Spanning-tree root switch determination

STP, RSTP and MSTP are well known solutions for both loop detection and redundancy. Spanning-tree will determine the root switch and from that point it will calculate the link cost to the device. After the determination of the shortest path it will disable the more costly paths.

But this won’t always result in the best network topology. So how is this root switch appointed? Root switches are chosen because of two things their priority settings and after that who has the lowest MAC-address. This is why if you just let it determine the paths it won’t always be the best topology.

HP ProCurve (and maybe also other) switches are configured with the default priority 8. This way when you enable STP on them they just look at the lowest MAC-address. To optimize the topology you might want to set the priority of your main switch lower as the default priority (8). This way your switch is chosen as root switch and all paths are calculated from there.

The way to change the priority on HP ProCurve switches from CLI goes as follows:

[hostname]#config
[hostname](config)#spanning-tree priority [n]
[hostname](config)#write mem

[n] can be anything from 0 to 15. 0 being the highest priority and 15 being the lowest.

Beacon probing resulting in excessive broadcasts

Not so long ago I posted a link to a VMWare blog in which beacon probing was demystified. This article stated that you only should use beacon probing when there is no link state tracking on the physical switches and you could consider beacon probing as a nice software solution for replacing it.

Well we’ve got our ESX environment set up by a local supplier and they advised us to use beacon probing instead of link state tracking. But for some reason beginning from that moment I got major events from my ProCurve switches stating excessive broadcasts. This wasn’t often, but especially during peak hours I was getting this notification.

When I started sniffing the network packets on the uplink of the switches I noticed what kind of packets it were. It was an almost continuous flow of RARP packets coming from the ESX servers. RARP packets are meant as MAC address table updates for switches. This way when a node is suddenly available on a different MAC address the switch already knows the new path. This is also what happens when a virtual switch detects a link is not functional. It will switch the uplink and notify the switches. When beacon probing isn’t working as expected ESX constantly thinks the uplink isn’t functional so it is constantly switching the uplink and as a result constantly sending out RARP packets.

While you can define both the notify switches parameter and the network failure detection, it isn’t good to just put notify switches to ‘No’. This way the failover is still constantly changing uplinks and this can result in errors (in my case timeouts with TFTP). The real problem was the failure detection. Beacon probing for some reason just didn’t work in our environment. When I changed the failure detection to ‘Link Status only’ all the RARP packets disappeared and my excessive broadcasts were gone.

In my case just link status only is sufficient, but I can imagine there are cases where you would want to use beacon probing. If you enable beacon probing and this results in excessive broadcasts (or just more broadcasts) I do advise to look if you could find those RARP packets. This can indicate that beacon probing is just not working correctly in your environment.

I want to thank Scott Lowe for giving me a push in the right direction.

Switch meshing ProCurve switches unless…

… you are using those switches for routing. Great so the only option for improving my networking environment for load balancing my ESX environment was crushed by these two HP documents: Switch Meshing and LAN Aggregation Through Switch Meshing.

Apparently there is a whole list of requirements for switch meshing. This list is most clearly defined in Switch Meshing (starting from page to 5 and ending at page 7). Just a short beginning of the list:

• A meshed switch can have some ports in the meshed domain and other ports outside the meshed domain. That is, ports within the meshed domain must be configured for meshing, while ports outside the meshed domain must not be configured for meshing.
• Meshed links must be point-to-point switch links.
• On any switch, all meshed ports belong to the same mesh domain.
• A switch can have up to 24 meshed ports.
• A mesh domain can include up to 12 switches.
• Up to five inter-switch, meshed hops are allowed in the path connecting two nodes through a switch mesh domain. A path of six or more meshed hops between two nodes is unusable. However, in most mesh topologies, there would normally be a shorter path available, and paths of five hops or fewer through the same mesh will continue to operate.
• ….

I have four HP ProCurve 5300 series switches and they all do routing. Switch meshing was my only option left after exploring NIC teaming articles from Scott Lowe and Lukas Kubin. Those articles describe perfectly how you could do NIC teaming and VLAN trunking on one ProCurve switch, but not how to do it with two of them. After some searching the only option to accomplish what I wanted was a HP patented technique called switch meshing.

So this is not an article explaining how to configure switch meshing for use with ESX, but to warn people for all the requirements. If your environment complies with all the requirements made for switch meshing the two documents named above will do sufficiently as far as it goes how to configure and what switch meshing exactly is.

If I ever end up in an environment where I am able to configure switch meshing I will certainly post how I did it. During my search for what switch meshing was I found out about another technology called XRRP. With that technology at least I can make a really good failover for my gateways, but later more on that.