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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
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ODA030010 BGP RoutingProtocol
ISSUE 1.4
Page 1HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
Upon completion of this course, you
will be able to:
Understand the principle of BGP
Understand the features of
Attributes
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ChapterChapter 11 Principle of BGPPrinciple of BGP
ChapterChapter 22 BGP Route AttributesBGP Route Attributes
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Overview Of BGP
BGP is an exterior routing protocol, used to transmit routing
information between ASs
It is a kind of distance-vector routing protocol and avoids the
occurrence of loop in design. It provides additional attribute
information for the route
Transfer protocol: TCP; port No.: 179
It supports Classless Inter-Domain Routing (CIDR)
Route updating: transmit incremental routes only
Abundant route filtering and routing policies
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Autonomous System
What is an Autonomous System (AS)?
which routing protocol running inside the AS
Which routing protocol running between ASs
The Autonomous System (AS) refers to a set of routers, which aremanaged by the same technical management organization and adopt
the unified routing strategy. Each AS has a unique AS number, whichis allocated by the management organization authorized by theInternet.
IGP routing protocol such as static route, OSPF , IS-IS etc
BGP only
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Working Mechanism Of BGP
AS1
AS7
AS6
AS5
AS4
AS3
AS2
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IBGP Neighbor & EBGP Neighbor
EBGP
RTB
RTC
IBGP
RTA
RTD
RTE
EBGP
AS100
AS200
AS300
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Route Advertising Principles of BGP
BGP Speaker only selects the best one for its own use
BGP Speaker only advertises the routes used by itself to its
neighbors
For the routes obtained from EBGP, the BGP Speaker will
advertise them to all its neighbors (including EBGP and IBGP) For the routes obtained from IBGP, the BGP Speaker will not
advertise them to its IBGP neighbors
For the routes obtained from IBGP, whether the BGP Speaker will
advertise them to its EBGP neighbors depends on the
synchronization state of IGP and BGP
Once the connection is established, the BGP Speaker will
advertise all its BGP routes to the new neighbors
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BGP Synchronization
EBGP EBGP
RTB
RTC
IBGP
RTA
RTD
RTE
RTF
E0:10.1.1.1/24
S0
S1
AS100
AS200
AS300
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Full Dynamic Redistribution
OSPF discovers route 18.0.0.1/8
Dynamically redistribute the route discovered by IGP (OSPF)
into the BGP routing table of RTB
18.0.0.1/8
OSPFRTB
AS200
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Semi Dynamic Redistribution
OSPF discovers the route 18.0.0.1/8
Semi-dynamically redistribute the route discovered by IGP
(OSPF) into the BGP routing table of RTB
18.0.0.1/8
RTB
AS200
OSPF
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Static Redistribution
Manually configure the static route 18.0.0.1/8
Redistribute the static route manually configured into the BGP
routing table of RTB
18.0.0.1/8
AS200
RTB
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BGP Messages
There are four types of BGP messages:
Open: greeting--"hello, let's make friends!"
Keepalive: I'm alive, don't leave me alone
Update: fresh news...
Notification: i won't play with you any more!
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BGP Message Header
BGP messages have the same format: "message header +
packet body"
The BGP header is given as follows:
Marker(16 byte)
Length(2 byte) Type(1 byte)
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Open Message
Open Message consists of the Message header and the
structure below
Optional Parameters
Length (1byte)
Hold Time (2byte)
Version(1byte)
My Autonomous System (2byte)
BGP Identifier (4byte)
Optional Parameters
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Keepalive Message
Keepalive Message only has the header
Marker(16 byte)
Length(2 byte) Type(1 byte)
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Update Message
An Update Message consists of the header and the following
structure
Withdrawn Routes (variable length)
Unfeasible Routes Length (2byte)
Path Attribute Length (2byte)
Network Layer Reachability Information (variable length)
Path Attributes (variable length)
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Notification Message
A Notification packet consists of the header and the structure below
Data (variable length)
Error Code(1byte)
Error Subcode(1byte)
Header
error
1
OPEN
Massage
error
2
UPDATE
Massage
error
3
Hold
time
expiry
4
State
Machine
error
5
ExitError Type
6Error Code
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Finite State Machine of BGP
Active
Open-sent
Open-confirm Established
Idle
Connect
Connect-Retrytimer expiry
TCP connection fails
Connect-Retry
timer expiry
Start
Others
TCP connection fails
Error
Error Error
KeepAlivetimer expiry
KeepAlive packet
received
1. KeepAlivetimer expiry
2. Update received
3. KeepAlive received
Correct OPEN
packet received
TCP connection setup
TCP connection setupOthers
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Application of Messages in BGP
The Open message is sent when establishing a BGP
connection via TCP
After the connection is established, the UPDATE message is
sent to notify the peer of the routing information if a route
needs to be sent or route change occurs After stabilization, it is necessary to send the KEEPALIVE
message periodically to keep the validity of the BGP
connection
When an error is found during the running of local BGP,
NOTIFICATION message shall be sent to notify the BGP peer
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ChapterChapter 11 Principle of BGPPrinciple of BGP
ChapterChapter 22 BGP Route AttributesBGP Route Attributes
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BGP Path Attributes
A Path Attribute is a characteristic of an advertised BGP route.
Each Path Attribute falls into one of four categories:
Well-known mandatory
Well-known discretionary
Optional transitive
Optional nontransitive
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Path AttributeWell-known mandatory
ORIGIN
AS-Path
Next hop
Well-known discretionary
Local-Preference
Atomic-Aggregate
Optional transitive
Aggregator
Community
Optional nontransitive
Multi-Exit-Disc (MED)
ORIGINATOR-ID
Cluster-List
Destination Pref (MCI)
Advertiser (Baynet)
Rcid-Path (Baynet)
MP_Reach_NLRI
MP_Unreach_NLRI
Extended_Communities
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ORIGIN Attribute
ORIGIN specifies the origin of the routing update. When BGP has
multiple routes, it uses ORIGIN as one factor in determining the
preferred route.
