Strange behaviors with ATT setting in IS-IS

jiafan.wang · ‎03-18-2010

Products: Cisco IOS Software
IOS Type: c7200-adventerprisek9-mz.122-33.SRC1.bin
Version 12.2(33)SRC1, Compiled Tru 22-May -08

We are doing some research about IS-IS with the network as below:
R0 ------ R1
R0 is in area 20, 40
R1 is in area 10, 30, 20
Both the R0 and R1 are Level-1-2 IS-IS routers.

It should be an easily understand network, however we notice strange behaviors with the ATT bit setting on the Routers' Level-1 LSP. That is:

(1) From the very beginning, we start the R0 and R1 at the same time and see R0's Level-1 LSP with ATT bit setted.

Also then a default route 0.0.0.0/0 is provided by IS-IS Level-1 on R1.

R1>show isis d
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num LSP Checksum LSP Holdtime      ATT/P/OL
R0.00-00              0x00000004   0x6871        1073            1/0/0
R1.00-00            * 0x00000003   0xA9CB       1120             0/0/0
R1.01-00            * 0x00000001   0x8C96        346               0/0/0

(2) After 4 times' SPF computation or you can say, after 4 times' log information printed by R0 as follow,

the ATT bit set on R0's Level-1 LSP has been cleared and also disappeared the default route 0.0.0.0/0 provided by IS-IS Level-1 on R1.

*Mar 18 16:25:19.895: %CLNS-4-AREALOST: ISIS: Too many IS-IS areas--can't route to area 40

R1>show isis d
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num LSP Checksum LSP Holdtime      ATT/P/OL
R0.00-00              0x00000005   0x5E82        1164              0/0/0
R1.00-00            * 0x00000003   0xA9CB        1042             0/0/0
R1.01-00            * 0x00000002   0x8A97        1150              0/0/0

It's hard to figure out whether there is anything wrong with the network configuration or the software itself has any special work besides the RFC1142's statement?

Any body ideas? Thanks to reply. The detailed configuration is pasted

Giuseppe Larosa · ‎03-18-2010

Hello Jiafan,

the use of multiple nets under router isis is meant for migration purposes only and it is not representative of a working network.

the two systems build a L1 adjacency on the net in common they share.

They should also build a L2 adjacency on the same link.

so sh clns neigh should show L1L2 as adjacency type on the Fas interface

but there are no other active interfaces on the routers taking part in ISIS

I would try to add at least one loopback interface with ip router isis on each router to see if there is any change

Edit:

in other words, to which L2 backbone could R0 claims to be attached (ATT bit) if there is no other interface with ISIS enabled ?

>> A Level-2 IS that indicates that it has one or more Level-2 neighbors in other areas may be used by Level-1 routers in the same area as the path of last resort, also called the default route. The Level-2 IS indicates its attachment to other areas by setting an attached bit (ATT) in its Level-1 LSP 0.

http://www.cisco.com/en/US/docs/ios/iproute_isis/configuration/guide/irs_ovrvw_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1054973

Hope to help

Giuseppe