as-path versus as-set attributes

Answered Question
Apr 9th, 2009

Well I am studying BGP more intensely these days. Does anyone have a simple explanation of the difference between an as-path and an as-set?

I have this problem too.
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Correct Answer by Jon Marshall about 7 years 9 months ago

Seth

AS-PATH = ordered list of AS's that describe= uo the path to a destination

AS-SET = unordered list of AS's that describe the path to a destination

So why use an AS-SET. Well it is used when a router has aggregated a number of routes before advertising them on.

The AS-PATH is used to ensure there are no routing loops in the path ie. the same AS should not be seen more than once. But if a router aggregates a number of routes then the AS-PATH info is lost.

So the router that is aggregating the routes can include the actual AS's for the routes it is aggregating in an AS-SET. It doesn't need to be in order because to detect a routing loop you only need to check for the occurence of an AS more than once not the actual order.

Jon

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Correct Answer
Jon Marshall Thu, 04/09/2009 - 13:09

Seth

AS-PATH = ordered list of AS's that describe= uo the path to a destination

AS-SET = unordered list of AS's that describe the path to a destination

So why use an AS-SET. Well it is used when a router has aggregated a number of routes before advertising them on.

The AS-PATH is used to ensure there are no routing loops in the path ie. the same AS should not be seen more than once. But if a router aggregates a number of routes then the AS-PATH info is lost.

So the router that is aggregating the routes can include the actual AS's for the routes it is aggregating in an AS-SET. It doesn't need to be in order because to detect a routing loop you only need to check for the occurence of an AS more than once not the actual order.

Jon

CriscoSystems Thu, 04/09/2009 - 17:45

And am I correct to assume that the numbers in the AS-PATH are in order because the luxury of having them in order is available for each specific route, but that luxury is lost in route aggregation because the AS-PATHS of each specific route can vary from each other...

I understand the list doesn't have to be in order to prevent loop-free routing, which kinda makes me ask, is there a point to having the AS-PATH in order in the first place?

royalblues Thu, 04/09/2009 - 22:46

When a BGP router originates a route it appends its AS number in the AS path. This is true for all the subsequent who advertise this ebgp route and hence it becomes a ordered list

But when you agregate a route it is not possible to have a ordered list. This is because the route could aggregate the prefix received from multiple neighbors.

For eg.

R1

|

R2

|

R3 ----R4

|

R5

R3 might be aggregating the prefixes received from R1&R2 nd R5 into a single prefix while advertising it to R4. So at R4 it cannot be a ordered list of AS paths.

Narayan

Giuseppe Larosa Thu, 04/09/2009 - 22:47

Hello Stuey,

>> which kinda makes me ask, is there a point to having the AS-PATH in order in the first place?

It provides very useful information :

the leftmost /first AS is the peer AS the one that has passed the route to the local AS.

the rightmost/last AS is the origin AS, that is the AS that generated the prefix.

The second information can be used to find out the AS number owner of a public prefix.

The AS numbers in the middle being in order provides insight of the interconnections of the different ISPs

Actually, there are studies of Internet connectivity that use the AS path information.

Hope to help

Giuseppe

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