16-2 summary-address ip-address mask not-advertise

16-2 summary-address ip-address mask not-advertise

Syntax Description:

• ip-address ” IP address of the summary route.

• mask ” Subnet mask used to generate the summary.

Purpose: OSPF can summarize routes that have been redistributed into OSPF. The summary can be applied to routes redistributed from a dynamic routing protocol, static routes, and connected routes. The router performing the redistribution is called an Autonomous System Boundary Router (ASBR). The summary-address command can be used on an ASBR or an Area Border Router (ABR). When used on an ABR, only external OSPF routes can be summarized. Using the not-advertise keyword suppresses the advertisement of the summary route by the ASBR or ABR.

Initial Cisco IOS Software Release: 10.0

Configuration Example 1: Summarizing Routes Redistributed into OSPF

In Figure 16-1, Router B will summarize the four networks that are being redistributed into OSPF. For this example we will simulate four networks on Router B using loopback interfaces. These four networks will then be redistributed into OSPF. The router performing the redistribution is an ASBR.

Figure 16-1. An ASBR Can Summarize Redistributed Routes into OSPF

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/1   ip address 10.1.1.1 255.255.255.252   clockrate 64000   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0  _______________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Loopback1   ip address 156.26.32.1 255.255.255.240   !   interface Loopback2   ip address 156.26.32.17 255.255.255.240   no ip directed-broadcast   !   interface Loopback3   ip address 156.26.32.33 255.255.255.240   !   interface Loopback4   ip address 156.26.32.49 255.255.255.240   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   no ip directed-broadcast   !   router ospf 1   network 10.1.1.0 0.0.0.3 area 0    redistribute connected subnets   

Before summarizing the routes, inspect the IP routing table on Router A to verify that the four networks are being advertised.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR 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      2.0.0.0/32 is subnetted, 1 subnets O E2    2.2.2.2 [110/20] via 10.1.1.2, 00:03:17, Serial0/1      156.26.0.0/28 is subnetted, 4 subnets  O E2    156.26.32.32 [110/20] via 10.1.1.2, 00:03:17, Serial0/1   O E2    156.26.32.48 [110/20] via 10.1.1.2, 00:03:17, Serial0/1   O E2    156.26.32.0 [110/20] via 10.1.1.2, 00:02:52, Serial0/1   O E2    156.26.32.16 [110/20] via 10.1.1.2, 00:03:17, Serial0/1  10.0.0.0/30 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0/1 

A 26-bit subnet mask is required to summarize the four loopback addresses being advertised by Router B. Modify the configuration on Router B in order to summarize the four loopback addresses.

 Router B  router ospf 1    summary-address 156.26.32.0 255.255.255.192    network 10.1.1.0 0.0.0.3 area 0  

Verification

Verify that the four loopback networks have been summarized by Router B by inspecting the IP routing table on Router A.

 rtrA#  show ip route  Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default        U - per-user static route, o - ODR 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      2.0.0.0/32 is subnetted, 1 subnets O E2    2.2.2.2 [110/20] via 10.1.1.2, 00:05:36, Serial0/1  156.26.0.0/26 is subnetted, 1 subnets   O E2    156.26.32.0 [110/20] via 10.1.1.2, 00:00:21, Serial0/1  10.0.0.0/30 is subnetted, 2 subnets C       10.1.1.0 is directly connected, Serial0/1 

Troubleshooting

Step 1. Verify that there is a neighbor relationship between the OSPF routers by using the show ip ospf neighbor command.

Step 2. The summary-address command will only work on an OSPF ASBR.

Step 3. Verify that you are using the correct IP address and mask with the summary-address command.