IS-IS Lab 8 Prefix Summarization
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Download Lab: VIRL | EVE-NG | GNS3
Image requirements:
Cisco IOSv (vios-adventerprisek9-m.vmdk.SPA.156-2.T)
Introduction:
Some of the internal routers in the IS-IS topology simulate multiple subnets belonging to the classful networks of 172.16.0.0/16 and 172.17.0.0/16 by using loopback interfaces. Those subnets have been advertised to the L1/L2 peers. Now L1/L2 routers in each area advertise individual /24 subnets over L2 adjacency to each other. You have to summarize these routes into two single /20 prefixes on the routers A2 and B1 as illustrated in the diagram. After this task successfully has been accomplished you will remove IS-IS configuration from the loopback interface on one of the internal routers and introduce these routes to the IS-IS domain as the external prefixes via redistribution of connected routes with the help of route-map. When propagation of external routes verified across the topology, you will configure a one /21 summary route into the level-1 area on the internal router.
Nodes Access:
Enable: cisco
Username: cisco
Password: cisco
Topology:
Lab Tasks:
1. Verify the presence of the /24 subnets of the loopback interfaces in the routing tables of the L1/L2 IS-routers A2 and B1.
2. Check detail database information for IS nodes (A1, A3, B1) on the router A2.
3. Check detail database information for IS nodes (B2, B3, A2) on the router B1.
4. Configure the /20 summary route on router A2.
5. Configure the /20 summary route on router B1.
6. Confirm that both summary routes are advertised by routers A2 and B1 to each other. A2 has to have a one /20 summary route of area B200 and /24 routes of area A100 in its routing table. The opposite has to occur on the router B1.
7. Verify connectivity between A1 and B3 subnets of loopback 172 interfaces.
8. Remove summary prefix from router A2, then check if router B1 has individual /24 subnets of area A100 in its routing table.
9. Disable IS-IS on the loopback 172 interface of the router A1.
10. On the router A1, configure route-map named CONNECTED to match interface loopback 172. In the IS-IS configuration mode redistribute connected using the route-map you just enabled.
11. Verify that router A2 receives routes you just redistributed as L1 /24 prefixes and router B1 receives them as L2 /24 prefixes.
12. Configure /21 summary route for external prefixes on the router A1.
13. Verify that router A2 receives an external summary route of A1 as L1 prefix, and router B1 as L2 prefix.
14. Configure router A2 to summarize /24 subnets of A3 as /21 prefix.
Lab procedure:
Step1: Issue the command to observe the is-is routes on the router A2.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 01:59:09, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L1 172.16.0.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.1.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.2.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.3.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.4.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.5.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.6.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.7.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.8.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.9.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.10.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.11.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.12.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.13.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.14.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.15.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
172.17.0.0/24 is subnetted, 16 subnets
i L2 172.17.0.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.1.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.2.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.3.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.4.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.5.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.6.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.7.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.8.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.9.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.10.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.11.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.12.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.13.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.14.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.15.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
192.168.0.0/32 is subnetted, 6 subnets
i L1 192.168.0.1 [115/11] via 10.0.0.1, 01:59:27, GigabitEthernet0/1
i L2 192.168.0.2 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L1 192.168.0.4 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L2 192.168.0.9 [115/20] via 10.1.128.2, 01:59:09, GigabitEthernet0/2
i L2 192.168.0.10 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
A2#
You can see that the routing table of A2 has /24 subnets of Area A100 as L1 routes and of area B200 as L2 routes. Next check the routing table of B1.
Step2: Display the content of the IS-IS databases for multiple nodes on the router A2 starting with database for the A1:
A2#show isis database detail A1.00-00
IS-IS Level-1 LSP A1.00-00
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL
A1.00-00 0x00000010 0x0197 997/1199 0/0/0
Area Address: 49.a100
NLPID: 0xCC
Hostname: A1
Metric: 1 IS-Extended A3.01
IP Address: 192.168.0.1
Metric: 1 IP 10.0.0.0/16
Metric: 10 IP 192.168.0.1/32
Metric: 10 IP 172.16.0.0/24
Metric: 10 IP 172.16.1.0/24
Metric: 10 IP 172.16.2.0/24
Metric: 10 IP 172.16.3.0/24
Metric: 10 IP 172.16.4.0/24
Metric: 10 IP 172.16.5.0/24
Metric: 10 IP 172.16.6.0/24
Metric: 10 IP 172.16.7.0/24
The output shows entries of the subnets of A1's loopback172 interface and its metrics. Verify the databases for the rest of the routers as indicated in the lab task number 2.