IGP NLRI (Network layer Reachability Information) was learned
from a protocol internal to the originating AS. BGP routes are given
an origin of IGP if they are learned from an IGP routing table viathe network statement.
EGP NLRI was learned from the Exterior Gateway Protocol.
Incomplete NLRI was learned by some other means. Incomplete
imply that the information for determining the origin of the route is
incomplete. Routes that BGP learns through redistribution carry the
incomplete origin attribute.
Which one is preferred? IGP > EGP > Incomplete
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AS_PATH Attribute
AS-PATH uses a sequence of AS numbers to describe the inter-AS
path or route to the destination specified by the NLRI.
AS-PATH describes all AS it has passed through ,beginning with the
most recent AS and ending with the originating AS.
D(18.0.0.0/8)AS200
AS300
AS400
AS100 AS500
RTA
RTB
30.0.0.1
30.0.0.2D (400 300 200)
D (500 200)
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AS_PATH Attribute
The Function of AS-PATH
AS can influence its incoming traffic by changing the AS_PATH of
its advertising route
AS_PATH can be used for loop avoidance
D(18.0.0.0/8)AS200
AS300
AS400
AS100AS500
RTA
RTB
30.0.0.1
30.0.0.2D (400 300 200)
D (500 200,200,200)
D (200 200 200)
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Next Hop Attribute
18.0.0.0/8
20.0.0.0/8
RTA
RTC
RTB
RTD19.0.0.0/8
21.0.0.2
21.0.0.1
10.0.0.2
10.0.0.3
10.0.0.1
AS100
AS200
IBGP
IBGPEBGP
RTBI can reach 18.0.0.0/8 via the next hop 10.0.0.2I can reach 20.0.0.0/8 via the next hop 10.0.0.3
RTAI can reach 18.0.0.0/8 via the next hop10.0.0.2I can reach 20.0.0.0/8 via the next hop 10.0.0.3I can reach 19.0.0.0/8 via the next hop 21.0.0.1
RTCI can reach 19.0.0.0/8 via the next hop 10.0.0.1I can reach 20.0.0.0/8 via the next hop 10.0.0.3
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LOCAL_PREF Attribute
LOCAL_PREF is used to communicate a BGP routers degree
of preference for an advertised route.
LOCAL_PREF is only in updates between internal BGP peers
and it is not passed to other AS.
If an internal BGP speaker receives multiple routes to the samedestination, it compares the LOCAL_PREF attribute of the
routes. The route with highest LOCAL_PREF is selected.
The LOCAL_PREF attribute affects only traffic leaving the AS.
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LOCAL_PREF Attribute
D local-pref1 100
D local-pref2 200 RTA will select local-pref2 that has higher local preference
AS400
AS100
AS300AS200
RTA
RTB RTC
RTD RTE
RTF
D (18.0.0.0/8)
30.0.0.1
30.0.0.2
20.0.0.1
20.0.0.2
D
local-pref1 100 D
local-pref2 200
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MULTI-EXIT-DISC (MED) Attribute
MED is carried in EBGP updates and allows an AS to inform
another AS of its preferred ingress points. It is meant only for a
single AS to demonstrate a degree of preference when it has
multiple ingress points.
MED attribute affects only the incoming traffic to the AS.
If all else is equal , an AS receiving multiple routes to the same
destination compare the MED of the routes. The lowest MED value
is prefered. MEDs are not compared if two routes to the same
destination are received from two different AS.
The MED is passed between internal peers of the receiving AS but
not passed beyond the receiving AS. MED is used only to
influence traffic between two directly connected AS.
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MULTI-EXIT-DISC (MED) Attribute
D(18.0.0.0/8)
RTA
RTB RTC
30.0.0.1
30.0.0.2
20.0.0.1
20.0.0.2
AS100
AS200
D,metric1 10
D,metric2 20
D,metric1 10 D,metric2 20
RTA will select the lower metric
IBGP
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Community Attribute
Community is designed to simplify policy enforcement. It identifies a
destination as a member of some community of destinations that share
one or more common properties.
The COMMUNITY attribute is a set of four octets values(32 bits)
AA:NN. AA is AS number. NN is an administratively defined identifier.
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Well-known Community
NO_EXPORT
Routes received carrying this value cannot be advertised to real
EBGP peers (still can be advertised within a confederation)
NO_ADVERTISE
Routes received carrying this value cannot be advertised at all
to either EBGP or IBGP peers.
LOCAL_AS
Routes received carrying this value cannot be advertised to
EBGP peers and other AS in conferation
INTERNET
All routes belong to this community by default. Received routes
belonging to this community are advertised freely
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BGP Route Selection Procedure
In general, the procedure of local BGP route selection is:
1. If the next hop of this route is unreachable, this route is not selected.
2. Select the route with a higher local preference.
3. Select the originated route by the local router (same local
precedence).
4. Select the route with shortest AS path.
5. Select the route with lowest origin code (IGP lower than EGP, EGP
lower than Incomplete ).
6. Select the route with smallest MED .
7.Performing load sharing on multiple routes according to the
configured number of routes (in case load sharing is configured and
there are multiple external routes to the same AS)
8. Select the route with smallest Router ID .
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Summary
Feature of BGP
Difference with the IGP and EGP
Principle of BGP
The route attributes of BGP
Microsoft Excel
Worksheet
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