Path Attributes of BGP

BGP Path Attributes

BGP uses BGP Path Attributes to determine the Best Path to a destination. According to BGP Best Path Algorithm, BGP Path Attributes are used to determine the BGP Best Path selection.

 

With these Path Attributes, network administrators can manage the BGP Traffic and determine way of the BGP traffic flow.

 

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You can also test yourself on BGP Quizes Sectionfor your BGP Path Attribute knowledge.

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BGP Path Attributes divide into two groups. These classes are Well-Known and Optional path attributes. These two groups divide also into two subgroups again. These four BGP Path Attribute sub groups are:

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

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Well-Known BGP Path Attributes are supported on all BGP software implementations. And, Well-Known Mandatory Attributes are must be included in every BGP Update Message. But, Well-Known Discretionary Path Attributes may or may not appear in BGP 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 Preference, Atomic_AggregateOptional BGP Path Attributes, may or may not be supported by all BGP implementations. Here, Optional Transtive Attributes are sent in an update message, but not recognized by the receiver, it should be passed on to the next AS. The Optional Non-transtive BGP Path Attributes 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 BGP Path Attribute. This attribute is, Cisco Proprietary, BGP Weight Attribute.BGP Weight Attribute is only used on Cisco router and it is the first step of BGP Path Selection algorithm for Cisco devices.

 

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You can also test yourself on BGP Quizes Sectionfor your BGP Path Attribute knowledge.

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

 

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• 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.

 

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We will discuss these BGP Path Attributes detailed and with Configuration examples in the following posts as our previous lesson, BGP Configuration on Packet Tracer.

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BGP Administrative Distance

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

 

By the way “Administrative Distance (AD)” term is used in Cisco terminology. For Juniper, Nokia and Huawei, instead of Administrative Distance (AD), “Preference” is used.

 

 

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

• Cisco: 20(eBGP) and 200(iBGP)
• Juniper: 170
• Nokia: 170
• Huawei: 255(eBGP) and 255(iBGP)

 

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.

 

Lastly, for Huawei, Preference values are 255 for both EBGP and IBGP.