oda030010 bgp routing protocol issue1.4
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ODA030010 BGP Routing Protocol
ISSUE 1.4
Page 2HUAWEI 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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Chapter 1 Principle of BGPChapter 1 Principle of BGP
Chapter 2 Chapter 2 BGP Route AttributesBGP Route Attributes
Page 4HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
Overview Of BGP
BGP is an exterior routing protocol, used to transmit routing inf
ormation between ASs
It is a kind of distance-vector routing protocol and avoids the oc
currence of loop in design. It provides additional attribute infor
mation 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 are managed by the same technical management organization and adopt the unified routing strategy. Each AS has a unique AS number, which is allocated by the management organization authorized by the Internet.
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
Page 10HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
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 r
outing 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)
Error Code 1 2 3 4 5 6
Error TypeHeader
error
OPEN
Massage
error
UPDATE
Massage
error
Hold
time
expiry
State
Machine
error
Exit
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Finite State Machine of BGP
Active
Open-sent
Open-confirm Established
Idle
Connect
Connect-Retrytimer expiry
TCP connection fails
Connect-Retrytimer expiry
Start
Others
TCP connection fails
Error
Error Error
KeepAlivetimer expiry
KeepAlive packet received
1. KeepAlive timer expiry2. Update received3. KeepAlive received
Correct OPENpacket 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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Chapter 1 Principle of BGPChapter 1 Principle of BGP
Chapter 2 Chapter 2 BGP Route AttributesBGP Route Attributes
Page 22HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
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 via the 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 pat
h or route to the destination specified by the NLRI.
AS-PATH describes all AS it has passed through ,beginning with the m
ost 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
AS100 AS500
RTA
RTB
30.0.0.1
30.0.0.2D (400 300 200)
D (500 200,200,200)
D (200 200 200)
Page 27HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
Next Hop Attribute
18.0.0.0/8
20.0.0.0/8
RTARTC
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
IBGPIBGPEBGP
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
RTC I 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
Page 28HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
LOCAL_PREF Attribute
LOCAL_PREF is used to communicate a BGP router’s 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 same
destination, 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 anoth
er AS of its preferred ingress points. It is meant only for a single A
S to demonstrate a degree of preference when it has multiple ingre
ss points.
MED attribute affects only the incoming traffic to the AS.
If all else is equal , an AS receiving multiple routes to the same de
stination compare the MED of the routes. The lowest MED value is
prefered. MEDs are not compared if two routes to the same destin
ation 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 influenc
e 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 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
EBGP peers and outside of the 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 including peers in other AS within a confederation.
INTERNET
− All routes belong to this community by default. Received routes
belonging to this community are advertised freely
Page 34HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
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 .
Page 35HUAWEI TECHNOLOGIES CO., LTD. All rights reserved
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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Thank You
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