NETWORK LESSON

Network Engineers Guide to IS-IS Configurations

Published by

Recto Orpia

on

On this post, I will be exploring how to configured IS-IS routing protocol and provide some basic concepts on how IS-IS routing protocol works.

I will be using the below topology to illustrate the basic concept and configurations of IS-IS.

Legend:

R1 & R4 – Level-1 routers
R2 & R3 – Level 1-2 routers

So the question is what is IS-IS routing protocol? It was originally developed by DEC for Connectionless-Mode Network Service. It’s one of the two Link-State routing protocol ( aside from OSPF). There are couple of differences between the two, e.g. there is no backbone 0 for IS-IS and this provides flexibility of deployment ( no more virtual-link).

But before moving to the configurations part, we need to understand some concepts about IS-IS.

There are 3 types of IS-IS routers:

  1. Level 1 System– This is an intra-area router . It can only formed neighbor adjacency with another L1 router.
  2. Level 2 System – This is a backbone router. It knows about the prefix on intra-area and inter-area.
  3. Level 1-2 System – By the name itself, it can act as Level 1 or Level 2 router.  This router usually is considered the backbone connecting two different areas as it could function as L1 or L2 system router.
There is no backbone 0 for IS-IS unlike OSPF. Backbone could be in any area number but the backbone router could either be a Level 1-2 or Level 2 system.
If you still remember LSA’s for OSPF, the equivalent of this for IS-IS is LSP or Link State Packet. So Link State Packet contained the following information’s when they are exchange with the neighboring router:

  1. Prefixes
  2. Neighbor
  3. Metric

Each IS-IS router must have a unique identifier called Network Entity Title ( NET) . NET have two major parts called IDP or Initial Domain Part consisting of Authority and Format Identifier (AFI) and Initial Domain Identifier (IDI).  AN AFI of 49 signifies a private networks and usually the IDI represents the IS-IS area.   The second part of NET is the DSO or the Domain Specific Part which consist of High Order DSP, System ID and NSEL.  (Reference  RFC 1195 – https://tools.ietf.org/html/rfc1195)

Considerations for this laboratory:

  • AFI is set to 49
  • System ID will be 0000.0000.000X where X is the router number.

I will go through the configurations part and will explore the LSP Level 1 and Level 2 database on all the routers.

R1 Configurations:

R1#show ip int brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        unassigned      YES unset  administratively down down
GigabitEthernet1/0     172.16.12.1     YES manual up                    up
GigabitEthernet2/0     unassigned      YES unset  administratively down down
Loopback1              11.11.11.11     YES manual up                    up

router isis
 net 49.0012.0000.0000.0001.00
 is-type level-1
 log-adjacency-changes

interface GigabitEthernet1/0
 ip address 172.16.12.1 255.255.255.0
 ip router isis
 negotiation auto

interface Loopback1
 ip address 11.11.11.11 255.255.255.255
 ip router isis

R2 Configurations:

R2#show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        unassigned      YES unset  administratively down down
GigabitEthernet1/0     172.16.12.2     YES manual up                    up
GigabitEthernet2/0     172.16.23.2     YES manual up                    up
Loopback0              unassigned      YES unset  up                    up
Loopback2              22.22.22.22     YES manual up                    up

router isis
 net 49.0012.0000.0000.0002.00
 log-adjacency-changes

interface Loopback2
 ip address 22.22.22.22 255.255.255.255
 ip router isis

interface GigabitEthernet1/0
 ip address 172.16.12.2 255.255.255.0
 ip router isis
 negotiation auto
!
interface GigabitEthernet2/0
 ip address 172.16.23.2 255.255.255.0
 ip router isis
 negotiation auto

Note:  I have configured R2 as Level 1-2 system router but the configurations is not showing this details as seen above. By default a Cisco router is a Level 1-2 system.

R2(config)#router isis
R2(config-router)#is-type level-1-2

R3 Configurations:

R3#show ip int brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        unassigned      YES unset  administratively down down
GigabitEthernet1/0     172.16.34.3     YES manual up                    up
GigabitEthernet2/0     172.16.23.3     YES manual up                    up
Loopback3              33.33.33.33     YES manual up                    up
R3#

interface Loopback3
 ip address 33.33.33.33 255.255.255.255
 ip router isis

interface GigabitEthernet1/0
 ip address 172.16.34.3 255.255.255.0
 ip router isis
 negotiation auto
!
interface GigabitEthernet2/0
 ip address 172.16.23.3 255.255.255.0
 ip router isis
 negotiation auto
!
router isis
 net 49.0034.0000.0000.0003.00
 log-adjacency-changes

Similarly above, R3 is also a Level 1-2 system and its a default IS-IS configs for Cisco device.

R4 Configurations:

R4#show ip int brief
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        unassigned      YES unset  administratively down down
GigabitEthernet1/0     172.16.34.4     YES manual up                    up
GigabitEthernet2/0     unassigned      YES unset  administratively down down
Loopback0              unassigned      YES unset  up                    up
Loopback4              44.44.44.44     YES manual up                    up
R4#

interface Loopback4
 ip address 44.44.44.44 255.255.255.255
 ip router isis
!
interface GigabitEthernet1/0
 ip address 172.16.34.4 255.255.255.0
 ip router isis
 negotiation auto
!
router isis
 net 49.0034.0000.0000.0004.00
 is-type level-1

Let’s check the IS-IS neighbor. IS-IS neighbor is established between R1 and R2.

R1#show isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L1   Gi1/0       172.16.12.2     UP    6        R2.01
R1#

Let’s check the R1 Link State Packet L1 database. As seen below, R1 have received LSP for the prefix which I highlighted in red from R2.

R1#show isis database verbose

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000010   0x3402        873               0/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R1
  IP Address:   11.11.11.11
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 11.11.11.11 255.255.255.255
  Metric: 10         IS R2.01
R2.00-00              0x0000000E   0xE97E        545               1/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R2
  IP Address:   22.22.22.22
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 22.22.22.22 255.255.255.255
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IS R2.01
R2.01-00              0x00000009   0x6CDE        1063              0/0/0
  Metric: 0          IS R2.00
  Metric: 0          IS R1.00
R1#

Let’s check the IS-IS routes on R1.

  • A default route is created on R1. This is one rule created for IS-IS in which a prefix from other Level 1 router on a different area will not be advertise on another Level 1 system from a different area.  In order to established connectivity a default route will be introduced
  • IS-IS Administrative distance is 115 with link metrics of 10. A metric of 10 is constant irregardless of the link bandwidth. 
  • The prefix 22.22.22.22/32 from R2 is learned via IS-IS with Admin Distance of 115 but since its two hops away from R1 , the metrics is 20.
  • It also learned the prefix ( 172.16.23.0/24) used between the Level 1-2 routers ( R2 & R3) with a admin distance of 115 and metric of 20.

R1#show ip route isis
Codes: L – local, C – connected, S – static, 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
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is 172.16.12.2 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 172.16.12.2, 00:38:12, GigabitEthernet1/0
      22.0.0.0/32 is subnetted, 1 subnets
i L1     22.22.22.22 [115/20] via 172.16.12.2, 01:44:10, GigabitEthernet1/0
      172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
i L1     172.16.23.0/24 [115/20] via 172.16.12.2, 00:39:19, GigabitEthernet1/0
R1#

By this time, lets check R2,

R2#show isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1             L1   Gi1/0       172.16.12.1     UP    25       R2.01
R3             L2   Gi2/0       172.16.23.3     UP    8        R3.01
R2#

Note:

  • There are two LSP database created on R2, one is for Level-1 Link State Database and the other 1 is for Level-2 Link State Database.
  • The rules is that all directly connected interfaces that are configured for IS-IS are added to both Level-1 LSP’s and Level-2 LSP’s.
  • Also Level 1-2 routers will add a prefixes it learned from Level 1 LSP’s to their Level 2 LSP’s.
  • Level 1 LSP’s are only flooded within an area while Level 2 LSP’s are flooded within the backbone.
  • One more thing as we have observed on R1, it generates a default routes towards Level 1-2 routers when it sees the attached bit.  The attached bit is set by the Level 1-2 router on the Level 1 LSP update. We have to take note that Level 1 LSP’s are not allowed between backbone but the Level 1 LSP’s is copied to Level 2 LSP’s with an altered bit to indicate that its a prefix coming from Level 1 router. This attached bit prefix is advertised to the other backbone router in other area which eventually been sync with the LSP database on the Level 1 router of that area. It created a default route when he sees this attached bit to ensure reachability with the other Level-1 router.

R2 has the following Link State Packet database:

R2#show isis database verbose

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x00000011   0x3203        1166              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R1
  IP Address:   11.11.11.11
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 11.11.11.11 255.255.255.255
  Metric: 10         IS R2.01
R2.00-00            * 0x0000000F   0xE77F        780               1/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R2
  IP Address:   22.22.22.22
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 22.22.22.22 255.255.255.255
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IS R2.01
R2.01-00            * 0x00000009   0x6CDE        532               0/0/0
  Metric: 0          IS R2.00
  Metric: 0          IS R1.00

IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00            * 0x00000005   0x2A80        562               0/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R2
  IP Address:   22.22.22.22
  Metric: 10         IS R3.01
  Metric: 20         IP 11.11.11.11 255.255.255.255
  Metric: 10         IP 22.22.22.22 255.255.255.255
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 172.16.23.0 255.255.255.0
R3.00-00              0x0000000A   0x840A        1001              0/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R3
  IP Address:   33.33.33.33
  Metric: 10         IS R3.01
  Metric: 10         IP 33.33.33.33 255.255.255.255
  Metric: 20         IP 44.44.44.44 255.255.255.255
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IP 172.16.34.0 255.255.255.0
R3.01-00              0x00000004   0x27AC        944               0/0/0
  Metric: 0          IS R3.00
  Metric: 0          IS R2.00
R2#

R2#show ip route isis
Codes: L – local, C – connected, S – static, 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
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      11.0.0.0/32 is subnetted, 1 subnets
i L1     11.11.11.11 [115/20] via 172.16.12.1, 02:15:02, GigabitEthernet1/0
      33.0.0.0/32 is subnetted, 1 subnets
i L2     33.33.33.33 [115/20] via 172.16.23.3, 01:02:35, GigabitEthernet2/0
      44.0.0.0/32 is subnetted, 1 subnets
i L2     44.44.44.44 [115/30] via 172.16.23.3, 00:57:43, GigabitEthernet2/0
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
i L2     172.16.34.0/24 [115/20] via 172.16.23.3, 00:57:49, GigabitEthernet2/0
R2#

Let’s check R3 as well,

R3#show isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R2             L2   Gi2/0       172.16.23.2     UP    25       R3.01
R4             L1   Gi1/0       172.16.34.4     UP    9        R4.01
R3#

