The%20need%20for%20BGP

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The need for BGP

• AfNOG Workshops
• Philip Smith

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Keeping Local Traffic Local
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IGP

• Interior Gateway Protocol
• Within a network/autonomous system
• Carries information about internal infrastructure
  prefixes
• Examples OSPF, ISIS, EIGRP

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Why do we need an IGP?

• ISP Backbone Scaling
• Hierarchy
• Modular infrastructure
• Limiting scope of failure
• Healing of faults with fast convergence

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EGP

• Exterior Gateway Protocol
• Used to convey routing information between
  networks/ASes
• De-coupled from the IGP
• Current EGP is BGP4

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Why Do We Need an EGP?

• Scaling to large network
• Hierarchy
• Limit scope of failure
• Define administrative boundary
• Policy
• Control reachability of prefixes

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Interior vs. Exterior Routing Protocols

• Interior
• Automatic neighbour discovery
• Generally trust your IGP routers
• Routes go to all IGP routers
• Binds routers in an AS together

• Exterior
• Specifically configured peers
• Connecting with outside networks
• Set administrative boundaries
• Binds ASes together

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Interior vs. Exterior Routing Protocols

• Interior
• Carries ISP infrastructure addresses only
• ISPs aim to keep IGPs small for scalability and
  efficiency

• Exterior
• Carries customer prefixes
• Carries Internet prefixes
• EGPs are independent of ISP network topology

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Overview

• Typical small ISP
• Direct connections with other ISPs
• Routing protocol requirements
• Scaling things up

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Typical Small ISP

• Local network
• May have multiple POPs
• Line to Internet
• International line providing transit connectivity
• Very, very expensive

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Typical Small ISP
Upstream ISP
Static routes
Static and/or default routes
Small ISP
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Other ISP in Country

• Similar setup
• Traffic between you and them goes over
• Your expensive line
• Their expensive line
• Traffic can be significant
• Same language/culture
• Traffic between your and their customers

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Other ISP in Country
Europe
Upstream ISP
Small ISP
Small ISP
Africa
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Bringing down costs

• Local (national) links much cheaper than
  international ones
• Might be interesting to get direct link between
  you and them
• Saving traffic on expensive lines
• better performance, cheaper
• No need to send traffic to other ISP down the
  street via New York!

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Keeping Local Traffic Local
Europe
Upstream ISP
Small ISP
Small ISP
Africa
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Terminology peer and transit

• Peer getting connectivity to network of other
  ISP
• and just that network, no other networks
• Frequently at zero cost (zero-settlement)
• Transit getting connectivity through network of
  other ISP to other networks
• getting connectivity to rest of world (or part
  thereof)
• Usually at cost (customer-provider relationship)

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Large ISP in Country

• Large ISP multi-homes to two or more upstream
  providers
• multiple connections
• to achieve
• redundancy
• connection diversity
• increased speeds

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Large ISP in Country
Upstream ISP
Upstream ISP
USA
Europe
Large ISP
Africa
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Making it work

• Just getting direct line is not enough
• Need to work out how to do routing
• Need to get local traffic between ISPs/peers
• Need to make sure the peer ISP doesnt use us for
  transit
• Need to control what networks to announce, what
  network announcements to accept to upstreams and
  peers

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Not using static routes

• ip route lttheir_networkgt lttheir_gatewaygt
• Does not scale
• The more external networks you connect to, the
  more complex and harder to maintain this becomes
• No failover in case of link failure
• Have to manually reconfigure
• Often needs assistance of upstream too

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Not using IGP (OSPF)

• Using an IGP gives serious operational
  consequences
• If the other ISP has a routing problem, you will
  have problems too
• Instability in their backbone becomes an
  instability in your backbone
• Leaked prefixes into their backbone become
  leakages into your backbone
• Wont scale
• Etc
• Very hard to filter routes so that we dont
  inadvertently give transit

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Using BGP instead

• BGP Border Gateway Protocol
• BGP is an Exterior Gateway Protocol
• Focus on routing policy, not topology
• BGP can make groups of networks (Autonomous
  Systems)
• Good route filtering capabilities
• Ability to isolate from others problems

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Autonomous Systems

• Autonomous systems is a misnomer
• Nothing to do with freedom, independence,
• Just a handle for a group of networks that is
  under the same administrative control
• Uniquely identified by an AS number
• 16 bit integer
• Assigned by registries, just like IP addresses

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Autonomous System(AS)...

• Identified by AS number
• example AS16907 (ISPKenya)
• Public Private AS numbers
• Public (assigned by registries) 1-26623
• Private 64512 65534
• AS 0 and 65535 are reserved
• Typical applications
• Service provider
• Multi-homed customers
• Anyone needing policy discrimination

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Using AS numbers

• BGP can filter on AS numbers
• Get all networks of the other ISP using one
  handle
• Include future new networks without having to
  change routing filters
• AS number for new network will be same
• Can use AS numbers in filters with regular
  expressions
• BGP actually does routing computation on IP
  numbers

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Summary Why do I need BGP?

• Network scaling
• Multi-homing connecting to multiple providers
• upstream providers
• local networks regional peering to get local
  traffic
• Policy discrimination
• controlling how traffic flows