Introduction to BGP - PacNOG•Transit – carrying traffic across a network, usually for a fee...
Transcript of Introduction to BGP - PacNOG•Transit – carrying traffic across a network, usually for a fee...
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1© 2005, Cisco Systems, Inc. All rights reserved.Cisco ISPWorkshops
Introduction to BGPISP/IXP WorkshopsISP/IXP Workshops
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Border Gateway Protocol
• Routing Protocol used to exchange routinginformation between networks
exterior gateway protocol
• Described in RFC1771work in progress to updatewww.ietf.org/internet-drafts/draft-ietf-idr-bgp4-26.txt
• The Autonomous System is BGP’s fundamentaloperating unit
It is used to uniquely identify networks with commonrouting policy
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BGP
• Path Vector Protocol
• Incremental Updates
• Many options for policy enforcement
• Classless Inter Domain Routing (CIDR)
• Widely used for Internet backbone
• Autonomous systems
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Path Vector Protocol
• BGP is classified as a path vector routingprotocol (see RFC 1322)
A path vector protocol defines a route as apairing between a destination and theattributes of the path to that destination.
12.6.126.0/24 207.126.96.43 1021 0 6461 7018 6337 11268 i
AS PathAS Path
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Path Vector Protocol
AS6461
AS7018
AS6337 AS11268
AS500
AS600
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Definitions
• Transit – carrying traffic across a network,usually for a fee
• Peering – exchanging routing informationand traffic
• Default – where to send traffic when there isno explicit match in the routing table
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Default Free Zone
The default free zone is madeup of Internet routers which
have explicit routinginformation about the rest of the
Internet, and therefore do notneed to use a default route.
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Peering and Transit example
provider A
provider C
provider B
Backbone Provider D
IXP-West
IXP-East
A and B can peer, but need transitarrangements with D to get packets to/from C
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Autonomous System (AS)
• Collection of networks with same routing policy
• Single routing protocol
• Usually under single ownership, trust andadministrative control
• Identified by a unique number
AS 100
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Autonomous System Number (ASN)
• An ASN is a 16 bit number1-64511 are assigned by the RIRs64512-65534 are for private use and should never appear onthe Internet0 and 65535 are reserved
• 32 bit ASNs are coming soonwww.ietf.org/internet-drafts/draft-ietf-idr-as4bytes-09.txt
• ASNs are distributed by the Regional Internet RegistriesAlso available from upstream ISPs who are members of oneof the RIRsCurrent ASN allocations up to 38911 have been made to theRIRs
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AS 100 AS 101
AS 102
DMZNetwork
AA
BB
CC
DD
EE
• Shared network between ASes
Demarcation Zone (DMZ)
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BGP Basics
AS 100 AS 101
AS 102
EE
BB DD
AA CC
Peering
BGP speakersare called peers
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BGP General Operation
• Learns multiple paths via internaland external BGP speakers
• Picks the best path and installs inthe forwarding table
• Policies applied by influencing thebest path selection
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Constructing the Forwarding Table
• BGP “in” processreceives path information from peers
results of BGP path selection placed in the BGP table“best path” flagged
• BGP “out” processannounces “best path” information to peers
• Best paths installed in forwarding table if:prefix and prefix length are uniquelowest “protocol distance”
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Constructing the Forwarding Table
BGP inprocess
BGPtable
BGP outprocess
accepted
discarded
bgppeer
forwardingtable
in
out
best paths
everything
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eBGP & iBGP
• BGP used internally (iBGP) and externally(eBGP)
• iBGP used to carrysome/all Internet prefixes across ISP backboneISP’s customer prefixes
• eBGP used toexchange prefixes with other ASesimplement routing policy
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BGP/IGP model used in ISP networks
• Model representation
IGP
iBGP
IGP
iBGP
IGP
iBGP
IGP
iBGP
eBGP eBGP eBGP
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External BGP Peering (eBGP)
AS 100 AS 101CC
AA
• Between BGP speakers in different AS
• Should be directly connected
• Never run an IGP between eBGP peers
BB
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Configuring External BGP
Router A in AS100
interface ethernet 5/0 ip address 102.102.10.2 255.255.255.240!router bgp 100 network 100.100.8.0 mask 255.255.252.0 neighbor 102.102.10.1 remote-as 101 neighbor 102.102.10.1 prefix-list RouterC in neighbor 102.102.10.1 prefix-list RouterC out!
ip address onethernet interface
ip address of RouterC ethernet interface
Local ASN
Remote ASN
Inbound andoutbound filters
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Configuring External BGP
Router C in AS101
interface ethernet 1/0/0 ip address 102.102.10.1 255.255.255.240!router bgp 101 network 100.100.8.0 mask 255.255.252.0 neighbor 102.102.10.2 remote-as 100 neighbor 102.102.10.2 prefix-list RouterA in neighbor 102.102.10.2 prefix-list RouterA out!
ip address onethernet interface
ip address of RouterA ethernet interface
Local ASN
Remote ASN
Inbound andoutbound filters
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Internal BGP (iBGP)
• BGP peer within the same AS
• Not required to be directly connectedIGP takes care of inter-BGP speakerconnectivity
• iBGP speakers need to be fully meshedthey originate connected networks
they do not pass on prefixes learned fromother iBGP speakers
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Internal BGP Peering (iBGP)
• Topology independent• Each iBGP speaker must peer with
every other iBGP speaker in the AS
AS 100
AA
EE
BB
DD
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Peering to Loop-back Address
AS 100
• Peer with loop-back addressLoop-back interface does not go down – ever!
