IS-IS as a link-state routing protocol is a bit restrictive when it comes to filtering. All routers within an area require a synchronized level 1 database, the same thing applies to all level 2 routers. The level 2 database has to be the same on all routers. Once an LSP is generated, you can’t filter it anymore.
There are two methods how you can filter something:
- Distribute-list inbound filtering.
- Filtering between level 1 and level 2.
Inbound filtering is possible, this doesn’t prevent an LSP from being installed in the database but it does prevent an LSP from being installed in the routing table. It is also possible to filter level 1 LSPs from being copied to the level 2 database.
In this lesson, I will show you both examples.
Here is the topology we will use:
We have three routers in area 123 and one in area 4. R1 has a loopback interface with a prefix that we will filter.
Want to take a look for yourself? Here you will find the startup configuration of each device.
hostname R1 ! ip cef ! interface Loopback0 ip address 220.127.116.11 255.255.255.255 ip router isis ! interface GigabitEthernet0/1 ip address 192.168.12.1 255.255.255.0 ip router isis ! router isis net 49.0123.0000.0000.0001.00 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 49.0123.0000.0002.00 is-type level-1 log-adjacency-changes ! end
hostname R3 ! ip cef ! interface GigabitEthernet0/1 ip address 192.168.34.3 255.255.255.0 ip router isis ! interface GigabitEthernet0/2 ip address 192.168.23.3 255.255.255.0 ip router isis ! router isis net 49.0123.0000.0000.0003.00 log-adjacency-changes ! end
hostname R4 ! ip cef ! interface GigabitEthernet0/1 ip address 192.168.34.4 255.255.255.0 ip router isis ! router isis net 49.0004.0000.0000.0004.00 is-type level-2-only log-adjacency-changes ! end
Let’s get started.
Distribute-list Inbound filtering
We’ll start with the distribute-list which allows us to prevent something from being installed in the routing table. Let’s take a look at R2:
R2#show ip route isis 18.104.22.168/32 is subnetted, 1 subnets i L1 22.214.171.124 [115/20] via 192.168.12.1, 00:38:16, GigabitEthernet0/1 i L1 192.168.34.0/24 [115/20] via 192.168.23.3, 00:37:26, GigabitEthernet0/2
Let’s get rid of the 126.96.36.199/32 prefix. I will use an access-list for this:
R2(config)#ip access-list standard R1_L0 R2(config-std-nacl)#deny host 188.8.131.52 R2(config-std-nacl)#permit any
We can enable the access-list with the distribute-list command:
R2(config)#router isis R2(config-router)#distribute-list R1_L0 in
When you look at the level 1 database, you will see that the prefix is still there:
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 0x00000002 0xEA6C 1007 0/0/0 Area Address: 49.0123 NLPID: 0xCC Hostname: R1 Metric: 10 IS R2.01 IP Address: 184.108.40.206 Metric: 10 IP 220.127.116.11 255.255.255.255 Metric: 10 IP 192.168.12.0 255.255.255.0
We can’t remove it from the database but it will be gone from the routing table:
R2#show ip route isis i*L1 0.0.0.0/0 [115/10] via 192.168.23.3, 00:03:29, GigabitEthernet0/2 i L1 192.168.34.0/24 [115/20] via 192.168.23.3, 00:03:39, GigabitEthernet0/2
Since it’s still in the database, other routers will learn about. For example, here’s R3:
R3#show ip route isis 18.104.22.168/32 is subnetted, 1 subnets i L1 22.214.171.124 [115/30] via 192.168.23.2, 00:42:35, GigabitEthernet0/2 i L1 192.168.12.0/24 [115/20] via 192.168.23.2, 00:42:47, GigabitEthernet0/2
This introduces a problem. Since R2 is a transit router, R3 will never be able to reach 126.96.36.199/32. That’s something to keep in mind…
Level 1 to Level 2 filtering
Let’s continue. R3 and R4 still have 188.8.131.52/32 in their routing tables. Let’s see if we can prevent this prefix from being installed on R4. Right now it does have this route in its routing table:
R4#show ip route isis 184.108.40.206/32 is subnetted, 1 subnets i L2 220.127.116.11 [115/40] via 192.168.34.3, 00:42:26, GigabitEthernet0/1 i L2 192.168.12.0/24 [115/30] via 192.168.34.3, 00:42:26, GigabitEthernet0/1 i L2 192.168.23.0/24 [115/20] via 192.168.34.3, 00:42:26, GigabitEthernet0/1
R4 has learned this from the level 2 LSP that R3 has generated. We can see it here:
R3#show isis database level-2 verbose R3.00-00 IS-IS Level-2 LSP R3.00-00 LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R3.00-00 * 0x00000003 0x1660 899 0/0/0 Area Address: 49.0123 NLPID: 0xCC Hostname: R3 Metric: 10 IS R3.01 IP Address: 192.168.23.3 Metric: 10 IP 192.168.23.0 255.255.255.0 Metric: 10 IP 192.168.34.0 255.255.255.0 Metric: 30 IP 18.104.22.168 255.255.255.255 Metric: 20 IP 192.168.12.0 255.255.255.0
R3 added 22.214.171.124/32 by copying it from its level 1 database to its level 2 database. Let’s see if we can prevent that from happening…
There are two methods. You can use a distribute-list with extended access-list numbers or a route-map. I prefer the route-map since it allows you to use named access-lists. Let’s create an access-list that matches the loopback interface of R1:
R3(config)#ip access-list extended R1_L0 R3(config-ext-nacl)#deny ip host 126.96.36.199 any R3(config-ext-nacl)#permit ip any any
Let’s add this access-list in a route-map:
R3(config)#route-map L1_L2_FILTER permit 10 R3(config-route-map)#match ip address R1_L0
The only thing left to do is to activate it. This is done with the redistribute command:
R3(config)#router isis R3(config-router)#redistribute isis ip level-1 into level-2 route-map L1_L2_FILTER
This tells R3 to redistribute everything from level 1 to level 2 except for the things that we added in our route-map. Let’s take another look at R3’s level 2 database: