Smooth Dual core migration to Distributed trunking solution

Another day in network engineer life.
I was working for a hospital and have to propose some enhancements to their network. They already had a pair of HP 5306 with a STP looped topology for redundancy between access switchs.

Physical topology before making changes

Decision was made to convert this topology to a Distributed trunking one. Configuration is pretty simple, but I had a lot of trouble with some old versions 5406 sofware. The new topology is something like that :

Distributed trunking topology

All links are now in forwarding state, each downstream DTP switch sees the grouped link as a classical link aggregation. The cores are seen as a single device for them.
One link had to be added (peer link) between core to ensure this thing is working properly.

Question was : How to change the topology without disturbing too much end users in this hospital ?

I am not going to explain the configuration, because this is straightforward.

Core configuration :
– Change the actual link aggregation between cores, to ISC (interswitch connect) link -> no impact
– Add the peer link (Vlan + IP conf) between the core -> no impact

Results in logfile :
I 12/03/19 17:52:57 03305 DT: ISC port Trk1 is deleted from keepalive VLAN 999
I 12/03/19 17:52:57 03242 DT: Distributed-trunking keep-alive receive socket
bind is successful. UDP port 1024 is used.
I 12/03/19 17:48:58 00076 ports: port E8 is now on-line
I 12/03/19 17:48:55 00842 stp: CIST starved for a BPDU Rx on port E8 from
I 12/03/19 17:48:00 05201 DT: Switch-interconnect link is In Sync.

Adding access Switchs to DT topology :
Slightly more cumbersome because you can easily create loop during these steps. I recommand starting with a switch close to Core, so you can plug your console cable quickly !
1) Disable the port toward Sw1 on Core 1, traffic is passing through Core 2
2) Configure this port as dt-lacp or dt-trunk
3) Configure a trunk (lacp or static) on Sw1 with only the port toward Core1
4) Disable the port toward Sw1 on Core 2, traffic is no more passing through
5) Enable the port toward Sw1 on Core 1, if the config is correct, traffic should pass through Core 1 on DT-trunk. Verify :

LTP2-CR1# sho lacp
LACP Trunk Port LACP Admin Oper
Port Enabled Group Status Partner Status Key Key
----- ------- ------- ------- ------- ------- ------ ------
A21 Active Trk21 Up Yes Success 0 310
A22 Active Trk20 Up Yes Success 0 309
A23 Active Trk26 Up Yes Success 0 315
A24 Active Trk23 Up Yes Success 0 312

6) Finish the configuration on Core 2, configure the port en Sw1, enable the port on Core 2 and you are done. Do some checks:

LTP1-CR1# sho lacp distributed
Distributed LACP
Local Port Status:
LACP Trunk Port LACP Admin Oper
Port Enabled Group Status Partner Status Key Key
A21 Active Trk32 Up Yes Success 0 321
Remote Port Status:
LACP Trunk Port LACP Oper
Port Enabled Group Status Partner Status Key
A21 Active Trk21 Up Yes Success 310

Sources :S

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Author: Kévin SAS

Hello, I'm Kévin SAS, an experienced Network and Security Engineer based in France. Over the years, I have had the privilege of working with a diverse range of clients, providing expert solutions in areas such as Wireless, LAN, Datacenter, and VoIP. I have hands-on experience with leading brands like Cisco, HPE, Aruba, Palo Alto, and many others. Having previously worked for a reputable service provider, I now find myself in a dynamic role as part of an integrator team, where I continuously expand my knowledge and skills. This blog serves as a repository of technical notes and insights that I am enthusiastic about sharing with fellow professionals and enthusiasts.

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