ATT-bit Filtering with CLNS Filter-Set Configuration Example

Introduction

This document provides a configuration example to filter the attach-bit (ATT-bit). When you use Intermediate System-to-Intermediate System (IS-IS) as the routing protocol in a network, the Level 1 (L1)/Level 2 (L2) router (R2) sets the ATT-bit on its L1 Link State Packets (LSPs). An L1/L2 router sets the ATT-bit automatically. The purpose of an ATT-bit is to accomplish Inter-Area Routing. When an L1/L2 router is connected to more than one area, it sets the ATT-bit on its L1 LSP. If multiple L1/L2 routers exist, then routers in L1 choose the nearest L1/L2 router.

In some cases it might not be desirable for an L1/L2 router to always set the ATT-bit. For example, in the topology shown in the Network Diagram section, R2 is the L1/L2 router. It forms an L2 adjacency with two different areas - 49.0003 and 49.0004. As shown, there is a connection to an ISP in Area 49.0003 only. You do not want R2 to set the ATT-bit in its L1 LSPs when the connection to area 49.0003 is down. The default behavior is that the R2 continues to set the ATT-bit even when it loses the connection with Area 49.0003. This is because it is still an L1/L2 router and it has peering with more than one area. This document provides a configuration example of how to filter an L1/L2 router (R2) from setting the ATT-bit in it's L1 LSPs.

Note: For communication between 49.0001 and 49.0004, you need to redistribute L2 routes into the L1 domain in the absence of an ATT-bit.

Prerequisites

Requirements

Cisco recommends that you have knowledge of IS-IS. Connectionless Network Service (CLNS) routing must be enabled globally and under required interfaces. You will use CLNS filter-set and hence CLNS routing has to be enabled.

Components Used

This document is not restricted to specific software and hardware versions.

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.

Configure

Network Diagram

A simple topology is shown here. The requirement is that you do not need an L1/L2 (R2) router in order to set the ATT-bit once it's connection to Area 49.0003 goes down.

Requirements

Basic IS-IS is already configured as per the topology. The network requirement is that R2 must no longer set the ATT-bit in its L1 database if it does not know about 49.0003 (the backbone area).

Default Behavior

R2 is the L1/L2 router and has peering with multiple area routers.

R2#show isis neighbors
Tag 1:
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1              L1    Et0/0      10.1.2.1        UP      29       R2.01
R3              L2    Et0/1      10.2.3.3        UP       7       R3.01
R4              L2    Et0/2      10.2.4.4        UP       9       R4.01

In the topology, since R2 is the L1/L2 router it sets the ATT-bit and provides a default route to R1 (Area 49.0001).

This can be seen in R2's L1 database.

R2#show isis database level-1
Tag 1:
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x0000000D      0x99B7        1178             0/0/0
R2.00-00            * 0x00000016      0x3274        1190             1/0/0  <<<<< ATTach
 bit Set.
R2.01-00            * 0x00000008      0xE4BF        1181             0/0/0

If the interface between R2 and R3 is shut, the R2 does not have a connection to the backbone area and hence must not advertise the ATT-bit in its L1 LSP database as per our requirement.

!
R2(config)#int eth 0/1
R2(config-if)#shutdown
!

After the interface towards R3 (Eth0/1) is shut, it no longer peers with R3.

R2#show isis neighbors
Tag 1:
System Id      Type Interface   IP Address      State Holdtime Circuit Id
R1              L1   Et0/0       10.1.2.1         UP    21         R2.01
R4              L2   Et0/2       10.2.4.4         UP     9         R4.01

However, R2 still advertises the ATT-bit and R1 still receives a default route via R2. This is undesirable in this network topology.

