The purpose of this document is to demonstrate a basic configuration of Intermediate System-to-Intermediate System (IS-IS) for IP on Cisco routers. In addition to the configuration, how to monitor various IS-IS information is demonstrated, such as Designated Intermediate System (DIS) election information and IS-IS database information.
There are no specific requirements for this document.
The information in this document is based on Cisco IOS ® Software Release 12.1(5)T9.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Refer to Cisco Technical Tips Conventions for more information on document conventions.
In this section, you are presented with the information to configure the features described in this document.
In order to enable IS-IS for IP on a Cisco router and have it exchange routing information with other IS-IS enabled routers, you must perform these two tasks:
Enable the IS-IS process and assign area
Enable IS-IS for IP routing on an interface
Other configuration tasks are optional, however the above two tasks are required. For more information on optional configuration tasks, refer to Configuring Integrated IS-IS.
This document uses this network setup:
This document uses these configurations:
The sample configurations below configure all the routers in the above topology with these parameters:
Level 1 (L1) and Level 2 (L2) routers (this is the default unless otherwise specified)
No optional parameters
Running IS-IS for IP only
Loopback interfaces (loopbacks are advertised by IS-IS, not IS-IS enabled)
! interface Loopback0 ip address 172.16.1.1 255.255.255.255 !--- Creates loopback interface and assigns !--- IP address to interface Loopback0. ! interface Ethernet0 ip address 172.16.12.1 255.255.255.0 ip router isis !--- Assigns IP address to interface Ethernet0 !--- and enables IS-IS for IP on the interface. ! router isis passive-interface Loopback0 net 49.0001.1720.1600.1001.00 ! !--- Enables the IS-IS process on the router, !--- makes loopback interface passive !--- (does not send IS-IS packets on interface), !--- and assigns area and system ID to router.
! interface Loopback0 ip address 172.16.2.2 255.255.255.255 !--- Creates loopback interface and assigns !--- IP address to interface Loopback0. ! Interface Ethernet0 ip address 172.16.12.2 255.255.255.0 ip router isis !--- Assigns IP address to interface Ethernet0 !--- and enables IS-IS for IP on the interface. ! Interface Serial0 ip address 172.16.23.1 255.255.255.252 ip router isis !--- Assigns IP address to interface Serial0 !--- and enables IS-IS for IP on the interface. ! router isis passive-interface Loopback0 net 49.0001.1720.1600.2002.00 ! !--- Enables the IS-IS process on the router, !--- makes loopback interface passive !--- (does not send IS-IS packets on interface), !--- and assigns area and system ID to router.
! interface Loopback0 ip address 172.16.3.3 255.255.255.255 !--- Creates loopback interface !--- and assigns IP address to !--- interface Loopback0. ! Interface Serial0 ip address 172.16.23.2 255.255.255.252 ip router Isis !--- Assigns IP address to !--- interface Serial0 and enables !--- IS-IS for IP on the interface. ! router isis passive-interface Loopback0 net 49.0001.1234.1600.2231.00 ! !--- Enables the IS-IS process on the router, !--- makes loopback interface passive !--- (does not send IS-IS packets on interface), !--- and assigns area and system ID to router.
There are many show commands available for monitoring the state of IS-IS on a Cisco router. This document demonstrates some of the more basic commands based on the above router configurations.
Use the show clns neighbor command to display the adjacencies for a specific router. This is the output of this command from Router 1 (R1) and Router 2 (R2):
R1# show clns neighbor System Id Interface SNPA State Holdtime Type Protocol R2 Et0 0000.0c47.b947 Up 24 L1L2 ISIS R2# show clns neighbor System Id Interface SNPA State Holdtime Type Protocol R1 Et0 0000.0c09.9fea Up 24 L1L2 ISIS R3 Se0 *HDLC* Up 28 L1L2 ISIS
In the above example, R1 recognizes R2 on its E0 interface with the adjacency type being L1L2. Because R1 and R2 are configured with default configurations, they send and receive both L1 and L2 hellos.
R2 recognizes R1 on its E0 interface, and Router 3 (R3) on its S0 interface. The same explanation as above is true for the adjacency type.
