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Border Gateway Protocol (BGP)

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Border Gateway Protocol (BGP) W.lilakiatsakun BGP Basics BGP is the protocol which is used to make core routing decisions on the Internet It involves a table of IP ... – PowerPoint PPT presentation

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Title: Border Gateway Protocol (BGP)


1
Border Gateway Protocol (BGP)
  • W.lilakiatsakun

2
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

3
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

4
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

5
External BGP Peering (eBGP)
Between BGP speakers in different AS Should
be directly connected Never run an IGP between
eBGP peers
6
Configuring eBGP
7
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

8
Internal BGP peering
9
Configuring iBGP
10
BGP Attributes
  • Origin
  • AS-Path
  • Next Hop
  • Multi_Exit_Disc
  • Local Preference
  • Atomic_aggregrate
  • Aggregrator

11
Origin
  • The ORIGIN attribute is generated by the speaker
    that originates the associated routing information

12
AS_Path
  • This attribute identifies the autonomous systems
    through which routing information
  • carried in this UPDATE message has passed.

13
AS_Path Loop Detection
14
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

15
Next Hop -eBGP
16
Next Hop -iBGP
17
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.

18
Multi_Exit_Disc (2)
19
Multi_Exit_Disc (3)
20
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.

21
Local Preference (2)
22
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

23
Local Preference (4)
24
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

25
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.

26
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

27
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.

28
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.

29
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.

30
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.
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