OSPF loop with 6850 and 6860E Layer3 switches

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Idrissdiop
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Joined: 27 Feb 2016 20:15

OSPF loop with 6850 and 6860E Layer3 switches

Post by Idrissdiop »

Hello.
I have an OSPF loop made by 8 backbones layer 3 switches connected via Fiber Link.
but some of them have equal cost paths to the same network.
It was previously working correctly when the loop was only made of 6850 (AOS R 6.X) switches.
When i replaced one of them by a 6860E(AOS R 8), I got issues when trying from the new switch(6860E) to reach those equal cost path networks.
Pings toggle to UP/DOWN 50% of the time.
When I break the loop (by removing one of connections from any of core switches) => (only one path to each subnet...) it works fine.
When the loop is rebuilt the issue comes back.
Do I have to enable load balancing or something other on the new 6860E to support the loop?
Regards
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silvio
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Re: OSPF loop with 6850 and 6860E Layer3 switches

Post by silvio »

I assume all in switches in the same backbone area, the speed at the links are the same and between the switches are transfer networks/vlans. So ECMP is normal behavior from 6860 to the target network (both directions 3 hops). In my knowledge the decision at the 6860 for the direction (left or right) is made at the same way like within a LAG: using f.e. source/destination ip-address and tcp/udp port. So all packet for the same destination-ip should use the same way. For the other way (back) the 6850 makes its own decision (maybe the other way).
Please check that all your switches use the same spanning tree protocoll and mode f.e. all RSTP with 1x1 (R6) = perVlan (R8). Maybe a stp mismatch is the reason for this strange behavior (if f.e your 6860 is in flat mode).
best regards
Silvio
Idrissdiop
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Posts: 1
Joined: 27 Feb 2016 20:15

Re: OSPF loop with 6850 and 6860E Layer3 switches

Post by Idrissdiop »

Hi Silvio,
Thanks for your answer, you solved my problem
there were stp mismatches as you said.
-6850 switches are running with RSTP in "1x1" mode and MVRP disabled
-while 6860 are running with RSTP in "Flat" mode with MVRP enabled
On 6860, I changed the RSTP mode from "Flat" to "Per-VLAN" and I disabled MVRP. it is now working perfectly.
Thank you
Best regards
Last edited by Idrissdiop on 04 May 2021 05:15, edited 1 time in total.
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