Help me to understand IS-IS attached bit, thanks !

[gongya001]

VMX1(L2/L1) <------->eth0  R1 (L1/L2) eth0/1 <-------> eth0/1 R3 (L1)

R1 is a l1/l2 router. In my understanding, its outbounding L1 LSP should have attached bit turned on.  I captured the packets on its interface e0/1 to get the following.

R1#show int e0/1 | i Hardware
Hardware is AmdP2, address is aabb.cc00.3010 (bia aabb.cc00.3010)

R3#show int e0/1 | i Hardware
Hardware is AmdP2, address is aabb.cc00.1010 (bia aabb.cc00.1010)

I want to understand why the LSP sent from R1 has attached bit off,  but the LSP sent from R3 has attached bit on.

thanks !!

[Giuseppe Larosa]

Hello @gongya001 ,

what you see is correct an L1/L2 node will generate an LSP1 with attached bit set to 1 if and only if it has at least one L2 ISIS adjacency to another ISIS node in a different area.

In ISIS the backbone is a collection of L2 links and associated adjacencies that must be contiguous

in OSPF area 0 is  the backbone.

Hope to help

Giuseppe

[gongya001]

R1 does have connection to vMX1 (L2/L1).  R3 is L1 only router.

I looked the packets captured more closely to find something event more confused. The LSP sent from R1 to R3 has the attached bit off, but the LSP sent from R3 to R1 has the attached bit on.

thanks !!

[gongya001]

I got it. the first packet comes from pseudonode.