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> Chapter 21 - Networking > Intermediate System to Intermediate System Protocol (IS-IS) > Simple IS-IS example

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Simple IS-IS example

This is an example of a typical medium-sized network configuration using IS-IS routing.

Imagine a company with four FortiGate devices connected to one another. A FortiGate at one end of the network connects to two routers, each with its own local subnet; one of these routers uses OSPF and the other uses RIP.

Your task is to configure the four FortiGates to route traffic and process network updates using IS-IS, such that the farthest FortiGate (see ‘FGT4’ in Network layout and assumptions) receives route updates for the two routers at the opposite end of the network. Furthermore, FGT4 has been given a loopback subnet that must be identified by the router running RIP.

Since the internal networks use OSPF and RIP, those protocols will need to be redistributed through the IS-IS network. To keep the example simple, there will be no authentication of router traffic.

With IS-IS properly configured in this example, if a router fails or temporarily goes offline, the route change will propagate throughout the system.

This section includes the following topics:

Network layout and assumptions

 

  • It is assumed that each FortiGate is operating in NAT mode, running FortiOS 4.0MR2+.
  • All interfaces have been previously assigned and no static routes are required.
  • The AFI (Authority and Format Identifier) used is 49 : Locally administered (private).
  • The Area identifiers are 0048 and 0049.

Expectations

  • FGT4 must get the IS-IS route updates for RTR1 and RTR2 local subnets (10.1.1.0, 10.2.2.0, 10.3.3.0, 10.4.4.0).
  • RTR1 must receive (via RIP2) the loopback subnet of FGT4 (10.60.60.1/32).

CLI configuration

The following CLI configuration occurs on each FortiGate (as identified), including only the relevant parts.

FGT1

config router isis

config isis-interface

edit "port3"

set circuit-type level-1

set network-type broadcast

set status enable

next

end

config isis-net

edit 1

set net 49.0048.1921.6818.2136.00

next

end

config redistribute "connected"

end

config redistribute "rip"

set status enable

set level level-1

end

config redistribute "ospf"

set status enable

set level level-1

end

end

config router rip

config interface

edit "port2"

set receive-version 2

set send-version 2

next

end

config network

edit 1

set prefix 10.10.10.0 255.255.255.0

next

end

config redistribute "isis"

set status enable

end

end

 

FGT2

config router isis

config isis-interface

edit "port3"

set circuit-type level-1

set network-type broadcast

set status enable

next

edit "port2"

set network-type broadcast

set status enable

next

end

config isis-net

edit 1

set net 49.0048.1221.6818.2110.00

next

end

set redistribute-l1 enable

set redistribute-l2 enable

end

 

FGT3

config router isis

set is-type level-2-only

config isis-interface

edit "wan1"

set network-type broadcast

set status enable

next

edit "dmz1"

set network-type broadcast

set status enable

next

end

config isis-net

edit 1

set net 49.0048.1921.6818.2108.00

next

edit 2

set net 49.0049.1921.6818.2108.00

next

end

end

 

FGT4

config router isis

set is-type level-2-only

config isis-interface

edit "wan1"

set network-type broadcast

set status enable

next

end

config isis-net

edit 1

set net 49.0049.1721.0160.1004.00

next

end

config redistribute "connected"

set status enable

end

end

Verification

Once the network has been configured, you need to test that it works as expected. Use the following CLI commands on the devices indicated.

Verifying if RTR1 receives loopback subnet of FGT4

(RTR1) # get router info routing-table all

Result:

C    10.1.1.0/24 is directly connected, vlan1

C    10.2.2.0/24 is directly connected, vlan2

C    10.10.10.0/24 is directly connected, dmz1

R    10.40.40.0/24 [120/2] via 10.10.10.1, dmz1, 00:04:07

R    10.50.50.0/24 [120/2] via 10.10.10.1, dmz1, 00:04:07

R    10.60.60.1/32 [120/2] via 10.10.10.1, dmz1, 00:04:07

(*) If required, filtering out 10.50.50.0 and 10.40.40.0 from the routing table could be done with a route-map.

