and totally stubby areas)

LSA Type 3/4 - Summary LSA

Generated by Area Border Router Describe links between Area Border Router and internal routers of local area Flooded through backbone area

Internal Routers Backbone Routers Area Border Router AS Border Router LSA Type 1 - Router LSA

Generated by each router for each area it belongs to. Describes the states of the routers links to that area Flooded only within this particular area

LSA Type 2 - Network LSA

Generated by Designated Router in multiaccess link Describe et of routers attached to that link Flooded only within the area containing that link

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LSA Type 5 - AS External LSA

Generated by AS Border Router Describe routes to destinations external to AS Flooded through an OSPF AS (Except for stub and totally stubby areas)

LSA Type 3/4 - Summary LSA

Generated by Area Border Router Describe links between Area Border Router and internal routers of local area Flooded through backbone area

Internal Routers Backbone Routers Area Border Router AS Border Router LSA Type 1 - Router LSA

Generated by each router for each area it belongs to. Describes the states of the routers links to that area Flooded only within this particular area

LSA Type 2 - Network LSA

Generated by Designated Router in multiaccess link Describe et of routers attached to that link Flooded only within the area containing that link

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Area Types

Area 0 / Backbone Area Stub Area Totally Stubby Area

Does not accept External LSAs Accept all LSAs Does not accept External or Summary LSAs

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Area Types

Area 0 / Backbone Area Stub Area Totally Stubby Area

Does not accept External LSAs Accept all LSAs Does not accept External or Summary LSAs

Areas serve to contain traffic, reduce size of LSDB in routers within area limit impact of topology changes make routing decisions lighter

36

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LSA Type 1: Router LSAs

  • We’ve already seen those

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LSA Type 2: Network LSAs

  • We’ll get to those in a bit, when talking

multi-access networks

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LSA Type 3: Summary LSAs

  • Originated by the ABR
  • Describes links between ABR and Internal

Routers of the area of the ABR

  • Flooded throughout the backbone (Area 0)

(thus, to other ABRs)

  • Routes learned via LSA type 3s are denoted

Inter-Area routes

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

LSA3 Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

41

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LSA Type 3: Summary LSAs

LSA3 Area 0 / Backbone Area Stub Area Totally Stubby Area

42

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LSA Type 3: Summary LSAs

LSA3 Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

LSA3 Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area LSA3

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area LSA3

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LSA Type 3: Summary LSAs

Area 0 / Backbone Area Stub Area Totally Stubby Area LSA3

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LSA Type 1+3:

  • Routers only see the topology of own area
  • When a link in one area changes, the adjacent routers
  • riginate in LSA 1‘s; flood them within the area,

causing intra-area (internal) routers to re-run the Dijkstra

  • ABRs do not announce topological information

between areas.

  • ABRs only inject routing information into other areas

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LSA Type 1+3:

  • ABRs calculate intra-area routes for directly attached

areas; announce them to all other areas as inter-area routes, using LSA Type 3

  • ABRs announces inter-area routes that were learned

from the backbone/area 0.

  • Backbone/area 0 serves as a repository for inter-area

routes.

  • This keeps OSPF safe from routing loops.

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LSA Type 5: AS External LSA

  • Originated by the ASBR
  • Describes destination networks, external to

this AS

  • Flooded throughout the backbone (Area 0)

(thus, to other ABRs) and all other areas except stub and totally stubby areas

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area LSA5

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area LSA5

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Ext. AS Area 0 / Backbone Area Stub Area Totally Stubby Area

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Ext. AS Area 0 / Backbone Area Totally Stubby Area

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Ext. AS Area 0 / Backbone Area Totally Stubby Area

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5

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Ext. AS Area 0 / Backbone Area Totally Stubby Area LSA5 LSA5 LSA4

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LSA Type 4: ASBR Summary-LSA

  • Originated by ABRs when an ASBR is

present to let other areas know where the ASBR is.

  • Handled just like summary-LSAs (Type 3)

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Multi-Access Media

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Multi-Access Media

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Multi-Access Media

E.g. Ethernet: create n(n-1)/2 adjacencies! Consider flooding process: lots of unnecessary LSAs

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Multi-Access Media

Elect Designated Router to represent network segment

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Multi-Access Media

Elect Designated Router to represent network segment

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Multi-Access Media

Elect Designated Router to represent network segment

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Designated Router

  • Represent the LAN segment
  • Elected as part of HELLO protocol
  • Send Network LSAs
  • List all local routers
  • Synchronizes (DB Exchange) with all local

routers

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Designated Router

  • The only router on segment sending LSAs to

the rest of the network

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Multi-Access Media

Backup Designated Router - as .... backup

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Multi-Access Media

Backup Designated Router - as .... backup

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Multi-Access Media

Backup Designated Router - as .... backup

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Backup and Designated Router

  • All routers establish adjacency to DR, BDR
  • BDR also establish adjacency to DR
  • Only DR sends LSAs, still, BDR is just there

if the DR fails

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DR and BDR Election

  • Starts if HELLO packets received do not list

DR/BDR

  • If DR/BDR already chosen, new router

(even with higher priority) does not change election

  • Yes, boot-up sequence matters ;)

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DR and BDR Election

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Autonomous Systems

  • A region of the Internet administered by a single

entity.

  • Examples of autonomous regions are:
  • X’s campus network
  • Frees backbone network
  • Routing is done differently within an autonomous

system (intradomain routing) and between autonomous system (interdomain routing).

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Intradomain routing

  • Internals of an AS never revealed outside AS
  • An EGP such as BGP does therefore not

seek optimal paths, but just any path

  • Intradomain routing is another, large,

subject; we will touch on that when we talk peering at Lecture 7 ....

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So what did we do?

  • Dijkstra is fine, but scaling world-wide is big:
  • ASes, IGB, EGP
  • Even ASes can become big:
  • Areas, different LSA types, area types
  • Some (flooding? topology?) reduction for

broadcast links

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So what will we do?

  • We saw some (flooding? topology?)

reduction for broadcast links (DR, BDR)

  • The exercises for this afternoon are simple:
  • Understand OSPF behavior well enough to

conceive of a modus operandi over non- transitive non-broadcast links.

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What Haven’t We Covered .....

  • Not So Stubby Areas (NSSAs)
  • Virtual Links
  • Non-Broadcast Multiple Access (NBMA)

links

  • Multicast
  • Security
  • ......

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