Step3: Is the same as Step2 but only for router B1, issue the same verification command as for step 2.
Step4: Configure summary route on the router A2 to represent subnets of A1 and A3 as one prefix when advertising to the area B200. You can accomplish this with a summary-address command under the IS-IS router configuration mode.
Configuration example:
!
A2(config)#router isis
A2(config-router)#summary-address 172.16.0.0 255.255.240.0
A2(config-router)#exit
A2(config)#
Step5: The same way as in the step4 configure router B1 to summarize the B200 area's subnets when advertising to A2 router.
Step6: Verifying the summary route implementation, check router A2' routing table fist then proceed to the router B1.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 02:34:40, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 17 subnets, 2 masks
i su 172.16.0.0/20 [115/11], 00:09:01, Null0
i L1 172.16.0.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.1.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.2.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.3.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.4.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.5.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.6.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.7.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.8.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.9.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.10.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.11.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.12.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.13.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.14.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.15.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
172.17.0.0/20 is subnetted, 1 subnets
i L2 172.17.0.0 [115/21] via 10.1.128.2, 00:02:20, GigabitEthernet0/2
192.168.0.0/32 is subnetted, 6 subnets
i L1 192.168.0.1 [115/11] via 10.0.0.1, 02:34:58, GigabitEthernet0/1
i L2 192.168.0.2 [115/21] via 10.1.128.2, 02:34:34, GigabitEthernet0/2
i L1 192.168.0.4 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L2 192.168.0.9 [115/20] via 10.1.128.2, 02:34:40, GigabitEthernet0/2
i L2 192.168.0.10 [115/21] via 10.1.128.2, 02:34:24, GigabitEthernet0/2
A2#
The output shows that the summary route 172.17.0.0/20 has been installed into the routing table of router A2, which is what expected to be. You also can verify if your router performs summarization by issuing this command:
A2#show ip protocols
Routing Protocol is "isis"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: isis
Address Summarization:
172.16.0.0/255.255.240.0 into level-2
Maximum path: 4
Routing for Networks:
Loopback0
GigabitEthernet0/1
GigabitEthernet0/2
Routing Information Sources:
Gateway Distance Last Update
192.168.0.9 115 00:07:12
192.168.0.1 115 00:09:51
192.168.0.4 115 00:09:51
Distance: (default is 115)
Step7: From router A1, ping the IP address of the router B3's loopback172 interface to check if the summarization effort did not disturb the connectivity between two areas.
A1#ping 172.17.15.1 source lo172
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.17.15.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms
A1#
Step8: This is where the second part of the lab begins, the second half of the lab is about summarization of external routes into the level-1 area as L1 prefixes. Router A1 will introduce to the IS-IS domain external routes. To begin this task you need to remove first the current summarization from router A2.
Verify summary configuration on A2:
A2#show running-config | section router isis
ip router isis
ip router isis
ip router isis
router isis
net 49.a100.1921.6800.0003.00
metric-style wide
summary-address 172.16.0.0 255.255.240.0
Remove summary address:
!
A2(config)#router isis
A2(config-router)#no summary-address 172.16.0.0 255.255.240.0
Save the running-config on the A2 and then verify router B1's routing table, there should be present /24 subnets of area A100.
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:04:54, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L2 172.16.0.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.1.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.2.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.3.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.4.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.5.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.6.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.7.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.8.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.9.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.10.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.11.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.12.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.13.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.14.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.15.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
--Output omitted--
Step9: Remote IS-IS configuration from router A1's interface loopback 172.
A1#show isis protocol
IS-IS Router: <Null Tag>
System Id: 1921.6800.0001.00 IS-Type: level-1
Manual area address(es):
49.a100
Routing for area address(es):
49.a100
Interfaces supported by IS-IS:
GigabitEthernet0/1 - IP
Loopback172 - IP
Loopback0 - IP
A1(config)#interface lo172
A1(config-if)#no ip router isis
Step10: Router A1, configure route-map and redistribution.
A1(config)#route-map CONNECTED permit 10
A1(config-route-map)#match interface loopback 172
A1(config-route-map)#exit
!
