BGP Aggregation

In this post we will try to understand how BGP advertise summary addresses and various options of doing that

This is our setup 

R1

R2

R3

R3

R4

Task 1

configure R1 to advertise a summary address that encompass all it’s loopback address

summary addresses are configured using aggregate-address command under the BGP process

by default BGP advertise   the summary address and all specific prefixes

so on R1

now let’s check the BGP on R4

we can clearly see that the 4 prefixes are being received along the summary address

now let’s have a closer look at the aggregate address  on R4

we can see another attribute called atomic-aggregate, the atomic aggregate attribute is well know discretionary attribute

The purpose of this attribute is to signal other BGP peers that some path information have been lost along the way

we might not be able to see this attribute in this scenario because R1 is the one doing the aggregation

so what happens if R2 is the router that does the aggregation

so on R1 we will remove the aggregate address and perform it on R2

now let’s have a look on BGP on R4

as we can see 1.1.0.0/22 AS_PATH doesn’t include AS 100

let’s take a closer look

To fix this we can use as-set when configuring the aggregation

so on R2

now let’s check BGP on R4 again

so we can see that AS 100 is now included and also the atomic-aggregate is not set anymore

Summary-only

configure R1 to advertise a summary only aggregate address without advertising the specific routes

now let’s check the BGP on R3

so R3 is only receiving the summary route but no specific routes

SUPPRESS-MAP

using a suppress map, configure R1 to suppress only 1.1.0.0/24 only but advertises the other 3 prefixes

Suppress maps can be used in  conjunction with summary-only keyword to suppress some specific prefixes

So the logic with Suppress maps is permit to be suppressed so whatever prefixes we allow in the route-map will be suppressed

everything else is advertised

Let’s see if we can get it working on R1

and we will match the prefix list with a route-map

and under BGP configuration on R1

we will call the route-map when configuring the aggregate address on R1

now let’s check the BGP table on R2

So as we can clearly see R2 received the summary route and all individual prefixes except 1.1.0.0/24 which we suppressed 

on R1

UNSUPRESS-MAP

unsuppress maps are used in conjunction with summary-only keyword in the aggregate-address to unsuppress some more specific prefixes of the

aggregate address

unsuppress maps contrary to suppress maps  need to be configured per BGP neighbor basis

using unsupress map on R2 to advertise the prefix 2.2.3.0/24 and the aggregate address 2.2.0.0/22 to R3, R1 should only receive

we will match the prefix using extended list this time

we will then match the access-list with route-map

then under the bgp configuration on R2 we will attach the unsuppress map to the neighbor R3

conf t

router bgp 200

nei 10.1.23.3 unsuppress-map UNSUPPRESS

aggregate-address 2.2.0.0 255.255.252.0 summary-only as set

no lets’ check the BGP table on R1

as we can see R1 is only receiving the summary address

and R3

while on R3 the summary address 2.2.0.0/22 and the more specific route 2.2.3.0/24 have been received