Transcript Border Gateway Protocol (BGP)

Border Gateway Protocol (BGP)
W.lilakiatsakun
BGP Basics (1)
 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 Basics (2)
 The Border Gateway Protocol makes routing
decisions based on paths, network policies or rulesets configured by a network administrator.
 The major enhancement in version 4 was support for
Classless Inter-Domain Routing and use of route
aggregation to decrease the size of routings.
BGP Messages (1)
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BGP Messages
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
BGP Messages (2)
 OPEN Message
 After a TCP connection is established, the first message
sent by each side is an OPEN message.
 If the OPEN message is acceptable, a KEEPALIVE
message confirming the OPEN is sent back.
BGP Messages (3)
 KEEP ALIVE
 BGP does not use any TCP-based, keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages
are exchanged between peers often enough not to cause the
Hold Timer to expire.
 A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval.
 KEEPALIVE messages MUST NOT be sent more frequently than
one per second.
BGP Messages (4)
 NOTIFICATION
 A NOTIFICATION message is sent when an error
condition is detected.
 The BGP connection is closed immediately after it is
sent.
BGP Messages (5)
 UPDATE
 UPDATE messages are used to transfer routing information
between BGP peers.
 The information in the UPDATE message can be used to
construct a graph that describes the relationships of the
various Autonomous Systems.
BGP Messages (6)
 UPDATE (con’t)
 An UPDATE message is used to advertise feasible routes
that share common path attributes to a peer, or to
withdraw multiple unfeasible routes from service
 An UPDATE message MAY simultaneously advertise a
feasible route and withdraw multiple unfeasible routes
from service.
BGP Operations (1)
 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
BGP Operations (2)
 BGP neighbors, called peers, are established by
manual configuration between routers to create
a TCP session on port 179.
 A BGP speaker sends 19-byte keep-alive messages
every 30 seconds to maintain the connection.
 Among routing protocols, BGP is unique in using TCP
as its transport protocol.
BGP Operations (3)
 When BGP runs between two peers in the
same autonomous system (AS), it is referred to
as Internal BGP (iBGP)
 When it runs between different autonomous systems,
it is called ExternalBGP (eBGP)
 The main difference between iBGP and eBGP peering
is in the way routes that were received from one peer
are propagated to other peers
BGP Operations (4)
 For instance, new routes learned from an eBGP peer are
typically redistributed to all other iBGP peers as well as all
eBGP peers (if transit mode is enabled on the router).
 However, if new routes were learned on an iBGP peering,
then they are re-advertised only to all other eBGP peers but
not advertising to all other iBGP peers .
 These route-propagation rules effectively require that all iBGP
peers inside an AS are interconnected in a full mesh.
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 (1)
 Well-known attributes – must be supported by every
BGP implementation
 Mandatory attributes – must be included with every
route entry. If one attribute is missing, it will result in
an error message
 Ex: ORIGIN, AS_PATH, NEXT_HOP, LOCAL_PREF
BGP Attributes (2)
 Discretionary attributes – every BGP router must
recognize, but they don’t have to be present with
every route entry
 Ex. ATOMIC_AGGREGATE
 Optional attributes – not necessarily supported by all
BGP implementations. It can be either transitive or
non-transitive.
 Ex. AGGREGATOR, COMMUNITY, MULTI_EXIT_DISC
BGP Attributes (3)
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Origin
AS-Path
Next Hop
Multi_Exit_Disc
Local Preference
Atomic_aggregrate (not often used)
Aggregrator (not often used)
Origin
 ORIGIN is a well-known mandatory attribute.
 The ORIGIN attribute is generated by the speaker that
originates the associated routing information.
 Three values: IGP, EGP, incomplete
 IGP generated by BGP network statement
 EGP generated by EGP
 Incomplete redistributed from another routing protocol
AS_Path (1)
 This attribute identifies
the autonomous
systems through which
routing information
carried in this UPDATE
message has passed.
AS_Path (2)
 AS_Path is Used for
 Loop detection
 Path metrics where the length of the AS Path is used as
in path selection
AS_Path (3)
AS_Path Loop Detection
AS_Path (4)
 When a BGP speaker propagates a route it learned
from another BGP speaker’s UPDATE message, it
modifies the route’s AS_PATH attribute based on the
location of the BGP speaker to which the route will be
sent only when a given BGP speaker advertises the
route to an external peer.
Next Hop (1)
 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
 Well known mandatory attribute
Next Hop (2)
The IP address to reach the next AS
– Router A advertise 150.10.0.0/16 and 160.10.0.0/16 to router B
in eBGP with next hop 150.10.1.1 (Change it to own IP)
– Router B will update Router C in iBGP keeping the next hop
unchanged
Next Hop (3)
 IOS default is for external next-hop to be propagated
unchanged to iBGP peers
 This means that IGP has to carry external next-hops
 ISP Best Practice is to change external next-hop to be
that of the local router
 neighbor x.x.x.x next-hop-self
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 fouroctet unsigned number, called a metric.
 All other factors being equal, the exit point with the
lower metric SHOULD be preferred.
Multi_Exit_Disc (2)
MED
MED
Multi_Exit_Disc (3)
 For prefix 120.68.1.0/24 Router B send MED 1000 and
router A send MED 2000 to eBGP neighbor
 Incoming traffic from AS200 will choose Router B
since lowest MED will win
Local Preference (1)
 Local preference is used to advertise to IBGP
neighbors only about how to leave their AS
(Outbound Traffic).
 Paths with highest preference value are most
desirable
 Local preference attribute is well-known and
discretionary and is passed only within the AS
 Cisco Default Local Pref is 100
Local Preference (2)
For destination 160.10.0.0/16 Router A advertise local
pref 500 and Router B advertise local pref 800 in iBGP
• 800 will win best path (Router B)
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)
 Step 1: Prefer highest weight (local to router)
 Step 2: Prefer highest local preference (global within
AS)
 Step 3: Prefer route originated by the local router
 Step 4: Prefer shortest AS path
 Step 5: Prefer lowest origin code (IGP < EGP <
incomplete)
Path Selection (3)
 Step 6: Prefer lowest MED (from other AS)
 Step 7: Prefer eBGP path over iBGP path
 Step 8: Prefer the path through the closest IGP
neighbor
 Step 9: Prefer oldest route for eBGP paths
 Step 10: Prefer the path with the lowest neighbor BGP
router ID