First time BGP setup. I have the basic BGP working fine on 1 of my 2 connections with cogent using full routes. Right now I am advertising a /24 out of one of my /21s and internet is working great. However Cogent will not allow me to advertise smaller than a /24. Is there a way around this? Here's my scenario. End result will be bgp at 2 separate fiber drops in 2 different towns. Between these 2 towns are 17 routers linked together via ospf and wireless backhauls. OSPF is already up and running I don't need help there. We will be sending some ips through one town, and other ips through another town. I need the option for those ips to reroute through the opposite town on the fly if needed. Some of these towns are small and require minimal ips so everything is broken up in various sizes as small as /29s in some cases.

So lets say I have 3 routers going out town 1 normally, and the rest go out the other. Can I take a /24 and split that up between those 3 routers but still advertise it through bgp as a /24? If so how?

First time BGP setup. I have the basic BGP working fine on 1 of my 2 connections with cogent using full routes. Right now I am advertising a /24 out of one of my /21s and internet is working great. However Cogent will not allow me to advertise smaller than a /24. Is there a way around this? Here's my scenario. End result will be bgp at 2 separate fiber drops in 2 different towns. Between these 2 towns are 17 routers linked together via ospf and wireless backhauls. OSPF is already up and running I don't need help there. We will be sending some ips through one town, and other ips through another town. I need the option for those ips to reroute through the opposite town on the fly if needed. Some of these towns are small and require minimal ips so everything is broken up in various sizes as small as /29s in some cases.

So lets say I have 3 routers going out town 1 normally, and the rest go out the other. Can I take a /24 and split that up between those 3 routers but still advertise it through bgp as a /24? If so how?

FYI... It is normal for eBGP (external bgp), to be setup not to accept any prefixes larger than /24 (or, any chunks of ip's smaller than a call 'c')
(notation / lingo: Prefix size is an increasing number for smaller groups of ip's, and a reducing number for larger groups of ip's)

How to do the network configuration that you are trying to accomplish is best described in this MUM presentation.
http://mum.mikrotik.com/presentations/H ... ktech1.pdf

So if I am reading this properly it cannot be done? I would need to have a /24 in each of my 3 routers?

If everyone would announce small subnets, the full routing table would become huge. It has already more than 500K routes.

I get that. I think im getting it. I can use aggregates to advertise as /24 but internally route them smaller right? Can someone show me the proper config to do that? Mikrotik manuals are headhurting some times.

I get that. I think im getting it. I can use aggregates to advertise as /24 but internally route them smaller right?

Yes, that's it.

Can someone kindly explain how to configure this?? The documentation is written as if I should already know all this. The following does not help me at all.

BGP allows the aggregation of specific routes into one route with. This menu ('/routing bgp aggregate') allows to specify which routes you want to aggregate, and what attributes to use for the route created by aggregation.


Property Description
advertise-filter (string;) name of the filter chain used to select the routes from which to inherit attributes
attribute-filter (string;) name of the filter chain used to set the attributes of the aggregate route
include-igp (yes | no; Default: ) By default, BGP aggregate takes into account only BGP routes. Use this option to take IGP and connected routes into consideration.
inherit-attributes (yes | no; Default: yes) whether to inherit BGP attributes from aggregated routes
instance (string;) the instance this network belongs to
prefix (IP prefix;) the aggregate prefix
summary-only (yes | no; Default: yes) whether to suppress advertisements of all routes that fall within the range of this aggregate
suppress-filter (string;) name of the filter chain used to select the routes to be suppressed

Read only status property:

routes-used (integer) aggregated route statistics.
in console- list of route console IDs used;
in winbox- number of routes used.