A1(config)#router isis
A1(config-router)#redistribute connected route-map CONNECTED level-1
Step11: Verify the routing table of routers A2 and B1 for the entries of external routes of router A1.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 00:19:43, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L1 172.16.0.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.1.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.2.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.3.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.4.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.5.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.6.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.7.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.8.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.9.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.10.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.11.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.12.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.13.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.14.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
--Output Omitted--
Step12: Now that router A1 propagates its external prefixes into the IS-IS domain, it's time to reduce the size of routing tables in the topology for external routes by replacing them with one summary route, router A1 will be configured to perform this operation.
A1(config)#router isis
A1(config-router)#summary-address 172.16.0.0 255.255.248.0 level-1
A1(config-router)#end
A1#show ip protocols
Routing Protocol is "isis"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: connected, isis
Address Summarization:
172.16.0.0/255.255.248.0 into level-1
Maximum path: 4
Routing for Networks:
Loopback0
GigabitEthernet0/1
Routing Information Sources:
Gateway Distance Last Update
192.168.0.3 115 00:14:28
192.168.0.4 115 00:14:28
Distance: (default is 115)
Step13: Verify that both router A2 and B1 receive external /21 summary prefix of router A1:
A2#show ip route isis
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 00:31:18, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
i L1 172.16.0.0/21 [115/1] via 10.0.0.1, 00:02:53, GigabitEthernet0/1
i L1 172.16.8.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.9.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.10.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.11.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.12.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.13.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.14.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.15.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
172.17.0.0/20 is subnetted, 1 subnets
--Output Omitted--
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:32:49, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
i L2 172.16.0.0/21 [115/2] via 10.1.128.1, 00:04:20, GigabitEthernet0/2
i L2 172.16.8.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.9.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.10.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.11.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.12.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.13.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.14.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.15.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
--Output Omitted--
Step14: Lastly you need to summarize router A3's subnets into the one /21 prefix when adverting to area B200. Router A2 will be configured to perform this.
A2(config)#router isis
A2(config-router)#summary-address 172.16.8.0 255.255.248.0
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:39:10, GigabitEthernet0/2
172.16.0.0/21 is subnetted, 2 subnets
i L2 172.16.0.0 [115/2] via 10.1.128.1, 00:10:41, GigabitEthernet0/2
i L2 172.16.8.0 [115/12] via 10.1.128.1, 00:00:59, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
i su 172.17.0.0/20 [115/11], 00:39:00, Null0
i L1 172.17.0.0/24 [115/11] via 10.1.0.1, 00:39:00, GigabitEthernet0/1
i L1 172.17.1.0/24 [115/11] via 10.1.0.1, 00:39:00, GigabitEthernet0/1
As you can see from the output now B1's routing table contains two summary entries instead of 16 subnets. The only way, you can distinguish between two entries by looking at their metrics, when redistributing into IS-IS the metric of external routes by default is 0, as the summary route traverse down to the router B1 it ends up with metric of 2, one for the broadcast segment and one for the link between A2 and B1.
Summary:
Route summarization is a great tool, on one hand, it helps you improve the stability of the network and conserve resources but on the other hand, you will end up with obstructed visibility when it comes to multipathing. In this topology, you configured summarization in the way that multiple /24 subnets funnel into the just a few smaller prefixes. In area B200, router B1 summarized routes of B2 and B3 into one /20 prefix because they all belong to the IS-IS domain, while area A100 has subnets diversity, router A2 has to be configured with two /21 prefixes, one for A1's external routes and another for A3's internal routes.
Download Lab: VIRL | EVE-NG | GNS3
Image requirements:
Cisco IOSv (vios-adventerprisek9-m.vmdk.SPA.156-2.T)
Introduction:
Some of the internal routers in the IS-IS topology simulate multiple subnets belonging to the classful networks of 172.16.0.0/16 and 172.17.0.0/16 by using loopback interfaces. Those subnets have been advertised to the L1/L2 peers. Now L1/L2 routers in each area advertise individual /24 subnets over L2 adjacency to each other. You have to summarize these routes into two single /20 prefixes on the routers A2 and B1 as illustrated in the diagram. After this task successfully has been accomplished you will remove IS-IS configuration from the loopback interface on one of the internal routers and introduce these routes to the IS-IS domain as the external prefixes via redistribution of connected routes with the help of route-map. When propagation of external routes verified across the topology, you will configure a one /21 summary route into the level-1 area on the internal router.
