IS-IS Summarization

IS-IS supports summarization but since it is a link-state routing protocol, you can’t do this within an area as the link-state database have to be the same on all routers within the area. You can only configure summarization on a “border”. That would be an area border router or a router that is doing redistribution..

In this lesson, I’ll walk you through both options.

Configuration

This is the topology we will use:

Above we have R1 and R2 in area 12. R3 is in area 3. On R1 we have two loopback interfaces. Loopback 0 will be advertised in IS-IS, loopback 1 will be redistributed. I will show you how to summarize both routes.

hostname R1
!
ip cef
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface Loopback1
 ip address 11.11.11.11 255.255.255.255
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
 ip router isis 
!
router isis
 net 49.0012.0000.0000.0001.00
 is-type level-1
 log-adjacency-changes
!
end

hostname R2
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
 ip router isis 
!
interface GigabitEthernet0/2
 ip address 192.168.23.2 255.255.255.0
 ip router isis 
!
router isis
 net 49.0012.0000.0000.0002.00
 log-adjacency-changes
!
end

hostname R3
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.23.3 255.255.255.0
 ip router isis 
!
router isis
 net 49.0003.0000.0000.0003.00
 log-adjacency-changes
!
end

Summarization

Let’s get started. First, we enable IS-IS on the loopback 0 interface:

R1(config)#interface Loopback 0
R1(config-if)#ip router isis

This loopback has the 1.1.1.1/32 prefix which will show up on R2’s level 1 database:

R2#show isis database level-1 verbose R1.00-00

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000007   0x1252        1164              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  Metric: 10         IS R2.01
  IP Address:   1.1.1.1
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 1.1.1.1 255.255.255.255

And it will be copied to R2’s level 2 database:

R2#show isis database level-2 verbose R2.00-00

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000007   0x5965        1146              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  Metric: 10         IS R2.02
  IP Address:   192.168.23.2
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 192.168.23.0 255.255.255.0
  Metric: 20         IP 1.1.1.1 255.255.255.255

This route will show up in the routing table of R2:

R2#show ip route isis                 

      1.0.0.0/32 is subnetted, 1 subnets
i L1     1.1.1.1 [115/20] via 192.168.12.1, 00:01:10, GigabitEthernet0/1

Let’s see if we can summarize this route so that R3 receives a summary:

R2(config)#router isis
R2(config-router)#summary-address 1.0.0.0 255.0.0.0 ?
  level-1    Summarize into level-1 area
  level-1-2  Summarize into both area and sub-domain
  level-2    Summarize into level-2 sub-domain
  metric     Set metric for summay route
  tag        Set tag
  <cr>

Let’s create a basic summary without specifying anything:

R2(config-router)#summary-address 1.0.0.0 255.0.0.0 

Now let’s take a look how this influences R2’s database. Nothing will change in its level 1 database but the LSP in the level 2 database will change:

R2#show isis database level-2 verbose R2.00-00 


IS-IS Level-2 LSP R2.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000008   0x8D33        1135              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  Metric: 10         IS R2.02
  IP Address:   192.168.23.2
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 192.168.23.0 255.255.255.0
  Metric: 20         IP 1.0.0.0 255.0.0.0

Above we see that R2 now shows 1.0.0.0 255.0.0.0 in its database. Let’s check its routing table:

R2#show ip route isis

Gateway of last resort is not set

      1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
i su     1.0.0.0/8 [115/20], 00:02:22, Null0
i L1     1.1.1.1/32 [115/20] via 192.168.12.1, 00:05:11, GigabitEthernet0/1

R2 has created a discard route to null 0 for the 1.0.0.0/8 summary. Let’s take a look at R3 now:

R3#show isis database level-2 verbose R2.00-00


IS-IS Level-2 LSP R2.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00              0x00000008   0x8D33        1002              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R2
  Metric: 10         IS R2.02
  IP Address:   192.168.23.2
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 192.168.23.0 255.255.255.0
  Metric: 20         IP 1.0.0.0 255.0.0.0

R3 receives R2’s LSP with the summary route so that’s what it will install in its routing table:

R3#show ip route isis

i L2  1.0.0.0/8 [115/30] via 192.168.23.2, 00:03:29, GigabitEthernet0/1
i L2  192.168.12.0/24 [115/20] via 192.168.23.2, 00:36:20, GigabitEthernet0/1

That’s all there is to it.

