BGP - Part 4 (BGP Administrative Distance and BGP Path Attributes) - :
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BGP Administrative Distance

As you know routing protocols have Administrative Distance values for preferability.And different vendors have different AD values for the same routing protocols.This is also valid for BGP.

Some main vendors and BGP AD(Preference) Values are below:

Cisco => 20(eBGP) and 200(iBGP)200
Juniper => 170
Alcatel-Lucent => 170

In Cisco routers two AD values used for BGP. For the routes learned outside the AS (eBGP routes) this value is 20, while the AD for iBGP and locally-originated routes is 200.

In Juniper and Alcatel-Lucent routers the Administrative Distance phrase changes to Preference value.

In Juniper routers Junos OS uses the same preference value for both EBGP and IBGP. This value is 170. However, difference between vendors has no operational impact because Junos OS always prefers EBGP routes over IBGP routes.

Preference value is also 170 in Alcatel-Lucent routers.

BGP Path Attributes

BGP uses path attributes to determine the best path to a destination. And the BGP path attributes lesson, is the most important and common lesson in BGP. With BGP path attributes, network administrator can manage the traffic flow and determine where the traffic will flow.

BGP path attributes divide into two main class. These classes are Well-Known and Optional path attributes. These two are also divide two again. These four classes are:

* Well-Known Mandatory
* Well-Known Discretionary
* Optional Transtive
* Optional Non-Transitive

Well-Known attributes are supported on all BGP software implementations. Well-Known Mandatory attributes are must be included in every BGP update. But, Well-Known Discretionary attributes may or may not appear in the update message, it is optional. The attributes name that are well-known are below:

* Well-Known Mandatory : AS_PATH, Origin, Next_Hop
* Well-Known Discretionary : Local-Pref, Atomic_Aggregate

Optional attributes, may or may not be supported by all BGP implementations. Here, Optional Transtive attribute is sent in an update message, but not recognized by the receiver, it should be passed on to the next AS. The Optional Non-transtive may or may not be supported too, but if it received, it is not required that the router pass it on. It may safely and quietly ignore the optional attribute. The attributes name that are optional are below:

* Optional Transtive : Aggregator, Community
* Optional Non-transtive : Multi-Exit-Discriminator (MED), Originator_ID, Cluster List

There is also one extra important attribute. This attribute is Cisco proprietary, Weight attribute.

The names and categories of these attributes were really confusing for me too:) To make this information more permanent for you, lets make an analogy.

– Well-known Mandatory: A famous (recognized) stage actor acts in all the stages of a theater play(Included in all BGP Update messages).
– Well-known Discretionary: A famous (recognized) stage actor plays and acts in some stages of a play(may or may not include in BGP update messages)
– Optional Transitive: A new stage actor is accepted as a player and send to other plays.
– Optional Non-transitive: A new stage actor ignored and not advertise to other plays.

We will discuss this attributes detailed and with configuration examples in the following posts.

BGP – Part 1
BGP – Part 2 (BGP Peers, BGP Sessions, BGP Messages)
BGP – Part 3 (IBGP, IBGP Topologies and EBGP)
BGP – Part 4 (BGP Administrative Distance and BGP Path Attributes)
BGP – Part 5 (Packet Tracer BGP Configuration Example)

About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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