Nodes Access:
Enable: cisco
Username: cisco
Password: cisco
Topology:
1. Verify the presence of the /24 subnets of the loopback interfaces in the routing tables of the L1/L2 IS-routers A2 and B1.
2. Check detail database information for IS nodes (A1, A3, B1) on the router A2.
3. Check detail database information for IS nodes (B2, B3, A2) on the router B1.
4. Configure the /20 summary route on router A2.
5. Configure the /20 summary route on router B1.
6. Confirm that both summary routes are advertised by routers A2 and B1 to each other. A2 has to have a one /20 summary route of area B200 and /24 routes of area A100 in its routing table. The opposite has to occur on the router B1.
7. Verify connectivity between A1 and B3 subnets of loopback 172 interfaces.
8. Remove summary prefix from router A2, then check if router B1 has individual /24 subnets of area A100 in its routing table.
9. Disable IS-IS on the loopback 172 interface of the router A1.
10. On the router A1, configure route-map named CONNECTED to match interface loopback 172. In the IS-IS configuration mode redistribute connected using the route-map you just enabled.
11. Verify that router A2 receives routes you just redistributed as L1 /24 prefixes and router B1 receives them as L2 /24 prefixes.
12. Configure /21 summary route for external prefixes on the router A1.
13. Verify that router A2 receives an external summary route of A1 as L1 prefix, and router B1 as L2 prefix.
14. Configure router A2 to summarize /24 subnets of A3 as /21 prefix.
Lab procedure:
Step1: Issue the command to observe the is-is routes on the router A2.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 01:59:09, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L1 172.16.0.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.1.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.2.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.3.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.4.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.5.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.6.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.7.0 [115/11] via 10.0.0.1, 01:26:20, GigabitEthernet0/1
i L1 172.16.8.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.9.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.10.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.11.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.12.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.13.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.14.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L1 172.16.15.0 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
172.17.0.0/24 is subnetted, 16 subnets
i L2 172.17.0.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.1.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.2.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.3.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.4.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.5.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.6.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.7.0 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L2 172.17.8.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.9.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.10.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.11.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.12.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.13.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.14.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
i L2 172.17.15.0 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
192.168.0.0/32 is subnetted, 6 subnets
i L1 192.168.0.1 [115/11] via 10.0.0.1, 01:59:27, GigabitEthernet0/1
i L2 192.168.0.2 [115/21] via 10.1.128.2, 01:59:03, GigabitEthernet0/2
i L1 192.168.0.4 [115/11] via 10.0.0.3, 01:59:17, GigabitEthernet0/1
i L2 192.168.0.9 [115/20] via 10.1.128.2, 01:59:09, GigabitEthernet0/2
i L2 192.168.0.10 [115/21] via 10.1.128.2, 01:58:53, GigabitEthernet0/2
A2#
You can see that the routing table of A2 has /24 subnets of Area A100 as L1 routes and of area B200 as L2 routes. Next check the routing table of B1.
Step2: Display the content of the IS-IS databases for multiple nodes on the router A2 starting with database for the A1:
A2#show isis database detail A1.00-00
IS-IS Level-1 LSP A1.00-00
LSPID LSP Seq Num LSP Checksum LSP Holdtime/Rcvd ATT/P/OL
A1.00-00 0x00000010 0x0197 997/1199 0/0/0
Area Address: 49.a100
NLPID: 0xCC
Hostname: A1
Metric: 1 IS-Extended A3.01
IP Address: 192.168.0.1
Metric: 1 IP 10.0.0.0/16
Metric: 10 IP 192.168.0.1/32
Metric: 10 IP 172.16.0.0/24
Metric: 10 IP 172.16.1.0/24
Metric: 10 IP 172.16.2.0/24
Metric: 10 IP 172.16.3.0/24
Metric: 10 IP 172.16.4.0/24
Metric: 10 IP 172.16.5.0/24
Metric: 10 IP 172.16.6.0/24
Metric: 10 IP 172.16.7.0/24
The output shows entries of the subnets of A1's loopback172 interface and its metrics. Verify the databases for the rest of the routers as indicated in the lab task number 2.
Step3: Is the same as Step2 but only for router B1, issue the same verification command as for step 2.