R3#show isis database verbose

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R3.00-00            * 0x0000000B   0x2BAB        1005              1/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R3
  IP Address:   33.33.33.33
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IP 33.33.33.33 255.255.255.255
  Metric: 10         IP 172.16.34.0 255.255.255.0
  Metric: 10         IS R4.01
R4.00-00              0x00000009   0xC42F        875               0/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R4
  IP Address:   44.44.44.44
  Metric: 10         IP 172.16.34.0 255.255.255.0
  Metric: 10         IP 44.44.44.44 255.255.255.255
  Metric: 10         IS R4.01
R4.01-00              0x00000005   0xB296        907               0/0/0
  Metric: 0          IS R4.00
  Metric: 0          IS R3.00
IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R2.00-00              0x00000007   0x2682        1169              0/0/0
  Area Address: 49.0012
  NLPID:        0xCC
  Hostname: R2
  IP Address:   22.22.22.22
  Metric: 10         IS R3.01
  Metric: 20         IP 11.11.11.11 255.255.255.255
  Metric: 10         IP 22.22.22.22 255.255.255.255
  Metric: 10         IP 172.16.12.0 255.255.255.0
  Metric: 10         IP 172.16.23.0 255.255.255.0
R3.00-00            * 0x0000000B   0x820B        846               0/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R3
  IP Address:   33.33.33.33
  Metric: 10         IS R3.01
  Metric: 10         IP 33.33.33.33 255.255.255.255
  Metric: 20         IP 44.44.44.44 255.255.255.255
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IP 172.16.34.0 255.255.255.0
R3.01-00            * 0x00000005   0x25AD        660               0/0/0
  Metric: 0          IS R3.00
  Metric: 0          IS R2.00
R3#


R3#show ip route isis
Codes: L – local, C – connected, S – static, 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
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      11.0.0.0/32 is subnetted, 1 subnets
i L2     11.11.11.11 [115/30] via 172.16.23.2, 01:03:25, GigabitEthernet2/0
      22.0.0.0/32 is subnetted, 1 subnets
i L2     22.22.22.22 [115/20] via 172.16.23.2, 01:03:25, GigabitEthernet2/0
      44.0.0.0/32 is subnetted, 1 subnets
i L1     44.44.44.44 [115/20] via 172.16.34.4, 00:58:37, GigabitEthernet1/0
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
i L2     172.16.12.0/24 [115/20] via 172.16.23.2, 01:03:25, GigabitEthernet2/0
R3#

And finally R4. As observed R4 has the default route in order to reach the Prefix advertise into IS-IS from R1.

R4#show isis neighbors

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R3             L1   Gi1/0       172.16.34.3     UP    25       R4.01

R4#show isis database verbose

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R3.00-00              0x0000000B   0x2BAB        931               1/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R3
  IP Address:   33.33.33.33
  Metric: 10         IP 172.16.23.0 255.255.255.0
  Metric: 10         IP 33.33.33.33 255.255.255.255
  Metric: 10         IP 172.16.34.0 255.255.255.0
  Metric: 10         IS R4.01
R4.00-00            * 0x00000009   0xC42F        808               0/0/0
  Area Address: 49.0034
  NLPID:        0xCC
  Hostname: R4
  IP Address:   44.44.44.44
  Metric: 10         IP 172.16.34.0 255.255.255.0
  Metric: 10         IP 44.44.44.44 255.255.255.255
  Metric: 10         IS R4.01
R4.01-00            * 0x00000005   0xB296        840               0/0/0
  Metric: 0          IS R4.00
  Metric: 0          IS R3.00
R4#


R4#show ip route isis
Codes: L – local, C – connected, S – static, 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
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is 172.16.34.3 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 172.16.34.3, 00:58:52, GigabitEthernet1/0
      33.0.0.0/32 is subnetted, 1 subnets
i L1     33.33.33.33 [115/20] via 172.16.34.3, 00:58:52, GigabitEthernet1/0
      172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
i L1     172.16.23.0/24 [115/20] via 172.16.34.3, 00:58:52, GigabitEthernet1/0
R4#

Let’s check the connectivity:

R4#ping 11.11.11.11 source 44.44.44.44
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.11.11.11, timeout is 2 seconds:
Packet sent with a source address of 44.44.44.44
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 60/83/112 ms

R4#traceroute 11.11.11.11 source 44.44.44.44
Type escape sequence to abort.
Tracing the route to 11.11.11.11
VRF info: (vrf in name/id, vrf out name/id)
  1 172.16.34.3 60 msec 32 msec 56 msec
  2 172.16.23.2 80 msec 80 msec 60 msec
  3 172.16.12.1 60 msec 60 msec 80 msec

##########################END#######################################

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