BGP / CCNP R&S ROUTE: Implementing IP Routing

[JMarquez99]

Hi, does anyone know what is the answer for this particular question?
A network administrator is configuring route summarization for received routes before advertising to the next peer with the commands: 

R1(config)# router bgp 65500 
R1(config-router)# aggregate-address 172.16.0.0 255.255.240.0 _____ summary-only 
R1(config-router)# end 

Complete the command, should we want the aggregate route be advertised with the previous BGP path information.

Would really appreciate to have your feedback.

TIA

[Giuseppe Larosa]

Hello @JMarquez99 ,

the additional keyword as-set is needed to achieve the desired behaviour

R1(config)# router bgp 65500 
R1(config-router)# aggregate-address 172.16.0.0 255.255.240.0 _____ summary-only  as-set
R1(config-router)# end 

without this keyword if the component routes have different AS path attributes the resulting AS path would be only the local AS of the router performing the aggregation unless all component routes share the same AS path that would be maintained. with as-set all the ASN in each single AS path are listed in a set that has no order to provide some info at routers downstream about the ASes that were in the component routes AS path attribute. The set of ASes counts as 1 in the AS path length computation regardless of how many ASes are listed as not all of them were in a single AS path attribute at the same time.

Hope to help

Giuseppe

[Vishnu Vardhan S]

I normally choose to use "attribute-map" to set the local preference to be lower than that of the routes with highest prefix matching.