The need for BGP
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Transcript of The need for BGP
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The need for BGP
Keeping local traffic local
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IGPInterior Gateway ProtocolWithin a network/autonomous
systemCarries information about internal
prefixesExamples - OSPF, ISIS, EIGRP
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EGPExterior Gateway ProtocolUsed to convey routing information
between networks/ASesDe-coupled from the IGPCurrent EGP is BGP4
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Why Do We Need an EGP?
Scaling to large network Hierarchy Limit scope of failure
Define administrative boundaryPolicy
Control reachability to prefixes
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Interior vs. Exterior Routing ProtocolsInterior
Automatic discovery
Generally trust your IGP routers
Routes go to all IGP routers
Exterior Specifically
configured peers Connecting with
outside networks Set administrative
boundaries
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OverviewTypical small ISPDirect connections with other ISP’sRouting protocol requirementsScaling things up
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Typical Small ISPLocal networkMay have multiple POPsLine to Internet
International line providing transit connectivity
Very, very expensive
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Typical Small ISP
Upstream ISP
Small ISP
Static, default routes
or IGP
Static, default routes
or IGP
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Other ISP in CountrySimilar setupTraffic 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
Upstream ISP
Small ISP Small ISP
Africa
Europe
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Bringing down costsLocal (national) links much cheaper
than international onesMight 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
Upstream ISP
Small ISP Small ISP
Africa
Europe
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Terminology: peer and transitPeer: getting connectivity to network of
other ISP … and just that network, no other networks Frequently at zero cost (zero-settlement)
Transit: getting connectivity though 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 CountryLarge ISP multi-homes to two or
more upstream providers multiple connections to achieve:
redundancyconnection diversityincreased speeds
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Large ISP in CountryUpstream
ISPUpstream
ISP
Africa
EuropeUSA
Large ISP
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Making it workJust getting direct line is not enoughNeed to work out how to do routing
Need to get local traffic between ISP’s/peers
Need to make sure the peer ISP doesn’t 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 their_network their_gatewayDoes not scale
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Not using IGP (OSPF)Serious operational consequences:
If the other ISP has a routing problem, you will have problems too
Very hard to filter routes so that we don’t inadvertently give transit
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Using BGP insteadBGP = Border Gateway ProtocolBGP is an EGP routing protocolFocus on routing policy, not topologyBGP can make ‘groups’ of networks
(Autonomous Systems)Good route filtering capabilitiesAbility to isolate from other’s problems
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Autonomous SystemsAutonomous 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
Identified by an AS number
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Autonomous System(AS)...
Identified by ‘AS number’ example: AS16907 (ISPKenya)
Public & Private AS numbers Public (assigned by registries): 1-22527 Private: 64512 - 65534
Examples: Service provider Multi-homed customers Anyone needing policy discrimination
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Autonomous System numbers16-bit number, 1-65534Assigned by registry, just like IP
numbersAS 0 and AS 65535 are reservedTop 1024 AS numbers (AS64512-
AS65534) are private numbers see RFC1930 for details
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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 filtersAS 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?Multi-homing - connecting to multiple
providers upstream providers local networks - regional peering to get
local trafficPolicy discrimination
controlling how traffic flows