Step4: Configure summary route on the router A2 to represent subnets of A1 and A3 as one prefix when advertising to the area B200. You can accomplish this with a summary-address command under the IS-IS router configuration mode.
Configuration example:
!
A2(config)#router isis
A2(config-router)#summary-address 172.16.0.0 255.255.240.0
A2(config-router)#exit
A2(config)#
Step5: The same way as in the step4 configure router B1 to summarize the B200 area's subnets when advertising to A2 router.
Step6: Verifying the summary route implementation, check router A2' routing table fist then proceed to the router B1.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 02:34:40, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 17 subnets, 2 masks
i su 172.16.0.0/20 [115/11], 00:09:01, Null0
i L1 172.16.0.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.1.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.2.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.3.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.4.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.5.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.6.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.7.0/24 [115/11] via 10.0.0.1, 02:01:51, GigabitEthernet0/1
i L1 172.16.8.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.9.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.10.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.11.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.12.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.13.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.14.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L1 172.16.15.0/24 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
172.17.0.0/20 is subnetted, 1 subnets
i L2 172.17.0.0 [115/21] via 10.1.128.2, 00:02:20, GigabitEthernet0/2
192.168.0.0/32 is subnetted, 6 subnets
i L1 192.168.0.1 [115/11] via 10.0.0.1, 02:34:58, GigabitEthernet0/1
i L2 192.168.0.2 [115/21] via 10.1.128.2, 02:34:34, GigabitEthernet0/2
i L1 192.168.0.4 [115/11] via 10.0.0.3, 02:34:48, GigabitEthernet0/1
i L2 192.168.0.9 [115/20] via 10.1.128.2, 02:34:40, GigabitEthernet0/2
i L2 192.168.0.10 [115/21] via 10.1.128.2, 02:34:24, GigabitEthernet0/2
A2#
The output shows that the summary route 172.17.0.0/20 has been installed into the routing table of router A2, which is what expected to be. You also can verify if your router performs summarization by issuing this command:
A2#show ip protocols
Routing Protocol is "isis"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: isis
Address Summarization:
172.16.0.0/255.255.240.0 into level-2
Maximum path: 4
Routing for Networks:
Loopback0
GigabitEthernet0/1
GigabitEthernet0/2
Routing Information Sources:
Gateway Distance Last Update
192.168.0.9 115 00:07:12
192.168.0.1 115 00:09:51
192.168.0.4 115 00:09:51
Distance: (default is 115)
Step7: From router A1, ping the IP address of the router B3's loopback172 interface to check if the summarization effort did not disturb the connectivity between two areas.
A1#ping 172.17.15.1 source lo172
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.17.15.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 3/3/4 ms
A1#
Step8: This is where the second part of the lab begins, the second half of the lab is about summarization of external routes into the level-1 area as L1 prefixes. Router A1 will introduce to the IS-IS domain external routes. To begin this task you need to remove first the current summarization from router A2.
Verify summary configuration on A2:
A2#show running-config | section router isis
ip router isis
ip router isis
ip router isis
router isis
net 49.a100.1921.6800.0003.00
metric-style wide
summary-address 172.16.0.0 255.255.240.0
Remove summary address:
!
A2(config)#router isis
A2(config-router)#no summary-address 172.16.0.0 255.255.240.0
Save the running-config on the A2 and then verify router B1's routing table, there should be present /24 subnets of area A100.
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:04:54, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L2 172.16.0.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.1.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.2.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.3.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.4.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.5.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.6.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.7.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.8.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.9.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.10.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.11.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.12.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.13.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.14.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
i L2 172.16.15.0 [115/12] via 10.1.128.1, 00:02:29, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
--Output omitted--
Step9: Remote IS-IS configuration from router A1's interface loopback 172.
A1#show isis protocol
IS-IS Router: <Null Tag>
System Id: 1921.6800.0001.00 IS-Type: level-1
Manual area address(es):
49.a100
Routing for area address(es):
49.a100
Interfaces supported by IS-IS:
GigabitEthernet0/1 - IP
Loopback172 - IP
Loopback0 - IP
A1(config)#interface lo172
A1(config-if)#no ip router isis
Step10: Router A1, configure route-map and redistribution.
A1(config)#route-map CONNECTED permit 10
A1(config-route-map)#match interface loopback 172
A1(config-route-map)#exit
!
