Atomic Aggregate & Aggregator | AS_SET | AS_SEQUENCE| BGP Attributes
Summary Insights:
- As some BGP attributes are not using for path selection but its very important to avoid loops & get summarization of route to achieve performance on BGP router. Atomic Aggregate, Aggregator & AS_Set are working together to avoid loop in aggregation of routes in BGP.
- AS_SET & AS_Sequence BGP attributes will cover as well.
Interview Questions & Answer:
- What is atomic aggregate & aggregator attributes?
- How loop create during summarization in BGP?
- How many types of AS Path attributes?
- Difference b/w AS_SET & AS_SEQUENCE BGP Attributes?
- Where will use AS_SET BGP attribute?
- Default behavior of AS Path? Is it AS_SET or AS_SEQUENCE?
- How to avoid loop in BGP summarization?
Atomic Aggregate & Aggregator Attribute:
As the summarization is very important feature of any routing protocol. Its reduce the routing table size then device processing will be improved. The summarization is also used in BGP to summarize the routes. Besides the advantages there is disadvantage when using in BGP.
As we know that BGP routing update contains attributes like AS-Path, community, MED etc. to perform some specific requirements. For example AS Path is used to avoid loop b/w autonomous systems. During summarization in BGP the new aggregated route will be generated but all BGP attributes will be lost & routing loop chances will be there. For the loop prevention in this case the atomic aggregate & aggregator BGP attributes need to configure.
Atomic Aggregate:
Atomic aggregate is well known Discretionary attribute is used to inform the BGP peer that route aggregation has implemented on this route & specific routes are not available in this.
How to avoid Loop:
you can see below picture if R3 advertise the summarized route to R4 then if R4 are getting specific route like 172.168.10.0/24 but it will not advertise to R3 as R3 already send aggregated information. By this way loop will avoid.
Create Loop without Atomic Aggregate:
Now if there is no aggregete command on R3, and R4 have learn specific routes (172.168.10.0/24) then it will advertise to R3 ,if link b/w AS200 & AS400 down then R4 will start to send toward R3 & R3 will return back to R4 in this case loop will create b/w R3&R4.
Aggregator:
Aggregator is optional transitive attribute & it’s showing that on which router the aggregation is configured like AS number & router id of the aggregator is mentioned.
Aggregator attribute will use during troubleshooting to identify the exact router where originating the summarized route throgh Router ID & AS number.
Aggregation is configured on R3 of AS#300 in below picture.
If we want to maintain the AS numbers with route then one parameter of AS Path called AS_Set will used to avoid loop.
As I already explained the AS Path attribute in my previous post that it’s using to avoid loop b/w AS, s, but here I need to discuss again with summarization & aggregate concept.Let me explain in details below.
There are main two types of AS Path BGP Attributes below:
- AS_SET.
- AS_SEQUENCE.
AS_SEQUENCE :
It’s adding the AS numbers in sequence when passing through multiple AS. This is using by default like the new AS will be added to the leftmost side.
Example Flow:
- R1 in AS100 advertises
10.1.1.0/24→ R2 in AS200 → R3 in AS300. - R3 sees AS_PATH: 100,200
This means:
- First, AS100 originated it.
- Then it passed through AS200.
- Now received in AS300.
in simple words its normal situation like AS number is adding on left side during traversing from one AS to another.
AS_SET:
In AS-SET random/unordered addition of AS numbers enclosed in {} in the BGP table, As during aggregation the prefix will receive from different AS then in this no sequence will follow to make AS Path list. To avoid loop during summarization of prefixes in BGP the AS_SET will use with aggregation to maintain all AS list with routes.
AS_SET is specifically using in route aggregatoin/summarization schenario.
R3 is receiving route from AS100 & AS200 then it will summarize by this way AS Path will change to AS_Set as its summarization like below
{100, 200} 300
Where:
{100, 200}= AS_SET (unordered, representing all origin ASes of the summarized prefixes),as there is no sequence like with one is left or right side just put in bracket that will represent AS_SET summrized.
Where to use AS_Sequence BGP Attribute & AS_SET BGP Attribute:
AS_SEQUENCE : Best for routing policy and path selection . preserves path length and order.
AS_SET : Prevents loops when summarizing routes from different ASes, but loses path order and may cause less optimal path decisions.
AS_CONFED_SEQUENCE & AS_CONFED_SET are the same concept as above but using in confederation that I will explain in next posts.
you can learn other BGP Attributes and BGP Route Reflector(RR) in details