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Classless Inter-Domain Routing (CIDR) and Border Gateway Protocol (BGP), Slides of Computer Networks

Jaypee University of Engineering & TechnologyComputer Networks

Classless inter-domain routing (cidr) is a method used to aggregate router entries in a routing table and reduce the size of routing tables at the backbone level. It enables the use of variable-length prefixes and enables hierarchical aggregation of routing information through border gateway protocol (bgp). The issues addressed by cidr, the concept of cidr prefixes, and the role of bgp in interdomain routing.

Typology: Slides

2012/2013

Uploaded on 04/27/2013

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Classless Inter Domain Routing
• (CIDR) – Classless Interdomain Routing
• Issues address:
– Large routing table at the backbone
– Exhaustion of address space
• Enables aggregation of router
– A single entry in a routing table
– Tells how to reach a number of Networks
– Configures allocation of router
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Classless Inter Domain Routing

  • (CIDR) – Classless Interdomain Routing • Issues address:
    • Large routing table at the backbone – Exhaustion of address space

Enables aggregation of router

  • A single entry in a routing table – Tells how to reach a number of Networks –

Configures allocation of router

•(CIDR) – Classless Interdomain Routing•Issues address:

•Large routing table at the backbone•Exhaustion of address space

  • Enables aggregation of router

•A single entry in a routing table•Tells how to reach a number of Networks• Configures allocation of router

Classless Inter Domain Routing

CIDR (contd.)

Example

192.4.16 through 192.4.

Top 20 bits are the same

Router entry for top 20 bits as Network number

Basically uses a common network prefix < length, value>pairs

Border Gateway Routing

• Assumes Internet is organised as an

Autonomous system

  • Each under the control of a single

administration entity

  • Enables hierarchical aggregation of routing

information

BGP (contd.)

Interdomain routing problem – ASes sharereachability information each other

Reduces routing information at each AS

Use default routes

Example tenet Gate Border router – Any packets destined for outside

(at a router inside tenet) sends to

tenet gateway

  • Finally reaches a backbone provides who knows how to

reach all Networks

Border Gateway Protocol

Assumes Internet is an arbitrary connection of ASes

Large corporation

Consumer ISP

Peering point

Backbone Service

provided

Peering point

Consumer ISP

Consumer ISP Small corporation

Autonomous

Systems

BGP (contd.)

• Types of ASes:

  • Stub AS: Single connection to one otheAS
    • Example: Small Corporation
      • only local traffic
        • Multihomed AS: AS has connections to

multiple Ases

  • but does not carry transit traffic • Example: large corporation
    • Transit AS: Connection to more than one AS
        • carries both transit and local traffic • - backbone provider

R

1

R

3

R

2

R

4

R

6

R

5

Find any path tointended destination

Border Gateway Protocol (contd.)

BGP Goals:

Border Gateway Protocol (contd.)

• Configuring BGP:

  • BGP speaker
    • Spokesperson for entire AS • Establish session with other BGP speakers • Identify border “Gatewa • Routers through which packets enter/ leave A •

Example R

2

, R

4

“Gateway” – An IP router forwarded packetsbetween ASes

Border Gateway Protocol

• BGP – Neither DV or LSP

  • Advertises complete paths • Enumerated list of ASes
    • To reach a network
      • Enable policy decisions • Enable detection of routing loops

Border Gateway Protocol(contd.)

  • AS
    • AS Regional provider B
      • AS Regional provider A
        • Customer - AS
          • Customer 2AS
          • Customer - AS
          • Customer - AS

Issues in looping:

AS

AS

AS

Example:

AS1 learns it can reach network 1 via AS

Advertises (AS1, AS2) to AS

Now AS3 advertises to AS

- (AS3, AS1, AS2) to reach network P

AS2 – see it

ignores

Border Gateway Protocol (contd.)

Border Gateway Protocol

(contd.)

• BGP – designed to cope with classless

addresses

Networks advertised in BGP are actually prefixes of any length

Addresses contain prefix and length 142.4.16 /

  • Complexity of BGP
    • Depends on number of ASes

Border Gateway Protocol

(contd.)

• Issues backbone routers:

  • Inject prefixed learnt from another AS into its

intra domain

  • Complex

Overcome this?

  • IBGP (Interior Border Gateway Routing

Protocol)