A1(config)#router isis
A1(config-router)#redistribute connected route-map CONNECTED level-1
Step11: Verify the routing table of routers A2 and B1 for the entries of external routes of router A1.
A2#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 00:19:43, GigabitEthernet0/2
172.16.0.0/24 is subnetted, 16 subnets
i L1 172.16.0.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.1.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.2.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.3.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.4.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.5.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.6.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.7.0 [115/1] via 10.0.0.1, 00:00:12, GigabitEthernet0/1
i L1 172.16.8.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.9.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.10.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.11.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.12.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.13.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
i L1 172.16.14.0 [115/11] via 10.0.0.3, 00:19:45, GigabitEthernet0/1
--Output Omitted--
Step12: Now that router A1 propagates its external prefixes into the IS-IS domain, it's time to reduce the size of routing tables in the topology for external routes by replacing them with one summary route, router A1 will be configured to perform this operation.
A1(config)#router isis
A1(config-router)#summary-address 172.16.0.0 255.255.248.0 level-1
A1(config-router)#end
A1#show ip protocols
Routing Protocol is "isis"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: connected, isis
Address Summarization:
172.16.0.0/255.255.248.0 into level-1
Maximum path: 4
Routing for Networks:
Loopback0
GigabitEthernet0/1
Routing Information Sources:
Gateway Distance Last Update
192.168.0.3 115 00:14:28
192.168.0.4 115 00:14:28
Distance: (default is 115)
Step13: Verify that both router A2 and B1 receive external /21 summary prefix of router A1:
A2#show ip route isis
i L2 10.1.0.0/17 [115/11] via 10.1.128.2, 00:31:18, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
i L1 172.16.0.0/21 [115/1] via 10.0.0.1, 00:02:53, GigabitEthernet0/1
i L1 172.16.8.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.9.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.10.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.11.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.12.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.13.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.14.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
i L1 172.16.15.0/24 [115/11] via 10.0.0.3, 00:31:20, GigabitEthernet0/1
172.17.0.0/20 is subnetted, 1 subnets
--Output Omitted--
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:32:49, GigabitEthernet0/2
172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
i L2 172.16.0.0/21 [115/2] via 10.1.128.1, 00:04:20, GigabitEthernet0/2
i L2 172.16.8.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.9.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.10.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.11.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.12.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.13.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.14.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
i L2 172.16.15.0/24 [115/12] via 10.1.128.1, 00:30:24, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
--Output Omitted--
Step14: Lastly you need to summarize router A3's subnets into the one /21 prefix when adverting to area B200. Router A2 will be configured to perform this.
A2(config)#router isis
A2(config-router)#summary-address 172.16.8.0 255.255.248.0
B1#show ip route isis
10.0.0.0/8 is variably subnetted, 5 subnets, 4 masks
i L2 10.0.0.0/16 [115/2] via 10.1.128.1, 00:39:10, GigabitEthernet0/2
172.16.0.0/21 is subnetted, 2 subnets
i L2 172.16.0.0 [115/2] via 10.1.128.1, 00:10:41, GigabitEthernet0/2
i L2 172.16.8.0 [115/12] via 10.1.128.1, 00:00:59, GigabitEthernet0/2
172.17.0.0/16 is variably subnetted, 17 subnets, 2 masks
i su 172.17.0.0/20 [115/11], 00:39:00, Null0
i L1 172.17.0.0/24 [115/11] via 10.1.0.1, 00:39:00, GigabitEthernet0/1
i L1 172.17.1.0/24 [115/11] via 10.1.0.1, 00:39:00, GigabitEthernet0/1
As you can see from the output now B1's routing table contains two summary entries instead of 16 subnets. The only way, you can distinguish between two entries by looking at their metrics, when redistributing into IS-IS the metric of external routes by default is 0, as the summary route traverse down to the router B1 it ends up with metric of 2, one for the broadcast segment and one for the link between A2 and B1.
Summary:
Route summarization is a great tool, on one hand, it helps you improve the stability of the network and conserve resources but on the other hand, you will end up with obstructed visibility when it comes to multipathing. In this topology, you configured summarization in the way that multiple /24 subnets funnel into the just a few smaller prefixes. In area B200, router B1 summarized routes of B2 and B3 into one /20 prefix because they all belong to the IS-IS domain, while area A100 has subnets diversity, router A2 has to be configured with two /21 prefixes, one for A1's external routes and another for A3's internal routes.
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