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Lecture 3

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What is a BGP Path Attribute?

• BGP uses a set of parameters known as path attributes to characterize a route.

• BGP path attributes are similar in nature to the IGP metrics.• BGP Update message contains variable number of path attributes.

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BGP Path Attributes

1 Origin Well-known, mandatory

2 AS_Path Well-known, mandatory

3 Next_Hop Well-known, mandatory

4 Multi_Exit_Disc (MED) Optional non-transitive

5 Local_Pref Well-known, discretionary

6 Atomic_Aggregate Well-known, discretionary

7 Aggregator Optional, transitive

8 Community Optional, transitive)

… … …

Attribute NameAttribute Type Code Category

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BGP Path Attributes (Well-known)

• Each Path Attribute belongs to one of following four categories:• Well-known Mandatory

Attribute is recognized by all BGP implementations Included in each Update message

• Well-known Discretionary Attribute is recognized by all BGP implementations It may or may not be present in an Update message

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BGP Path Attributes (Optional)

• Well-known attributes are always transitive (i.e., to be propagated to peers). The Optional attributes can be transitive or non-transitive.

• Optional Transitive Attribute is not recognized by all BGP implementations It may or may not be present in an Update message If present and transitive flag is set, the receiving BGP speaker should pass it on to other peers

• Optional Non-transitive Attribute is not recognized by all BGP implementations It may or may not be present in an Update message If present and the non-transitive flag is set, the BGP speaker should NOT pass it on to other peers

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Path Attributes Encoding

• Each Path Attribute is encoded as a triple <type(T), length(L),value(V)> or TLV format.

• Attribute Type field (2 byte)• Contains subfields (1 byte each) namely Attribute Flags and Attribute Type Code• The attribute Flags field is used for categorization of the attribute (e.g., Well-known

Mandatory, Well-known Discretionary, …)• The attribute type code value is used to identify a particular Path Attribute (e.g., type

code = 1 means Origin, type code = 2 means AS_Path, …)

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Path Attribute Encoding

b0 = 0 means Well-known attribute b0 = 1 means Optional attributed b1 = 0 means the Optional attribute is non-transitive b1 = 1 means the Optional attribute is transitive

Attribute Flags

Attribute Type Codeb0 b1 b2 b3 Unused bits

1. Origin (Attribute Type Code =1)2. AS_Path (Attribute Type Code = 2)3. Next_Hop (Attribute Type Code = 3)…

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Routing Information Bases (RIBs)

• BGP stores its routes in RIB. BGP routing information bases can be further categorized as:• Adj-RIBs-In

Stores the routes learned via Update messages from peers• Adj-RIBs-Out

Stores the routes selected for advertisement to peers• Loc-RIB

Stores routes that have been selected by the decision process• BGP keeps its forwarding state in forwarding information base (FIB).

• FIB is the routing information that is used for forwarding.

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BGP Decision Process

• BGP takes Adj-RIBs-In as input and carries out a three phase decision function for selecting and advertising the best routes.• Phase I: determine a degree of preference (e.g., using Local_Pref path attribute or

preconfigured local policy) for each route learned from neighbors• Phase II: select the best route out of all available routes for each destination and

install it in the Loc-Rib. All routes with unfeasible BGP next hop addresses (i.e., Next_Hop address) are excluded from the second phase decision function.

• Phase III: take Loc-RIB as input, processes it (e.g., filter, manipulate path attributes) to influence routing behavior, generate a corresponding entry in the Adj-RIBs-Out.

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Adj-RIBs-Out

Adj-RIBs-In

Input filtering and Pathattribute manipulation

Output filtering and Pathattribute manipulation

Decision Process

Loc-RIB

FIB

Locally originated routes

Adj-RIBs-In

Loc-RIB

FIB

Adj-RIBs-Out

Input filtering and Pathattribute manipulation

Output filtering and Pathattribute manipulation

Decision Process

Locally originated routes

Advertise routes via Update message

Advertise routes via Update message

BGP Speaker 1 BGP Speaker 2

BGP Peers

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AS100AS200

eBGP

IGP Static Routes

BGP

IGP

BGP

IGP/static routesinjected into BGP

Update (NLRI, attributes)

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BGP Route Selection

• BGP uses path attributes to select best routes and influence decision process of peers. The selection of a most preferable route is performed as follows:1. From the routes available in Adj-RIBs-In, exclude routes with inaccessible next hop address.2. Select the routes with largest local preference.3. If local preference is same, select a locally (by the router) originated route.4. If still multiple candidates, select the route with the shortest AS_Path.5. If the AS_Path is same, select the route with lowest value of origin type (0=IGP, 1=EGP,

2=Incomplete).6. If origin type same, select the route with lowest MED.7. …..

