Overview

This is an example of BGP route summarization configuration using the network command. network command generates known network addresses in the routing table as BGP routes.

Related articles

The BGP network command is explained in the following article.

N-Study
 
BGP Basic Configuration and Verification Commands | N-Study
https://www.n-study.com/en/bgp-detail/bgp-basic-configuration/
The basic configuration process for BGP consists of the following three steps1. Enable the BGP routing process2. Configuring the Neighbor3. Generating BGP routes

Please also refer to the following article about BGP route summarization with the network command.

N-Study
BGP Route Summary : network command | N-Study
https://www.n-study.com/en/bgp-detail/bgp-aggregation-by-network-command/
The network command can be used to generate BGP routes for known network addresses in the routing table. This section describes the generation of BGP aggregate route by the network command.

Network Diagram

Figure: BGP Route Summarization network command configuration example network diagram
Figure: BGP Route Summarization network command configuration example network diagram

Configuring and Verifying route summarization using network command

Only network commands for route summarization

First, verify the BGP table and routing table on R1.

R1 BGP Table/Routing Table

R1#show ip bgp
-- omitted --

   Network          Next Hop            Metric LocPrf Weight Path
r>i100.100.1.0/24   3.3.3.3                  0    100      0 i
r>i100.100.2.0/24   3.3.3.3                  0    100      0 i
r>i100.100.3.0/24   3.3.3.3                  0    100      0 i
R1#show ip route
-- omitted --

Gateway of last resort is not set

     1.0.0.0/32 is subnetted, 1 subnets
C       1.1.1.1 is directly connected, Loopback0
C    192.168.13.0/24 is directly connected, FastEthernet0/1
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/3] via 192.168.13.3, 01:57:43, FastEthernet0/1
     100.0.0.0/24 is subnetted, 3 subnets
O       100.100.1.0 [110/2] via 192.168.13.3, 01:57:43, FastEthernet0/1
O       100.100.2.0 [110/2] via 192.168.13.3, 01:57:43, FastEthernet0/1
O       100.100.3.0 [110/2] via 192.168.13.3, 01:57:43, FastEthernet0/1
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/2] via 192.168.13.3, 01:57:44, FastEthernet0/1
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.1.0 is directly connected, FastEthernet0/0
O    192.168.23.0/24 [110/2] via 192.168.13.3, 01:57:46, FastEthernet0/1

R1 is learning routes in the AS with OSPF. It is also learning 100.100.1.0/24, 100.100.2.0/24, and 100.100.3.0/24 as IBGP routes from R3. To summarize these three routes and advertise to ISP1, configure as follows

Note that the IBGP routes 100.100.1.0/24, 100.100.2.0/24, and 100.100.3.0/24 are in RIB Failure state. This is because they are registered as OSPF routes in the routing table.

Related article

Please also refer to the following article about RIB Failure.

N-Study
BGP Route RIB Failure | N-Study
https://www.n-study.com/en/bgp-detail/bgp-rib-failure/
If the best-path BGP route is not registered in the routing table, the route is placed in RIB Failure status.This section describes RIB Failure of BGP route.

R1 Generate summarized route

router bgp 100
 network 100.100.0.0 mask 255.255.0.0

However, this network command does not work because there is no 100.100.0.0/16 route in the routing table, and looking at the BGP table, no 100.100.0.0/16 route is generated.

R1 BGP Table

R1#show ip bgp
BGP table version is 31, local router ID is 1.1.1.1
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
r>i100.100.1.0/24   3.3.3.3                  0    100      0 i
r>i100.100.2.0/24   3.3.3.3                  0    100      0 i
r>i100.100.3.0/24   3.3.3.3                  0    100      0 i

Add static route destined toward Null0

The network command generates a BGP route for a known network address in the routing table. Network addresses that are not in the routing table cannot be generated as BGP routes on an arbitrary basis. So, to make the network command work, configure a static route with null0 as the output interface.

R1 static route destined toward Null0 

ip route 100.100.0.0 255.255.0.0 null 0

If a static route is configured with null0 as the output interface, the route 100.100.0.0/16 is placed in the BGP table and is found to be the best path. Summarized route 100.100.0.0/16 is then advertised to R2.

R1 BGP Table

R1#show ip bgp
BGP table version is 32, local router ID is 1.1.1.1
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
*> 100.100.0.0/16   0.0.0.0                  0         32768 i
r>i100.100.1.0/24   3.3.3.3                  0    100      0 i
r>i100.100.2.0/24   3.3.3.3                  0    100      0 i
r>i100.100.3.0/24   3.3.3.3                  0    100      0 i

Figure BGP Route Summarization network command configuration example
Figure BGP Route Summarization network command configuration example

How the BGP works