ilab Lab 3 Dynamic Routing Static Routing TCP/ IP ISO/ OSI - - PowerPoint PPT Presentation

Lehrstuhl für Netzarchitekturen und Netzdienste

Institut für Informatik Technische Universität München

ilab

Lab 3 Dynamic Routing

2 ilab: Dynamic Routing

Static Routing

Ciscorouter, IPv4, ICMP, ARP

Session Presentation Application Physical Datalink Network Transport Network Transport Application Host-to-Net TCP/ IP ISO/ OSI

Concepts, Hardware, Software

• 1. Setup - static routing
• 2. The routing table
• 3. The default gateway
• 4. Packet forwarding
• 5. Further configuring of the Cisco router
• 6. Checking if everything is set up correctly
• 7. Watching the packets travel
• 8. One more interesting experiment...
• 9. Suggestions/ complaints
• 10. Please remove the Cables

Minicom

3 ilab: Dynamic Routing

Dynamic Routing

RIP, OSPF, BGP Session Presentation Application Physical Datalink Network Transport Network Transport Application Host-to-Net TCP/ IP ISO/ OSI

Concepts, Hardware, Software

• 1. Setup - Dynamic routing
• 2. RIP (Cisco/ Linux)

2.4. What did RIP do? 2.5. Changing the setup a little... 2.6. Configuring the serial link 2.7. RIP done.

• 3. OSPF (without/ with areas)

3.3. Distance values 3.6. Compare: OSPF with and without areas 3.7. Ad-/ Disadvantages of OSPF area routing 3.8. Inspecting OSPF packets

• 4. BGP

4.1. Autonomous systems Cisco Serial Link

Zebra

4 ilab: Dynamic Routing

Routing – Problem definition



Given: Graph



Main problem:

• How to determine the shortest

path tree in order to forward packets to their destination



Subproblems:

• 1. Information gathering - which information do we need
• 2. Path calculation
• 3. Forwarding – how does a node decide?
• Based on its routing table

Routing algorithms solve subproblems 1 and 2

A E D C B F

2 2 1 3 1 1 2 5 3 5

5 ilab: Dynamic Routing

Distance Vector Routing

Approach



Information gathering

• Neighboring nodes share their

routing information (destination, costs)



Path calculation

• Distance table is updates if routing information

has changed

• e.g. costs have changed



Outcome

• Update routing table if the best (least expensive) entry in distance table has

changed

Properties



distributed – each nodes only shared information with its neighbor



Iterative - algorithm terminates after all information has been exchanged

A E D C B F

2 2 1 3 1 1 2 5 3 5

B costs 2 C costs 4 D costs 1 E costs 2 F costs 4 A costs 1 B costs 2 C costs 3 E costs 1 F costs 3

?

6 ilab: Dynamic Routing

Distance Table

D () A B C D A 1 7 6 4 B 15 8 9 11 D 5 5 4 2

E

Costs to destination via

7 8 1 2 1 2

A B C D E

D (Y,Z)

X Distance from X to Y via Z c(X,Z) + min {D (Y,w)}

Z

= =

D (C,D)

E

c(E,D) + min {D (C,w)}

D w

=

= 2+2 = 4

Loop!



Example



Distance table contains unwanted routes

• Loops result in endlessly circulating packets (until the TTL field expires)

D (C,A)

E

c(E,A) + min {D (C,w)}

A w

=

= 1+5 = 6

7 ilab: Dynamic Routing

Distance table -> Routing table D () A B C D A 1 7 6 4 B 15 8 9 11 D 5 5 4 2

E costs via

A B C D A,1 D,5 D,4 D,2

Exit link, costs

Distance table

• f E

Routing table

• f E

8 ilab: Dynamic Routing

Changes of the topology and of costs

 Rule:  good news travels fast  bad news travels slowly - “count to infinity” problem!

X

1 4 50 60

etc.

Y Z

9 ilab: Dynamic Routing

Split Horizon with poison reverse, path vector

 Split-Horizon

• Update messages of node A to D do not contain routes to nodes, which A

would route via D

 Poison Reverse

• Cost entry for such nodes is set to infinity
• Eliminates short loops

 Path vector routing

• Similar to distance vector protocol
• In order to avoid the count to infinity problem, nodes also include path

information in their update messages

A E D C B F

2 2 1 3 1 1 2 5 3 5

B costs 2 C costs ∞ D costs ∞ E costs ∞ F costs ∞

10 ilab: Dynamic Routing

Link-State Routing

Approach



Information gathering

• Nodes are aware of the network

topology due to the broadcasting of link properties



Path calculation

• Each node calculates its own shortest

path tree (itself being the root of the tree)

• Algorithm used:

Dijkstra or Bellman-Ford

• Derive routing table from shortest

path tree



Result: Routing table

A E D C B F

2 2 1 3 1 1 2 5 3 5

Flooding

A E D C B F

2 1 1 2 1

Shortest Path tree B B C-F D to via

11 ilab: Dynamic Routing

Algorithm of Dijkstra

1.

INPUT

2.

Q : set of all nodes

3.

a : node

4.

OUTPUT

5.

d : distance for each node

6.

p : predecessor of each node in shortest-path tree

7.

BEGIN

8.

FOR all nodes v DO d[v]=inf; ENDFOR

9.

d[a]=0;

10.

p[a]=null;

11.

N={a}; // MinHeap regarding d[]

12.

WHILE N.notEmpty() DO

13.

v = N.popMinimum();

14.

FOR all nodes u adjacent to v DO

15.

IF d[u] > d[v]+c(v,u) THEN

16.

d[u] = d[v]+c(v,u);

17.

p[u] = v;

18.

N.addOrUpdate(u);

19.

ENDIF

20.

ENDFOR

21.

