default route in isis

[weifeng.l]

Command "default-information originate" in isis enable L1L2 IS generates a 0.0.0.0/0 prefix in its Level-2 lsp.But what will happen if the L1L2 IS is unattached from backbone?

dut1-----L2(LAN1)--------dut2

| |

|______L1(LAN2)_________|

For example,dut2 has been set NET1 and NET2,dut1 is only set NET1,both dut1 and dut2 is configed as level-1-2 IS.dut1 and dut2 has L2 adj on lan1 and has L1 adj on lan2.dut2 has been config with "default-information originate".

dut1 will set ATT bit in his level-1 lsp,and will install a level-2 default route point to dut2.dut2 can NOT reach other area via level-2 topology,so dut2 think that it's unattached from backbone,so a level-1 default route is installed on dut2,the next-hop is dut1.Thus a route loop is created.

Is there some way to avoid this route loop?

[ruwhite]

The only way I could think of to prevent this is to attach a route map to the default-information originate on dut2, so it only advertises a default if it can see some route it should be learning from the l2 area. Most networks I've seen use a route map to condition the advertisement of the 0/0, or redistribute it at their dmz border in some way.

:-)

Russ.W