Aggregate address withdrawal

[ChrisNewnham_]

Hello

I was advertising an aggregate address of of 10.31.0.0/16 I have 2 other network statements that match both a directly connected and OSPF learned prefix (10.31.0.0/30 & 10.31.32.0/24).

Using a different device on-site, I advertised 10.31.129.0/24 to the router (via eBGP, unfortunately I cannot use iBGP).

The router then withdrew the 10.31.0.0/16 aggregate!

The 2 other prefixes (10.31.0.0/30 & 10.31.32.0/24) were still reachable, so why did it do this? Is there a rule that if a sub-prefix is learned from eBGP, it will withdraw the aggregate?

The command is "aggregate-address 10.31.0.0 255.255.0.0 as-set summary-only". Maybe it is the "as-set" that could also be causing the problem?

Thanks!

[Julio E. Moisa]

Hi

AS-Set parameter is used to add a list of the AS from the routes that are part of a summary route. It is a method to avoid loops. Do you have a diagram about how the routes are being advertised? 

Regards, 

[MHM Cisco World]

can I see 
show ip bgp 10.31.0.0/16 <<- in router originate the aggerate 

[ChrisNewnham_]

So this how it looks, whether 10.31.129.0/24 is present or not (doesn't change except "Advertised to...").

But what does change, is that the aggregate is withdrawn if 10.31.129.0/24 is present.

ASR1002-A#show bgp vpnv4 uni vrf MPLS 10.31.0.0
BGP routing table entry for 65518:65518:10.31.0.0/16, version 137
Paths: (2 available, best #2, table MPLS)
Advertised to update-groups:
12 15
Refresh Epoch 2
Local, (aggregated by 65518 10.254.0.126)
10.31.254.18 (via vrf MPLS) from 10.31.254.18 (10.254.0.126)
Origin incomplete, metric 0, localpref 100, valid, internal
Extended Community: OSPF DOMAIN ID:0x0005:0x000000640200
OSPF RT:0.0.0.51:2:0 OSPF ROUTER ID:10.254.0.126:0
rx pathid: 0, tx pathid: 0
Updated on May 25 2023 14:12:56 UTC
Refresh Epoch 1
{65250}, (aggregated by 65518 10.254.0.125)
0.0.0.0 (via default) from 0.0.0.0 (10.254.0.125)
Origin incomplete, localpref 100, weight 32768, valid, aggregated, local, best
Extended Community: OSPF DOMAIN ID:0x0005:0x000000640200
OSPF RT:0.0.0.51:2:0 OSPF ROUTER ID:10.254.0.125:0
rx pathid: 0, tx pathid: 0x0
Updated on May 25 2023 14:12:29 UTC

[ChrisNewnham_]

So I answered my own question in the end. I removed "as-set" and the aggregate was advertised once again.

[MHM Cisco World]

aggregate-address 10.31.0.0 255.255.0.0 as-set summary-only <<-

 With the as-set configuration command, the aggregate 
!--- inherits the attributes of the more-specific routes.

the prefix router learn use it in aggregate and inherit it path and add to as-path 

if am I right then the router will re-advertise to other peer, other peer see it AS in as_path and silent drop it. 
I will make lab and see how we can solve this issue 

[ChrisNewnham_]

I think I know what the problem is now, thanks. I have a NO_TRANSIT route-map with (^$) as an as-path. This aggregate then breached that rule so wasn't advertised.

I have tested and confirmed once I added in 10.31.0.0/16 separately, it all worked, also with the "as-path" command.

[MHM Cisco World]

you are so welcome