Random spanning tree question

So I have an odd issue and as it requires some interesting
theory discussion I thought this would be an appropriate place for it. To
start, this is involving a group of 6509 chassis and 2 MX960s. One of the 6509
chassis thinks it is the root of VLAN 1 despite it having a default priority
and the MX960 having a much better priority. The MX960s are connected with a
lag, and each MX960 has a lag facing a 6509. The 6509 chassis have lags out to
other 6500 chassis, for this discussion their irrelevant though. On every VLAN
the MX960 (01 chassis) sees itself as root. On VLAN 1 the MX960-01 is handling
the IP routing despite not participating in the same BPDUs for that VLAN -
which is just strange to me but I guess layer 2 vs layer 3... The 02 6509
chassis sees its root port facing the MX960-02 and is blocking on the port
facing the 6509-01. Both MX960s see the 01 chassis as root and all Cisco
devices see the 6509-01 chassis as root. Somehow the BPDUs are coming out of
the 6509-01 chassis, crossing both MX960s and arriving unmolested at the
6509-02 chassis in such a way that it sees that path as a better path than the
link directly connecting them.


So I guess the real question is "why" won't the
6500s see the MX960 as root on VLAN 1 and does it have anything to do with the
fact that they're running VTP. Also note I plan on fixing this by moving all
traffic off VLAN 1 and making it a non-routed VLAN because I think the whole
way this was designed is dumb but I found the whole thing of spanning-tree
freaking out on this VLAN fascinating.



Special notes: VTP v2 is running on all Cisco switches
for... reasons I guess. Don't flame me, I didn't design it, just trying to fix
it. The native VLAN is (obviously) not VLAN 1. All switches including the
MX960s, 6500s and some downstream QFXs are running PVST.


  • An MX960 is a service provider level Juniper router. You can think of it has half way between a ASR9010 and a Nexus 7k. It handles switching and spanning-tree closer to the Nexus and has remote line cards (QFXs, similar to Nexus FEX) but can do service provider routing, high level MPLS and they work well as boarder routers containing the entire internet routing table... Interestingly a lot of tier 1 providers use them in their MPLS core too - more common than Cisco for Service providers I think.

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