ENDWHILE

22.

END

Notation:

• c(i,j): costs from node I to node j
• d(v): distance from root a to v in the shortest

path tree

• p(v): Vorgänger von v im Kürzesten-Wege-

Baum

A E D C B F

2 2 1 3 1 1 2 5 3 5



Complexity: O(n2), n being the number of nodes

12 ilab: Dynamic Routing

Erstellen der Routing-Tabelle

 Erstellen der Routing-Tabelle aus dem Kürzeste-Wege-Baum  Wünschenswert ist es, nicht jedes Ziel einzeln eintragen zu müssen,

sondern Bereiche zusammenfassen zu können.

• siehe hierarchische Struktur der IP-Adressen

A E D C B F

2 2 1 3 1 1 2 5 3 5 Ziel Nächster Hop B B CDEF D

13 ilab: Dynamic Routing

Forward-Search Algorithm

 In practice, nodes use a forward search alogorithm based on the

• riginal algorithm of Dijkstra

 All nodes flood the network with their Link State Packets (LSP)  Nodes maintain a tentative and a confirmed list and calculate the

routing table directly after receiving the LSPs A C B

2 4 1 8 3

D

14 ilab: Dynamic Routing

Example for node D

Step 1 2 3 4 5 6 7 confirmed (D,0,-) (D,0,-) (D,0,-) (A,1,A) (D,0,-) (A,1,A) (D,0,-) (A,1,A) (B,3,A) (D,0,-) (A,1,A) (B,3,A) (D,0,-) (A,1,A) (B,3,A) (C,6,A) tentativ (A,1,A) (B,4,B) (C,8,C) (B,4,B) (C,8,C) (B,3,A) (C,8,C) (C,8,C) (C,6,A) Notes Read LSP of D populate tentative list Add the least expensive entry to the confirmed list (here A) Read its LSP and update the entries (here: path to B) Add the least expensive entry to the confirmed list Read its LSP and update the entries (here: route to C) Add the least expensive entry to the confirmed list

A C B

2 4 1 8 3

D

15 ilab: Dynamic Routing

Routing in the internet

16 ilab: Dynamic Routing

Internet: autonomous systems

AS X AS A AS D AS Z AS B AS C

Inter-AS- connection

Border Router Autonomous System X

AS X

Stub-AS Multihomed AS

Transittraffic

17 ilab: Dynamic Routing

Dynamic routing in the internet

 Autonomous System (AS):

Networks under one administrative organization

 Interior Gateway (IG):

Internal routers of an AS

 Exterior Gateway (EG):

Border routers Core Network EG EG IG IG IG IG

AS AS

IG

Interior Gateway Protocols (IGP) Exterior Gateway Protocols (EGP) Routing in the internet

• Exterior Gateway Protocol (EGP) - outdated
• Border Gateway Protocol (BGP)
• Routing Information Protocol (RIP)
• Open Shortest Path First (OSPF)

18 ilab: Dynamic Routing

Routing hierarchy – protocols

 Intra-Domain-Routing:

• OSPF (Open Shortest Path First)
• IAB recommended protocol
• „Link State“ protocol
• RIP (Routing Information Protocol) – for small networks
• Less robust (routing loops)
• Slower reaction on link changes
• Distance vector protocol
• IGRP: Interior Gateway Routing Protocol (Cisco propr.)

 Inter-Domain-Routing:

• BGP (Border Gateway Protocol)
• Path vector protocol
• BGP Version 4 (BGP4) also supports Classless Inter-Domain Routing

(CIDR)

19 ilab: Dynamic Routing

RIP ( Routing Information Protocol)



Distance-Vektor-Verfahren



since 1982 in BSD-UNIX



Metric: # of Hops (max = 15 Hops)



Distance vectors: updates/advertisements are sent every 30s via UDP



Each advertisement contains routes to max. 25 destination networks



Link declared unreachable after 180s without an update

• Routes via this neighbor are considered invalid
• New advertisements have to be sent to all neighboring nodes
• Sende neue Advertisements zu den Nachbarn
• Neighbors also send new advertisements on changes
• Poison-Reverse to avoid loops (infinity = 16 hops)

20 ilab: Dynamic Routing

OSPF (Open Shortest Path First)

 Link-State-Verfahren



OSPF-Advertisement beinhaltet einen Eintrag pro Nachbarrouter



Advertisements werden an ganzes AS geflutet



Sicherheit: alle OSPF-Messages authentisiert, über TCP-Verbindung



unterstützt Multiple-same-cost-Pfade



Für jeden Link, multiple Kostenmetriken abhängig vom TOS (Linkkosten zum Beispiel abhängig davon, ob Best-Effort oder Realtime-Verkehr)



Integrierte Unterstützung von Uni- und Multicast:

• Multicast OSPF (MOSPF) basiert auf gleicher Topologie-Datenbank wie

OSPF



Hierarchisches OSPF für große AS.

21 ilab: Dynamic Routing

Hierarchical OSPF

22 ilab: Dynamic Routing

Inter-AS-Routing

23 ilab: Dynamic Routing

BGP

 BGP (Border Gateway Protocol):  Standard protocol in the internet  Path-Vector-Protocol:

• Similar to distance vector
• Each border gateway broadcasts its neighbors the entire path

(sequence of AS numbers)

 Neighbors decide based on policies and costs which path to use

24 ilab: Dynamic Routing

Intra- vs. Inter-AS-Routing

 Policy:

• Inter-AS: Each administrator wants to control which traffic is routed

through the network

• Intra-AS: no need for policies

 Scalablity:

• Hierarchical routing reduces the size of the routing tables and the traffic

needed for exchanging routing information

 Performance:

• Intra-AS: focusses on performance
• Inter-AS: policies often more important than performance