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Origin Attribute

• Origin (Attribute Type Code = 1) is a Well-Known path attribute that defines the origin of the routing information contained in the Update message.• Origin attribute is created by the AS that originates routing information (i.e., NLRI)• The receiving speakers pass along the Origin parameter in their UPDATE messages when

advertising corresponding routing information to other speakers.• BGP considers following origin types:

• (0=)IGP - Network Layer Reachability Information (NLRI) is internal to the originating AS• (1=)EGP - NLRI is learned via EGP (e.g., BGP) in the originating AS• (2=)INCOMPLETE – NLRI is learned by some other means in the originating AS.

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Update (NLRI=128.32.0.0, Origin = IGP)2

R1R2

R3

128.32.0.0eBGP

eBGP

1

3Update (NLRI=128.32.0.0, Origin = IGP)

1 R1 learns about prefix (128.32.0.0) via IGP

2 R1 originates this route with Origin attribute set to IGP

3 After going through BGP decision process, R2 decides to advertise this route to its eBGP peer R3. R2 propagates the Origin attribute associated with the NLRI to R3.

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AS_Path Attribute

• AS_Path attribute is a Well-known mandatory attribute• AS_Path attribute identifies the AS’s traversed by the NLRI contained in the Update message• The traversed AS’s are recorded in two ways:

• Ordered sequence of AS numbers (i.e., AS_Sequence list), or• Unordered sequence of AS numbers (i.e., AS_Set list)

• When an iBGP speaker originates a route, it includes an empty AS_Path attribute list.• A BGP speaker includes its AS number in the AS_Path attribute when advertising or originating a

route to an eBGP peer.• When a BGP speaker advertises a route received from an eBGP peer to an iBGP peer, it does not

modify the AS_Path attribute.

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1 R6 originates a BGP route for 179.16.12.0/24 with empty AS_Path

2 R4 advertises this route for 179.16.12.0/24 with AS_Path = 200

3 R3 advertises this route to R1 with AS_Path = 200

4 R1 advertises this route to R7 with AS_Path = 300, 200.

AS 300

AS 200

R1

R2

R3

R4

R6

R5

iBGP

eBGP

iBGP

R7

eBGP

2

1

3

4

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AS_Path Attribute

• BGP uses AS_Path attribute to construct a loop-free AS topology graph.• For instance, BGP does not accept an Update message if its own AS number is included in the

AS_Path attribute.• BGP also uses the AS_Path attribute in its decision process for selecting a best route.

• For example, when there are several routes to chose from, BGP selects the route with the shortest AS_Path attribute.

• AS_Path attribute is also used identify routes for applying input/output route filtering.• In summary, manipulation of AS_Path attribute enable BGP to influence certain inter-

AS routing behavior.

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AS 100

AS 200

AS 300

172.64.16.0/16

1 Update (172.64..16.0/16, AS_Path = {AS100})

Update (172.64..16.0/16, A

S_P

ath = {A

S100, A

S200})

1

Update (172.64..16.0/16, AS_Path = {AS100, AS200, AS300})3

4 AS100rejects this Update message

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AS_Path and Routing Behavior

• We know that in case of a tie, a route with shorter AS_Path length is preferred.

• Thus AS_Path attribute can be used to influence inter-AS routing behavior.

• For example, BGP speaker can insert dummy AS numbers in the AS_Path attribute to increase its length to make it less preferable during the BGP decision process.

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Update (NLRI= 172.64.16.0/16,AS_Path{AS100,AS200})

AS 100

AS 200

AS 300

AS 500

DS3

DS3

OC3 ( 155 Mb/s)

OC3

AS 400

OC3

172.64.16.0/16

Update

(NLRI=

172.6

4.16.0

/16,

AS_Path

{AS10

0,AS30

0, As4

00}}

)

Traffic flow

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Update (NLRI= 172.64.16.0/16,AS_Path{AS100,AS100,AS100,AS00,AS200})

AS 100

AS 200

AS 300

AS 500

DS3

DS3

OC3 ( 155 Mb/s)

OC3

AS 400

OC3

172.64.16.0/16

AS100 inserts dummy AS numbers

Update

(NLRI=

172.6

4.16.0

/16,

AS_Path

{AS10

0,AS30

0, As4

00}}

)

Update (NLRI= 172.64.16.0/16,AS_Path{AS100,AS100, AS100})

Traffic Flow