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jorg_b
Beginner

summary-only option within the aggregate-address statement question

Greetings,

I have a question regarding summary-only option within the aggregate-address statements.

Here is my layout:

          [INTERNET]

          |                  |

     [ISP 1]         [ISP 2]

         |                   |

     [Loc 1]--------[Loc 2]

 

I'm getting Internet from 2 different backbone providers. Both backbone providers only send me a default route via BGP.

Loc 1 and Loc 2 are also connected to each other via a metro ethernet link and (iBGP). I'm also using ospf internally for some edge equipment.

Within my BGP settings on each router I have the following aggregate statements:

(the IPs have been changed for privacy reasons)

 aggregate-address 100.100.96.0 255.255.248.0 summary-only

 aggregate-address 100.100.104.0 255.255.248.0 summary-only

 aggregate-address 100.200.224.0 255.255.252.0 summary-only

 aggregate-address 100.200.228.0 255.255.255.0 summary-only

 aggregate-address 100.200.229.0 255.255.255.0 summary-only

 aggregate-address 100.200.230.0 255.255.255.0 summary-only

 aggregate-address 100.200.231.0 255.255.255.0 summary-only

 aggregate-address 100.200.232.0 255.255.255.0 summary-only

 aggregate-address 100.200.233.0 255.255.255.0 summary-only

 aggregate-address 100.200.234.0 255.255.255.0 summary-only

 aggregate-address 100.200.235.0 255.255.255.0 summary-only

 aggregate-address 100.200.236.0 255.255.255.0 summary-only

 aggregate-address 100.200.239.0 255.255.255.0 summary-only

Here is my output of "show ip bgp neighbors 123.234.0.1 advertised-routes" on the router at LOC 2 

BGP table version is 197644, local router ID is 10.255.0.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 100.100.96.0/21    0.0.0.0                            12345 i

*> 100.100.104.0/21   0.0.0.0                            12345 i

*> 100.200.224.0/22  0.0.0.0                            12345 i

*> 100.200.228.0/22  100.100.105.26            91         12345 i

*> 100.200.232.0/22  100.100.105.26            81         12345 i

*>i100.200.232.0/21  100.100.105.1              0     90      0 i

*> 100.200.236.0/24  100.100.105.26            71         12345 i

*> 100.200.239.0/24  100.100.105.4             22         12345 i

Total number of prefixes 8 

Right now, the inbound traffic from each upstream ISP is somewhat equal, as my announcements are the same on each router.

I want change the advertising of my IP space to my upstream providers to be more specific to each location. Some /24's are only handled by the router at LOC 1 and others are only handled by LOC 2. There is no reason to have inbound traffic for LOC 2 come into the router at LOC 1 and having to cross the metro ethernet link to get to the other router. However, I need redundancy, in case of an outage (hence the need to announce all IP space from both routers)

I understand that more specific route announcements win over less specific. So, the idea is to advertise more specific routes on each router that is handling the given IP space and be less specific on the other router (to preserve redundancy). My IP address layout was designed to be able to aggregate /24 into  /22 at each location. So, I have the flexibility to advertise more or less specific at each location.

Now my question...  Since I currently use the "summary-only" option within my  aggregate-address statements, my announcements are grouped in to smaller prefixes (see above), which will not work if I want to advertise my IP space more specific. 

If I omit the "summary-only" option, I assume that it announce the given IP space without any aggregations. Is there a downside or problem to omitting the "summary-only" option? 

I've noticed in my test setup that I may not get what I'm looking for, since the routes in my routing table will now show both, the /24's and the /22's and therefore my announcements to my upstream are not what I expected. Is is time for a "suppress map" to suppress what I'm advertising to each upstream peer?

Please advise.

Thanks a million

JB

1 REPLY 1
Masoud Pourshabanian
Collaborator

Hello,

If you remove summary-only, you receive both aggregated routes and actual routes which causes having a larger BGP data base.  If you are receiving the actual paths, the aggregated routes are necessary? If not, you can remove the aggregation completely.

If you intend to receive or send only aggregated routes and only some /24, you can use unsuppress-map along with aggregation.

Hope it helps,

Masoud