BGP As-set

[Jonn cos]

Hi all,

My topology is very simple

R1 ------------------------- R2 ------------------------R3 -------------------------- R4

R1 is advertising 3 subnets. 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24. All routers are in separate AS (1,2,3,4 respectively). Performing aggregation on R3

R3

router bgp 3

  aggregate-address 192.168.0.0 255.255.0.0 summary-only

on R4

R4#show ip bgp 192.168.0.0 255.255.0.0

BGP routing table entry for 192.168.0.0/16, version 39

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  3, (aggregated by 3 3.3.3.3)

    10.1.34.3 from 10.1.34.3 (3.3.3.3)

      Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best

Now on R3 if i use the keyword as-set, it removes the bolded atomic-aggregate, like below

R3

aggregate-address 192.168.0.0 255.255.0.0 as-set summary-only

on R4

BGP routing table entry for 192.168.0.0/16, version 40

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  3 2 1, (aggregated by 3 3.3.3.3)

    10.1.34.3 from 10.1.34.3 (3.3.3.3)

      Origin IGP, metric 0, localpref 100, valid, external, best (its gone)

R4#

Is this normal behavior ?

[John Blakley · Accepted solution]

Yes it is. Seeing the atomic-aggregate indicates to the receiving router that there are more AS's behind the one that's advertised it. If you'll notice that R3 has set itself as the only AS in the path and advertised that to R4. R4 sees it, but it only knows about R3. When you set as-set on the summary, it tells R3 to not aggregate all AS'es behind it, but instead pass the complete as-path to the upstream neighbor.

R4#show ip bgp 192.168.0.0 255.255.0.0

BGP routing table entry for 192.168.0.0/16, version 39

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

  3, (aggregated by 3 3.3.3.3)

    10.1.34.3 from 10.1.34.3 (3.3.3.3)

      Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best

R3

aggregate-address 192.168.0.0 255.255.0.0 as-set summary-only

on R4

BGP routing table entry for 192.168.0.0/16, version 40

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

  Not advertised to any peer

3 2 1, (aggregated by 3 3.3.3.3)

    10.1.34.3 from 10.1.34.3 (3.3.3.3)

      Origin IGP, metric 0, localpref 100, valid, external, best (its gone)

R4#

HTH,
John

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