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 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
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
eBGP & iBGP
• BGP used internally (iBGP) and externally
(eBGP)
• iBGP used to carry some/all Internet prefixes
across ISP backbone ISP’s customer prefixes
• eBGP used to exchange prefixes with other
Ases implement routing policy
External BGP Peering (eBGP)
• Between BGP speakers in different AS
• Should be directly connected
• Never run an IGP between eBGP peers
Configuring eBGP
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
Internal BGP peering
Configuring iBGP
BGP Attributes
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Origin
AS-Path
Next Hop
Multi_Exit_Disc
Local Preference
Atomic_aggregrate
Aggregrator
Origin
• The ORIGIN attribute is generated by the
speaker that originates the associated routing
information
AS_Path
• This attribute identifies the autonomous
systems through which routing information
carried in this UPDATE message has passed.
AS_Path Loop Detection
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
Next Hop -eBGP
Next Hop -iBGP
Multi_Exit_Disc (1)
• The MULTI_EXIT_DISC is an optional nontransitive 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.
Multi_Exit_Disc (2)
Multi_Exit_Disc (3)
Local Preference (1)
• LOCAL_PREF is an attribute that BGP speaker
SHALL calculate the degree of preference for
each external route based on the locallyconfigured policy, and include the degree of
preference when advertising a route to its
internal peers.
• The higher degree of preference MUST be
preferred.
Local Preference (2)
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
Local Preference (4)
Atomic_aggregrate
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Conveys the IP address of the router/BGP
Speaker generating the aggregate route
Useful for debugging purposes
Does not influence best path selection
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.
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
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.
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.
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.
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.