BGP Basics- Path Attribute

Path Attributes

A path attribute is a characteristic of an advertised BGP route. Path attributes are what allow BGP to set and communicate routing policy.

Each path attribute falls into one of four categories: 

•       Well-known mandatory
•       Well-known discretionary
•      Optional  transitive
•    Optional non-transitive

The ORIGIN Attribute- Well-known mandatory,

It specifies the origin of the routing update,

    IGP— The Network  was learned from a protocol  internal to the originating AS. An IGP origin                      gets the highest preference of the ORIGIN values.” i”

   EGP— The NLRI was learned from the Exterior Gateway Protocol. EGP is preferred second to

   IGP. “e”

    Incomplete— The NLRI was learned by some other means like disribution. Incomplete is the                                      lowest-preferred ORIGIN value. Incomplete does not imply that the route is in any                              way faulty, only that the information for determining the origin of the route is                                      incomplete “ ?”

 AS_PATH - Well-known mandatory

AS_PATH is a well-known mandatory attribute that 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 the  autonomous systems it has passed through, beginning with the most recent AS and ending with the originating AS

NEXT_HOP - Well-known mandatory

It describes the IP address of the next-hop router on the path to the advertised destination,

There are three condition for next_hop .

1.   If the advertising router and receiving router are in different autonomous systems (external

peers), the NEXT_HOP is the IP address of the advertising router's interface.

2. If the advertising router and the receiving router are in the same AS (internal peers), and the

NLRI of the update refers to a destination within the same AS, the NEXT_HOP is the IP

address of the neighbor that advertised the route.

3. If the advertising router and the receiving router are internal peers and the NLRI of the

update refers to a destination in a different AS, the NEXT_HOP is the IP address of the

external peer from which the route was learned

NEXT_HOP IP must be reachable to install route into routing table

LOCAL_PREF - well-known discretionary

LOCAL_PREF is short for local preference, is used only in updates between internal BGP peers; it is not passed to other autonomous systems,

 The attribute is used to communicate a BGP router's degree of preference for an advertised route.The route with the highest LOCAL_PREF is selected.

MULTI_EXIT_DISC -optional nontransitive

To influence incoming traffic, the MULTI_EXIT_DISC attribute, known as the MED for short, is used.

 This attribute is carried in EBGP updates and allows an AS to inform another AS of its preferred ingress points. The lowest MED value is preferred.

MED is considered a metric, and with a metric the lowest value—the lowest distance—is preferred

MEDs also are not compared if two routes to the same destination are received from two different 

autonomous systems

ATOMIC_AGGREGATE and AGGREGATOR Attributes

when summarization (route aggregation) is performed, some  route information is lost and routing can become less precise. When aggregation is performed in a BGP-speaking router, path detail is lost.

ATOMIC_AGGREGATE is a well-known discretionary attribute that is used to alert downstream routersthat a loss of path information has occurred.

In  summarization, BGP speaker must attach  the ATOMIC_AGGREGATE  attribute to the aggregate route, and receiver when advertising the route to other peers, the ATOMIC_AGGREGATE attribute must remain attached.

When the ATOMIC_AGGREGATE attribute is set, the BGP speaker has the option of also attaching theAGGREGATOR attribute, This is a optional transitive attribute provides information about where theaggregation was performed by including the AS number and the IP address of the router thatoriginated the aggregate route.

ORIGINATOR_ID and CLUSTER_LIST-- optional, nontransitive

ORIGINATOR_ID and CLUSTER_LIST are optional, nontransitive attributes used by route reflectors,

ORIGINATOR_ID is a 32-bit value created by a route reflector. The value is the router ID of the originator of the route in the local AS, If the originator sees its RID in the ORIGINATOR_ID of a received route, it knows that a loop has occurred, and the route is ignored.

CLUSTER_LIST is a sequence of route reflection cluster IDs through which the route has passed, If a route reflector sees its local cluster ID in the CLUSTER_LIST of a received route, it knows that a loop has occurred, and the route is ignored.

COMMUNITY Attribute,

COMMUNITY is an optional transitive attribute that is designed to simplify policy enforcement.

The COMMUNITY attribute is a set of four octet values.

RFC 1997 specifies that the first two octets are the autonomous system and the last two octets are an administratively defined identifier, giving a format of AA:NN.

The default Cisco format, on the other hand, is NN:AA. It can be changed  to the RFC 1997 format with the command ip bgp-community new-format.

We known communities are,

INTERNET

NO_EXPORT

NO_ADVERTISE

LOCAL_AS 

NO_EXPORT_SUBCONFED. 

Administrative Weight  -Cisco Specific

Administrative weight is a Cisco-specific BGP parameter that applies only to routes within an

individual router. It is not communicated to other routers. The weight is a number between 0 and

65,535 that can be assigned to a route; the higher the weight, the more preferable the route

All routes learned from a peer have a weight of 0, and all routes generated by the local router have a weight of 32,768.

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