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Chapter 4 roadmap. 4.1 Introduction and Network Service Models 4.2 Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet 4.5.1 Intra-AS routing: RIP and OSPF 4.5.2 Inter-AS routing: BGP 4.6 What’s Inside a Router?.

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Chapter 4 roadmap
Chapter 4 roadmap

4.1 Introduction and Network Service Models

4.2 Routing Principles

4.3 Hierarchical Routing

4.4 The Internet (IP) Protocol

4.5 Routing in the Internet

  • 4.5.1 Intra-AS routing: RIP and OSPF

  • 4.5.2 Inter-AS routing: BGP

    4.6 What’s Inside a Router?

Network Layer

Routing in the internet
Routing in the Internet

  • The Global Internet consists of Autonomous Systems (AS) interconnected with each other:

    • Stub AS: small corporation: one connection to other AS’s

    • Multihomed AS: large corporation (no transit): multiple connections to other AS’s

    • Transit AS: provider, hooking many AS’s together

  • Two-level routing:

    • Intra-AS: administrator responsible for choice of routing algorithm within network

    • Inter-AS: unique standard for inter-AS routing: BGP

Network Layer

Internet as hierarchy
Internet AS Hierarchy

Intra-AS border (exterior gateway) routers

Inter-ASinterior (gateway) routers

Network Layer

Intra as routing
Intra-AS Routing

  • Also known as Interior Gateway Protocols (IGP)

  • Most common Intra-AS routing protocols:

    • RIP: Routing Information Protocol

    • OSPF: Open Shortest Path First

    • IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

    • IS-IS: Intermediate System to Intermediate System

Network Layer

Rip routing information protocol
RIP ( Routing Information Protocol)

  • Distance vector algorithm

  • Included in BSD-UNIX Distribution in 1982

  • Distance metric: # of hops (max = 15 hops)

    • Can you guess why?

  • Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

  • Each advertisement: list of up to 25 destination nets within AS

Network Layer

Rip example
RIP: Example









Destination Network Next Router Num. of hops to dest.

w A 2

y B 2

z B 7

x -- 1

…. …. ....

Routing table in D

Network Layer

Rip example1









RIP: Example

Dest Next hops

w - -

x - -

z C 4

…. … ...


from A to D

Destination Network Next Router Num. of hops to dest.

w A 2

y B 2

z B A 7 5

x -- 1

…. …. ....

Routing table in D

Network Layer

Rip link failure and recovery
RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declared dead

  • routes via neighbor invalidated

  • new advertisements sent to neighbors

  • neighbors in turn send out new advertisements (if tables changed)

  • link failure info quickly propagates to entire net

  • poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

Network Layer

Rip table processing



RIP Table processing

  • RIP routing tables managed by application-level process called route-d (daemon)

  • advertisements sent in UDP packets, periodically repeated





network forwarding

(IP) table









Network Layer

Rip table example continued
RIP Table example (continued)


Destination Gateway Flags Ref Use Interface

-------------------- -------------------- ----- ----- ------ --------- UH 0 26492 lo0

192.168.2. U 2 13 fa0

193.55.114. U 3 58503 le0

192.168.3. U 2 25 qaa0 U 3 0 le0

default UG 0 143454

  • Three attached class C networks (LANs)

  • Router only knows routes to attached LANs

  • Default router used to “go up”

  • Route multicast address:

  • Loopback interface (for debugging)

Network Layer

Weaknesses of rip
Weaknesses of RIP

  • INFINITY defined as 15, thus RIP cannot be used in networks where routes are more than 15 hops

  • Difficulty in supporting multiple metrics (default metric: # of hops)

    • the potential range for such metrics as bandwidth, throughput, delay, and reliability can be large

    • thus the value for INFINITY should be large; but this can result in slow convergence of RIP due to count-to-infinity problem

Network Layer

Ospf open shortest path first
OSPF (Open Shortest Path First)