Redistribution Summarization

Now let’s see how we can summarize redistributed routes. I will redistribute the second loopback interface of R1 into IS-IS with a simple route-map:

R1(config)#route-map L1_ONLY
R1(config-route-map)#match interface Loopback 1

R1(config)#router isis
R1(config-router)#redistribute connected route-map L1_ONLY level-1

Without summarization, here’s what the level 1 database looks like:

R2#show isis database level-1 verbose R1.00-00


IS-IS Level-1 LSP R1.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000008   0xC75F        1180              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  Metric: 10         IS R2.01
  IP Address:   1.1.1.1
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 1.1.1.1 255.255.255.255
  Metric: 0          IP-External 11.11.11.11 255.255.255.255

R2 has installed this entry in its routing table:

R2#show ip route isis

      1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
i su     1.0.0.0/8 [115/20], 00:08:37, Null0
i L1     1.1.1.1/32 [115/20] via 192.168.12.1, 00:11:26, GigabitEthernet0/1
      11.0.0.0/32 is subnetted, 1 subnets
i L1     11.11.11.11 [115/10] via 192.168.12.1, 00:00:31, GigabitEthernet0/1

Let’s summarize this redistribute route on R1. You can do this with the same summary-address command:

R1(config)#router isis
R1(config-router)#summary-address 11.0.0.0 255.0.0.0 level-1

If you summarize into level 1, make sure you add the level-1 parameter or nothing will happen. Let’s check the routing table of R1:

R1#show ip route isis                 

i*L1  0.0.0.0/0 [115/10] via 192.168.12.2, 00:43:46, GigabitEthernet0/1
      11.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
i su     11.0.0.0/8 [115/0], 00:00:39, Null0
i L1  192.168.23.0/24 [115/20] via 192.168.12.2, 00:44:13, GigabitEthernet0/1

We can see that R1 has installed a discard route for this summary. Let’s check the level 1 database:

R2#show isis database level-1 verbose R1.00-00


IS-IS Level-1 LSP R1.00-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000009   0xD76F        1139              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC 
  Hostname: R1
  Metric: 10         IS R2.01
  IP Address:   1.1.1.1
  Metric: 10         IP 192.168.12.0 255.255.255.0
  Metric: 10         IP 1.1.1.1 255.255.255.255
  Metric: 0          IP-External 11.0.0.0 255.0.0.0

Above we can see that the route was summarized directly in the level 1 database. We can find the summarized route in the routing table of R2:

R2#show ip route isis                         

      1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
i su     1.0.0.0/8 [115/20], 00:11:37, Null0
i L1     1.1.1.1/32 [115/20] via 192.168.12.1, 00:14:26, GigabitEthernet0/1
i L1  11.0.0.0/8 [115/10] via 192.168.12.1, 00:01:10, GigabitEthernet0/1

That’s it.

hostname R1
!
ip cef
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip router isis 
!
interface Loopback1
 ip address 11.11.11.11 255.255.255.255
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
 ip router isis 
!
router isis
 net 49.0012.0000.0000.0001.00
 is-type level-1
 log-adjacency-changes
 summary-address 11.0.0.0 255.0.0.0 level-1
 redistribute connected route-map L1_ONLY level-1
!
route-map L1_ONLY permit 10
 match interface Loopback1
!
end

hostname R2
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.12.2 255.255.255.0
 ip router isis 
!
interface GigabitEthernet0/2
 ip address 192.168.23.2 255.255.255.0
 ip router isis 
!
router isis
 net 49.0012.0000.0000.0002.00
 log-adjacency-changes
 summary-address 1.0.0.0 255.0.0.0
!
end

hostname R3
!
ip cef
!
interface GigabitEthernet0/1
 ip address 192.168.23.3 255.255.255.0
 ip router isis 
!
router isis
 net 49.0003.0000.0000.0003.00
 log-adjacency-changes
!
end

Conclusion

IS-IS supports summarization but as a link-state routing protocol, there are some limitations. You can only configure summarization between areas or on a router that is doing redistribution.

If you configure summarization on an area border router for routes in the level 1 database then it will add the summary route in the level 2 LSP which is advertised to other areas. If you use redistribution, then the summary route will be added directly in the level 1 or level 2 LSP.