R2#show isis database level-1
Tag 1:
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x0000000D      0x99B7         974             0/0/0
R2.00-00            * 0x00000017      0x76D5        1188             1/0/0   <<< ATTach
 bit still set !
R2.01-00            * 0x00000008      0xE4BF         977             0/0/0

R1#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "isis", distance 115, metric 10, candidate default path, type level-1
  Redistributing via isis 1
  Last update from 10.1.2.2 on Ethernet0/0, 00:29:20 ago
  Routing Descriptor Blocks:
  * 10.1.2.2, from 10.2.4.2, 00:29:20 ago, via Ethernet0/0
      Route metric is 10, traffic share count is 1

As is shown in the previous examples, the default behavior is undesirable in regards to network requirements. Bring the interface Eth0/1 on R2 (connection to R3) back UP. Here is when you can use IS-IS ATT-bit filtering with the CLNS feature-set.

CLNS Routing Configuration

In order to configure CLNS routing, complete these steps:

1. Enable CLNS routing globally:

   !
   R1(config)#clns routing
   R2(config)#clns routing
   R3(config)#clns routing
   R4(config)#clns routing
   !

2. Enable CLNS routing on all the IS-IS enabled interfaces.

   R1(config-if)#clns router isis 1   <<< Here, 1 is the IS-IS tag.

CLNS Verification

Once CLNS is configured, check to see if R2 learns about the CLNS route.

R2#show clns route

C  49.0001.0000.0000.2222.00 [1/0], Local IS-IS NET
C  49.0001 [2/0], Local IS-IS Area

i  49.0003 [110/10]
      via R3, Ethernet0/1
i  49.0004 [110/10]
      via R4, Ethernet0/2

ATT-bit Filtering Configuration

In order to configure ATT-bit filtering, complete these steps:

1. Create the CLNS filter set.
   

   !
   clns filter-set ATT-BIT permit 49.0003
   !

2. Create the route-map.

   !
   route-map ATT permit 10
    match clns address ATT-BIT
   !

3. Configure the route-map under the IS-IS process on R2.

   !router isis 1
   
   set-attached-bit route-map ATT
   !

Verify

Use this section to confirm that your configuration works properly.

The Output Interpreter Tool (registered customers only) supports certain show commands. Use the Output Interpreter Tool in order to view an analysis of show command output.

With this configuration in place, the L1/L2 router R2 must NOT set the ATT-bit in the L1 database if the CLNS route to 49.0003 is lost.

When there is connectivity to the backbone, the CLNS route to 49.0002 exists on R2.

R2#show clns route 49.0003
 Routing entry for 49.0003
  Known via "isis 1", distance 110, metric 10, Dynamic Entry
  Routing Descriptor Blocks:
   via R3, Ethernet0/1
     isis 1, route metric is 10, route version is 22

Since the CLNS route exists, R2 must set the ATT-bit:

R2#show isis database level-1
Tag 1:
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00              0x0000000B   0x9DB5        815               0/0/0
R2.00-00            * 0x00000012   0x3A70        954               1/0/0
R2.01-00            * 0x00000007   0xE6BE        950               0/0/0
R4.00-00              0x00000003   0x7201        0 (756)           0/0/0
R4.01-00              0x00000002   0x6D06        0 (676)           0/0/0

Shut down the interface between R2 and R3.

R2#show clns route 49.0002

Routing entry for 49.0002

  Known via "isis 1", distance 110, metric 10, Dynamic Entry

  Routing Descriptor Blocks:

   via R3, Ethernet0/1, (Interface down), (Adjacency down)   <<<<<< Interface goes Down

     isis 1, route metric is 10, route version is 23 (Aging out: 23/24)   <<< The route
 is aging out

After the timeout, the route does not exist in the CLNS routing table.

R2#show clns route 49.0002
R2#

Check the database on R2.

R2#show isis database l1

Tag 1:

IS-IS Level-1 Link State Database:

LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL

R2.00-00            * 0x00000017   0xD6A7           1133              0/0/0  <<<< ATT
 bit not set.

R2.01-00            * 0x0000000E   0x79C9            901              0/0/0

R1.00-00              0x00000010   0xF74D            592              0/0/0

As seen in the database, R2 does not set the ATT-bit even though it still is an L1/L2 router.

R1#show ip route 0.0.0.0
% Network not in table

This is one way by which you can filter the ATT-bit as per the requirements.

Troubleshoot

There is currently no specific troubleshooting information available for this configuration.