Border Gateway Protocol (BGP)

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Border Gateway Protocol (BGP)

• W.lilakiatsakun

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BGP Basics

• BGP is the protocol which is used to make core
  routing decisions on the Internet
• It involves a table of IP networks or "prefixes"
  which designate network reachability
  among autonomous systems (AS).
• RFC 4271
• BGP version 4 is a De facto standard for
  exterior gateway protocol
• Run over TCP port 179

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BGP Operations

• Learns multiple paths via internal and external
  BGP speakers
• Picks the best path and installs in the
  forwarding table
• Best path is sent to external BGP neighbors
• Policies applied by influencing the best path
  selection

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eBGP iBGP

• BGP used internally (iBGP) and externally (eBGP)
• iBGP used to carry some/all Internet prefixes
  across ISP backbone ISPs customer prefixes
• eBGP used to exchange prefixes with other Ases
  implement routing policy

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External BGP Peering (eBGP)
Between BGP speakers in different AS Should
be directly connected Never run an IGP between
eBGP peers
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Configuring eBGP
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Internal BGP (iBGP)

• BGP peer within the same AS
• Not required to be directly connected
• IGP takes care of inter-BGP speaker connectivity
• iBGP speakers need to be fully meshed
• they originate connected networks
• They do not pass on prefixes learned from other
  iBGP speakers

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Internal BGP peering
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Configuring iBGP
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BGP Attributes

• Origin
• AS-Path
• Next Hop
• Multi_Exit_Disc
• Local Preference
• Atomic_aggregrate
• Aggregrator

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Origin

• The ORIGIN attribute is generated by the speaker
  that originates the associated routing information

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AS_Path

• This attribute identifies the autonomous systems
  through which routing information
• carried in this UPDATE message has passed.

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AS_Path Loop Detection
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Next Hop

• The NEXT_HOP defines the IP address of the router
  that SHOULD be used as the next hop to the
  destinations listed in the UPDATE message

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Next Hop -eBGP
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Next Hop -iBGP
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Multi_Exit_Disc (1)

• The MULTI_EXIT_DISC is an optional non-transitive
  attribute that is intended to be used on external
  (inter-AS) links to discriminate among multiple
  exit or entry points to the same neighboring AS.
• The value of the MULTI_EXIT_DISC attribute is a
  four-octet unsigned number, called a metric.
• All other factors being equal, the exit point
  with the lower metric SHOULD be preferred.

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Multi_Exit_Disc (2)
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Multi_Exit_Disc (3)
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Local Preference (1)

• LOCAL_PREF is an attribute that BGP speaker
  SHALL calculate the degree of preference for each
  external route based on the locally-configured
  policy, and include the degree of preference when
  advertising a route to its internal peers.
• The higher degree of preference MUST be preferred.

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Local Preference (2)
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Local Preference (3)

• Local to an AS non-transitive
• Default local preference is 100 (IOS)
• Used to influence BGP path selection
• determines best path for outbound traffic
• Path with highest local preference wins

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Local Preference (4)
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Atomic_aggregrate

• Conveys the IP address of the router/BGP
• Speaker generating the aggregate route
• Useful for debugging purposes
• Does not influence best path selection

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Path Selection (1)

• If the NEXT_HOP attribute of a BGP route depicts
  an address that is not resolvable, or if it would
  become unresolvable if the route was installed in
  the routing table, the BGP route MUST be excluded
  from the decision function.
• If the AS_PATH attribute of a BGP route contains
  an AS loop, the BGP route should be excluded from
  the decision function.
• AS loop detection is done by scanning the full AS
  path (as specified in the AS_PATH attribute), and
  checking that the autonomous system number of the
  local system does not appear in the AS path.

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Path Selection (2)

• Criteria for path selection
• a) the highest degree of preference of any route
  to the same set of destinations, or
• b) is the only route to that destination, or
• c) is selected as a result of the Phase 2 tie
  breaking rules

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Algorithm for remove path (1)

• a) Remove from consideration all routes that are
  not tied for having the smallest number of AS
  numbers present in their AS_PATH attributes.
• b) Remove from consideration all routes that are
  not tied for having the lowest Origin number in
  their Origin attribute.

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Algorithm for remove path (2)

• c) Remove from consideration routes with
  less-preferred MULTI_EXIT_DISC attributes.
• MULTI_EXIT_DISC is only comparable between routes
  learned from the same neighboring AS (the
  neighboring AS is determined from the AS_PATH
  attribute).
• Routes that do not have the MULTI_EXIT_DISC
  attribute are considered to have the lowest
  possible MULTI_EXIT_DISC value.

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Algorithm for remove path (3)

• d) If at least one of the candidate routes was
  received via EBGP, remove from consideration all
  routes that were received via IBGP.
• e) Remove from consideration any routes with
  less-preferred interior cost.
• The interior cost of a route is determined by
  calculating the metric to the NEXT_HOP for the
  route using the Routing Table.

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Algorithm for remove path (4)

• f) Remove from consideration all routes other
  than the route that was advertised by the BGP
  speaker with the lowest BGP Identifier value.
• g) Prefer the route received from the lowest peer
  address.