  • “open”: publicly available

  • Uses Link State algorithm

    • LS packet dissemination

    • Topology map at each node

    • Route computation using Dijkstra’s algorithm

  • OSPF advertisement carries one entry per neighbor router

  • Advertisements disseminated to entire AS (via flooding)

    • Carried in OSPF messages directly over IP (rather than TCP or UDP

Network Layer

Ospf advanced features not in rip
OSPF “advanced” features (not in RIP)

  • Security: all OSPF messages authenticated (to prevent malicious intrusion)

  • Multiple same-cost paths allowed (only one path in RIP)

  • For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)

  • Integrated uni- and multicast support:

    • Multicast OSPF (MOSPF) uses same topology data base as OSPF

  • Hierarchical OSPF in large domains.

Network Layer

Hierarchical ospf
Hierarchical OSPF

Network Layer

Hierarchical ospf1
Hierarchical OSPF

  • Two-level hierarchy: local area, backbone.

    • Link-state advertisements only in area

    • each nodes has detailed area topology; only know direction (shortest path) to nets in other areas.

  • Area border routers:“summarize” distances to nets in own area, advertise to other Area Border routers.

  • Backbone routers: run OSPF routing limited to backbone.

  • Boundary routers: connect to other AS’s.

Network Layer

Internet inter as routing bgp
Internet inter-AS routing: BGP

  • BGP (Border Gateway Protocol):the de facto standard

  • Path Vector protocol:

    • similar to Distance Vector protocol

    • each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to destination

    • BGP routes to networks (ASs), not individual hosts

    • E.g., Gateway X may send its path to dest. Z:

      Path (X,Z) = X,Y1,Y2,Y3,…,Z

Network Layer

Internet inter as routing bgp1
Internet inter-AS routing: BGP

Suppose: gateway X send its path to peer gateway W

  • W may or may not select path offered by X

    • cost, policy (don’t route via competitors AS), loop prevention reasons.

  • If W selects path advertised by X, then:

    Path (W,Z) = w, Path (X,Z)

  • Note: X can control incoming traffic by controlling it route advertisements to peers:

    • e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z

Network Layer

Bgp controlling who routes to you
BGP: controlling who routes to you

  • A,B,C are provider networks

  • X,W,Y are customer (of provider networks)

  • X is dual-homed: attached to two networks

    • X does not want to route from B via X to C

    • .. so X will not advertise to B a route to C

Network Layer

Bgp controlling who routes to you1
BGP: controlling who routes to you

  • A advertises to B the path AW

  • B advertises to X the path BAW

  • Should B advertise to C the path BAW?

    • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

    • B wants to force C to route to w via A

    • B wants to route only to/from its customers!

Network Layer

Bgp operation
BGP operation

Q: What does a BGP router do?

  • Receiving and filtering route advertisements from directly attached neighbor(s).

  • Route selection.

    • To route to destination X, which path )of several advertised) will be taken?

  • Sending route advertisements to neighbors.

Network Layer

Bgp messages
BGP messages

  • BGP messages exchanged using TCP.

  • BGP messages:

    • OPEN: opens TCP connection to peer and authenticates sender

    • UPDATE: advertises new path (or withdraws old)

    • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

    • NOTIFICATION: reports errors in previous msg; also used to close connection

Network Layer

Why different intra and inter as routing
Why different Intra- and Inter-AS routing ?


  • Inter-AS: admin wants control over how its traffic routed, who routes through its net.

  • Intra-AS: single admin, so no policy decisions needed


  • hierarchical routing saves table size, reduced update traffic


  • Intra-AS: can focus on performance

  • Inter-AS: policy may dominate over performance

Network Layer

Bgp fragility
BGP fragility

  • BGP can contribute to lot of the routing instability in the internet

  • Interactions between IGPs & EGPs poorly understood (OSPF timeouts etc.)

  • Incorrect information being fed from an IGP to an EGP (and vice versa) can result in catastrophic meltdown

  • BGP route flaps and associated dampening introduces more complexity than before

  • A routing protocol that does not work in the simple case?!

Network Layer