• iBGP session is not dependent on state of a singleinterface
• iBGP session is not dependent on physical topology
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Configuring Internal BGP
Router A in AS100
interface loopback 0 ip address 105.3.7.1 255.255.255.255
!router bgp 100
network 100.100.1.0 neighbor 105.3.7.2 remote-as 100 neighbor 105.3.7.2 update-source loopback0 neighbor 105.3.7.3 remote-as 100 neighbor 105.3.7.3 update-source loopback0!
ip address onloopback interface
ip address of RouterB loopback interface
Local ASN
Local ASN
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Configuring Internal BGP
Router B in AS100
interface loopback 0 ip address 105.3.7.2 255.255.255.255
!router bgp 100
network 100.100.1.0 neighbor 105.3.7.1 remote-as 100 neighbor 105.3.7.1 update-source loopback0 neighbor 105.3.7.3 remote-as 100 neighbor 105.3.7.3 update-source loopback0!
ip address onloopback interface
ip address of RouterA loopback interface
Local ASN
Local ASN
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Inserting prefixes into BGP
• Two ways to insert prefixes into BGPredistribute static
network command
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Inserting prefixes into BGP –redistribute static
• Configuration Example:router bgp 100
redistribute static
ip route 102.10.32.0 255.255.254.0 serial0
• Static route must exist before redistributecommand will work
• Forces origin to be “incomplete”• Care required!
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Inserting prefixes into BGP –redistribute static
• Care required with redistribute!redistribute <routing-protocol> meanseverything in the <routing-protocol> will betransferred into the current routing protocol
Will not scale if uncontrolled
Best avoided if at all possible
redistribute normally used with “route-maps”and under tight administrative control
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Inserting prefixes into BGP –network command
• Configuration Examplerouter bgp 100
network 102.10.32.0 mask 255.255.254.0
ip route 102.10.32.0 255.255.254.0 serial0
• A matching route must exist in the routingtable before the network is announced
• Forces origin to be “IGP”
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Configuring Aggregation
• Three ways to configure route aggregationredistribute static
aggregate-address
network command
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Configuring Aggregation
• Configuration Example:router bgp 100
redistribute static
ip route 102.10.0.0 255.255.0.0 null0 250
• static route to “null0” is called a pull up routepackets only sent here if there is no more specific match inthe routing tabledistance of 250 ensures this is last resort static
care required – see previously!
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Configuring Aggregation –Network Command
• Configuration Examplerouter bgp 100
network 102.10.0.0 mask 255.255.0.0
ip route 102.10.0.0 255.255.0.0 null0 250
• A matching route must exist in the routing tablebefore the network is announced
• Easiest and best way of generating an aggregate
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Configuring Aggregation –aggregate-address command
• Configuration Examplerouter bgp 100 network 102.10.32.0 mask 255.255.252.0 aggregate-address 102.10.0.0 255.255.0.0 [ summary-only ]
• Requires more specific prefix in BGP table beforeaggregate is announced
• {summary-only} keywordoptional keyword which ensures that only the summaryis announced if a more specific prefix exists in therouting table
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Historical Defaults – Auto Summarisation
• Disable historical default 1
• Automatically summarises subprefixes tothe classful network when redistributing toBGP from another routing protocol
Example:
61.10.8.0/22 → 61.0.0.0/8
• Must be turned off for any Internetconnected site using BGP
router bgp 100
no auto-summary
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Historical Defaults – Synchronisation
• Disable historical default 2• In Cisco IOS, BGP does not advertise a
route before all routers in the AS havelearned it via an IGP
• Disable synchronisation if:AS doesn’t pass traffic from one AS to another, orAll transit routers in AS run BGP, oriBGP is used across backbonerouter bgp 100 no synchronization
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SummaryBGP neighbour status
Router1>sh ip bgp sumBGP router identifier 100.1.15.224, local AS number 10BGP table version is 27, main routing table version 2714 network entries using 1582 bytes of memory14 path entries using 672 bytes of memory3/2 BGP path/bestpath attribute entries using 324 bytes of memory0 BGP route-map cache entries using 0 bytes of memory0 BGP filter-list cache entries using 0 bytes of memoryBGP using 2578 total bytes of memoryBGP activity 17/3 prefixes, 22/8 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd100.1.31.224 4 10 195 193 27 0 0 03:09:48 1100.1.63.224 4 10 71 70 27 0 0 01:05:31 1100.2.15.224 4 10 46 47 27 0 0 00:17:00 1...
BGP Version Updatessent andreceived
Updates waiting
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Summary
• BGP4 – path vector protocol
• iBGP versus eBGP• stable iBGP – peer with loopbacks
• announcing prefixes & aggregates
• no synchronization & no auto-summary
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Introduction to BGPISP/IXP WorkshopsISP/IXP Workshops