Since R1 and R2 are on the same Ethernet interface, there is a DIS for both L1 and L2. You can verify this using the show clns interface <int> command on Router 1, as shown below:
R1# show clns interface ethernet 0 Ethernet0 is up, line protocol is up Checksums enabled, MTU 1497, Encapsulation SAP Routing Protocol: ISIS Circuit Type: level-1-2 Interface number 0x0, local circuit ID 0x1 Level-1 Metric: 10, Priority: 64, Circuit ID: R2.01 Number of active level-1 adjacencies: 1 Level-2 Metric: 10, Priority: 64, Circuit ID: R2.01 Number of active level-2 adjacencies: 1 Next ISIS LAN Level-1 Hello in 5 seconds Next ISIS LAN Level-2 Hello in 1 seconds
In the above output, R2 is the DIS. It is the R2 (DIS) that generates the pseudonode Link State Packet (LSP) and is denoted with a non-zero LSP-ID - R2.01
Since the Metric/Priority are the same for both routers in L1/L2, the tiebreaker for the DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address. Other instance of data link addresses would be X.25 addresses and Frame Relay DLCI.
Notice that the DIS is elected for both levels, and that no backup DIS exists, as with Open Shortest Path First (OSPF), which has a backup Designated Router (DR).
Some other points of interest from the above output include:
Circuit Type: L1L2
L1 and L2 metrics and priorities are at default values: 10 and 64
L1 and L2 adjacencies: 1 (from R1 perspective on the Ethernet interface - it is R2 only)
IS-IS LAN hellos for L1 and L2
Maximum Transmission Unit (MTU): 1497. This is because the Open Systems Interconnection (OSI) IS-IS header is encapsulated inside a 3 byte 802.2 header.
The show isis database (detail) command displays the contents of the IS-IS database. This is the output of this command when issued on R2. Since IS-IS is a link state protocol, the link state database should be the same for any router in the same area.
R2# show isis database ISIS Level-1 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R1.00-00 0x0000008B 0x6843 55 0/0/0 R2.00-00 * 0x00000083 0x276E 77 0/0/0 R2.01-00 * 0x00000004 0x34E1 57 0/0/0 R3.00-00 0x00000086 0xF30E 84 0/0/0 ISIS Level-2 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R1.00-00 0x00000092 0x34B2 41 0/0/0 R2.00-00 * 0x0000008A 0x7A59 115 0/0/0 R2.01-00 * 0x00000004 0xC3DA 50 0/0/0 R3.00-00 0x0000008F 0x0766 112 0/0/0
There are a few things to notice in the above output. First, about the LSP-ID:
The LSP-ID, R1.00-00, can be broken down into three sections: R1/00/00
R1 = system ID
00 = non-zero value for the pseudonode. Notice R2.01-00 is the pseudonode LSP.
00 = fragment number. In this case, there are only fragment numbers of 00, which indicates that all the data fit into this LSP fragment, and there was no need to create more fragments. If there had been information that did not fit into the first LSP, IS-IS would have created more LSP fragments, such as 01, 02, and so on.
The * denotes the LSPs that were generated by this router, the router that the show command was issued on. Also, since this router is an L1 and L2 router, it contains an L1 and L2 database.
You can also look at a specific LSP and use the detail keyword to show more information. An example of this is shown here:
R2# show isis database R2.00-00 detail ISIS Level-1 LSP R2.00-00 LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R2.00-00 * 0x00000093 0x077E 71 0/0/0 Area Address: 49.0001 NLPID: 0xCC Hostname: R2 IP Address: 172.16.2.2 Metric: 10 IP 172.16.12.0 255.255.255.0 Metric: 0 IP 172.16.2.2 255.255.255.255 Metric: 10 IP 172.16.23.0 255.255.255.252 Metric: 10 IS R2.01 Metric: 10 IS R3.00 ISIS Level-2 LSP R2.00-00 LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL R2.00-00 * 0x0000009A 0x5A69 103 0/0/0 Area Address: 49.0001 NLPID: 0xCC Hostname: R2 IP Address: 172.16.2.2 Metric: 10 IS R2.01 Metric: 10 IS R3.00 Metric: 10 IP 172.16.23.0 255.255.255.252 Metric: 10 IP 172.16.1.1 255.255.255.255 Metric: 10 IP 172.16.3.3 255.255.255.255 Metric: 0 IP 172.16.2.2 255.255.255.255 Metric: 10 IP 172.16.12.0 255.255.255.0
The above output shows that the loopback address of this router is advertised with a value of 0. This is because the loopback is advertised with a passive-interface command under the router IS-IS process, and the loopback interface by itself is not enabled for IS-IS. All other IP prefixes have a value of 10, which is the default cost on the interfaces running IS-IS.
There is currently no verification procedure available for this configuration.
There is currently no specific troubleshooting information available for this configuration.
The Cisco Support Community is a forum for you to ask and answer questions, share suggestions, and collaborate with your peers.
Refer to Cisco Technical Tips Conventions for information on conventions used in this document.