What is a BGP Path Attribute

1
Lecture 3
2
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.

3
BGP Path Attributes
Attribute Name
Attribute Type Code
Category
4
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

5
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

6
Path Attributes Encoding

• Each Path Attribute is encoded as a triple
  lttype(T), length(L),value(V)gt 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, )

7
Path Attribute Encoding

• Origin (Attribute Type Code 1)
• AS_Path (Attribute Type Code 2)
• Next_Hop (Attribute Type Code 3)

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
8
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.

9
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.

10
(No Transcript)
11
Update (NLRI, attributes)
12
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
• From the routes available in Adj-RIBs-In,
  exclude routes with inaccessible next hop
  address.
• Select the routes with largest local preference.
• If local preference is same, select a locally
  (by the router) originated route.
• If still multiple candidates, select the route
  with the shortest AS_Path.
• If the AS_Path is same, select the route with
  lowest value of origin type (0IGP, 1EGP,
  2Incomplete).
• If origin type same, select the route with
  lowest MED.
• ..

13
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.

14
(No Transcript)
15
AS_Path Attribute

• AS_Path attribute is a Well-known mandatory
  attribute
• AS_Path attribute identifies the ASs traversed
  by the NLRI contained in the Update message
• The traversed ASs 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.

16
(No Transcript)
17
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.

18
(No Transcript)
19
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.

20
(No Transcript)
21
Update (NLRI 172.64.16.0/16, AS_PathAS100,AS100,
AS100, AS00,AS200)
Update (NLRI 172.64.16.0/16, AS_PathAS100,AS100,
AS100)
AS100 inserts dummy AS numbers
Traffic Flow
Update (NLRI 172.64.16.0/16, AS_PathAS100,AS300,
As400)