GLBP and STP ? 2

[rpast]

I’m new to some of the advanced features of MLS, and would like to ask what is undoubtedly a dumb question, but I can’t seem to get a grip on the concept. The question is, does STP have to be disabled in order to use GLBP? I want to add a second Distribution layer switch (3750)to our small network and enable GLBP on it and another 3750, for redundancy and load balancing. Since I will have layer 2 trunks extending from each access layer switch (2950) to each of the 3750s (one of which is the Root Bridge), PVST will put some of those ports in Blocking state for a given VLAN. And if that is the case, how will GLBP do it’s load balancing? Again, I apologize if this is an excessively lame question. If someone just wants to steer me to a good book that explains things better than what I’ve got, I’d gratefully accept that as an alternative. Thank you in advance.

 

[rpast]

OK, GLBP is not available on the 3750. Can I ask the same question about a pair of HSRP groups?

 

[Jsteve]

Do not disable STP!

If I understand you correctly, your topology is:

3750-------3750
|| ||
|| ||
|| ||
2950 2950

i would suggets to configure port-channel for the four uplinks, with trunk mode set to desiriable, and the same Natve Vlan configured for each link.

than set up your vlans on the 3750 to support HSRP. Keep in mind that this is just based on the info you have given so far. depending on other factors like the location of the devices. will the servers be dualholmed to both 2950's, i would likely suggest someting diferent.

 

[rpast]

Thank you for the response. Actually, each 2950 would link to both 3750s. Port-channel sounds like a good option, but I was considering redundancy as a priority over performance (port-channel). As for the servers, they are currently using their own proprietary teaming mechanisms on the NICs - that could change.

I must confess that my interest at this point is as much theoretical as practical. With STP assigning DBs for different segments (the 2950s linking to both 3750s), the other side of the link to the non-root-bridge 3750 will have at least one Blocking port. How does a Layer3 protocol like HSRP or GLBP overcome this with STP on?

Many thanks for your contributions.

 

[lambent]

Your VLAN spans across the 3750 also, so the HSRP/GLBP hello packets will traverse across the 3750-3750 link within that particular VLAN.

Same for packets from hosts to active/standby HSRP routers or active/active GLBP forwarders. That 3750-3750 link may be used to carry these traffic and thus we usually use port-channel to group a couple of ports to provide more apparent bandwidth.

 

[Jsteve]

lets say the first 3750 is the root, both 2950s will traverse the links while the other two links to the 2nd 3750 will be blocked(STP). Now if your first 3750 goes down, the block links to the second 3750 will be restored and the HSPR will take over on the 2nd 3750. The atvantage of this is that your DHCP only needs to be setup to give out the VIP for the gateway address.

The only way to achieve loadbalancing would be to set up diff priorities for your VLANs so that they distribute evenly accross both 3750, utilizing all of the four uplinks, For example you would setup the 1st 3750 to be the root bridge for Vlan1 (with HSRP active on this 3750 for Vlan1), and on the 2nd 3750 set up Vlan2 to be the rootbridge on that 3750(with HSRP active on this 3750 for Vlan2). You must know your VlAn utilization to set this up properly.

Again, do not disable STO. You will only casue your network to be unstable.

 

[rpast]

Thank you both. So if I understand you correctly, the textbook explanation of setting up two HSRP groups for one vlan doesn't cut it for load balancing, unless all the vlan ports are on one access layer switch. Otherwise, STP will block some ports -- same with GLBP. I.e. you need to get more creative when you have a vlan spanning multiple AL switches, if you want to keep STP.

 

[Jsteve]

that's correct.