ATOMIC_AGGREGATE vs AGGREGATOR

[Balajee Muggalla]

Hi,

   Just having AGGREGATOR attribute is not enough? If a prefix has this attribute, that means it is aggregated. Why to have another special attribute ATOMIC_AGGREGATOR just to tell this is an aggregator route?

Thx

[luis_cordova]

Hi @Balajee Muggalla ,

Review this similar discussion of the community:

https://community.cisco.com/t5/routing/atomic-aggregate-vs-aggregator/td-p/520417

I hope you find it useful.

Regards

[paul driver]

Hello

---

@Balajee Muggalla wrote:

Hi,

   Just having AGGREGATOR attribute is not enough? If a prefix has this attribute, that means it is aggregated. Why to have another special attribute ATOMIC_AGGREGATOR just to tell this is an aggregator route?

---

These are both used in when performing aggregation - My understanding is the Aggregator attrib lists the rtr that performed the aggregation and its own related ASN , And the atomic attirb informs the receiving rtr that some information of the prefixes within the aggregation maybe missing -, such as suppressed prefixes or the prefixes originating ASN, hence why when you perform aggregation it suggested you include the AS-SET command to overcome such hidden attribute information

[Balajee Muggalla]

Let me elaborate this question. Let us assume there are 2 cases. 1. a peer received route with just aggregator attribute 2. a peer received with both aggregator and atomic_aggregator attribute. Why doesnt bgp implementation consider the route is aggregated and some info like path asn is missing in the first case itself? For that, why to add an additional attribute? 

Thanks 

[Giuseppe Larosa]

Hello Balajee,

the ATOMIC_AGGREGATE is just a flag to signal that some ASNs  have been removed from the AS Path attribute caused by aggregation action. So it is set if at least one component route in the aggregate is originated in a different ASN or going via a different ASN.

The Aggregator-ID provides the BGP router-id and BGP ASN of the router that performed the aggregation.

As explained in the thread reported by Luis a missing ATOMIC_AGGREGATE should mean that all component routes share the same AS path attribute and in this case there is no info lost in aggregation in the AS path attribute.

Hope to help

Giuseppe