Verification on FGT2, which is the border between L1 and L2 routing levels; looking at IS-IS information

FGT2 # get router info isis interface

Result:
port2 is up, line protocol is up
Routing Protocol: IS-IS ((null))
Network Type: Broadcast
Circuit Type: level-1-2
Local circuit ID: 0x01
Extended Local circuit ID: 0x00000003
Local SNPA: 0009.0f85.ad8c
IP interface address:
10.40.40.2/24
IPv4 interface address:
Level-1 Metric: 10/10, Priority: 64, Circuit ID: 1221.6818.2110.01
Number of active level-1 adjacencies: 0
Level-2 Metric: 10/10, Priority: 64, Circuit ID: 1221.6818.2110.01
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-1 Hello in 6 seconds
Next IS-IS LAN Level-2 Hello in 1 seconds
port3 is up, line protocol is up
Routing Protocol: IS-IS ((null))
Network Type: Broadcast
Circuit Type: level-1
Local circuit ID: 0x02
Extended Local circuit ID: 0x00000004
Local SNPA: 0009.0f85.ad8d
IP interface address:
10.30.30.2/24
IPv4 interface address:
Level-1 Metric: 10/10, Priority: 64, Circuit ID: 1221.6818.2110.02
Number of active level-1 adjacencies: 1
Next IS-IS LAN Level-1 Hello in 2 seconds

FGT2 # get router info isis neighbor
Result:
  System Id Interface SNPA State Holdtime Type Protocol
  1921.6818.2108 port2 0009.0f04.0794 Up 22 L2 IS-IS
  1921.6818.2136 port3 0009.0f85.acf7 Up 29 L1 IS-IS
Verification on FGT3, which is border between 2 areas; looking at IS-IS information

IS-IS router CLI commands available:

FGT3 # get router info isis ?

Result:
  interface show isis interfaces
  neighbour show CLNS neighbor adjacencies
  is-neighbour show IS neighbor adjacencies
  database show IS-IS link state database
  route show IS-IS IP routing table
  topology show IS-IS paths

 

Example of interface status and neighbors:

FGT3 # get router info isis interface

Result:
wan1 is up, line protocol is up
Routing Protocol: IS-IS ((null))
Network Type: Broadcast
Circuit Type: level-1-2
Local circuit ID: 0x01
Extended Local circuit ID: 0x00000003
Local SNPA: 0009.0f04.0794
IP interface address:
10.40.40.1/24
IPv4 interface address:
Level-2 Metric: 10/10, Priority: 64, Circuit ID: 1221.6818.2110.01
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-2 Hello in 3 seconds
 
dmz1 is up, line protocol is up
Routing Protocol: IS-IS ((null))
Network Type: Broadcast
Circuit Type: level-1-2
Local circuit ID: 0x02
Extended Local circuit ID: 0x00000005
Local SNPA: 0009.0f04.0792
IP interface address:
10.50.50.1/24
IPv4 interface address:
Level-2 Metric: 10/10, Priority: 64, Circuit ID: 1721.0160.1004.01
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-2 Hello in 7 seconds
 
FGT3 # get router info isis neighbor
Result:
  System Id Interface SNPA State Holdtime Type Protocol
  1221.6818.2110 wan1 0009.0f85.ad8c Up 8 L2 IS-IS
  1721.0160.1004 dmz1 0009.0f52.7704 Up 8 L2 IS-IS
Verification on FGT4 that the remote subnets from RTR1 and RTR2 are in the routing table and learned with IS-IS

FGT4 # get router info routing-table all

Result:

Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP

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
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default

i L2 10.1.1.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.2.2.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.3.3.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.4.4.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46

i L2 10.10.10.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.11.11.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.20.20.0/24 [115/30] via 10.50.50.1, wan1, 00:12:46
i L2 10.30.30.0/24 [115/30] via 10.50.50.1, wan1, 00:13:55
i L2 10.40.40.0/24 [115/20] via 10.50.50.1, wan1, 00:15:30

C 10.50.50.0/24 is directly connected, wan1
C 10.60.60.1/32 is directly connected, loopback

Troubleshooting

The following diagnose commands are available for further IS-IS troubleshooting and will display all IS-IS activity (sent and received packets):

FGT # diagnose ip router isis level info

FGT # diagnose ip router isis all enable

FGT # diagnose debug enable

...to stop the debug type output:

FGT # diagnose ip router isis level none

Output and interpretation depends on the issue faced. You can provide this information to TAC if you open a support ticket.