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Community Member

GLBP really works well with STP?

Hi all,

I came across some discussions about GLBP with STP. Like Figure 41 at
http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/HA_campus_DG/hacampusdg.html#wp1108611

Cisco's description is wrong there. Even the link between the two distribution switches gets blocked. There must be another port in one of distribution switch gets blocked too to avoid the loop. That means for one same vlan, the access layer switches won't have two uplinks working at the same time. So there is no way you can utilize the load balance feature from GLBP. Yes, you can still load balance by tuning the STP, one Vlan one different uplinks. But for that purpose, HSRP will be more suitable.

Is there anyone who has the way to tune the STP working with GLBP to keep two uplinks work for same vlan? Thank you!

Lou

4 REPLIES
Cisco Employee

GLBP really works well with STP?

Hi Lou,

Can I comment on this . It is enough to block link between distibution if the link between distribution and core is L3. After that you will not have a loop anymore.

If link between distribution and core is L2 and there is a trunk between Core switches - then you will again have a loop posibility - but you can again block link between cores with an STP cost keeping links between Access and distr forwarding.

Nik

Community Member

GLBP really works well with STP?

Thanks Nik. The connection between Distribution and Core will be layer 3 OSPF. But even with the trunk between Distribution switches got blocked by raising the cost on secondary root switch, we will still have a loop for the same vlan like:

Access A --> Distribution B --> Access B --> Distribution A --> Access A

STP will avoid this loop by blocking one uplink from Access A or Access B. That's reasonable for STP. But for GLBP, this STP won't utilize the load balance feature well. We have only one uplink from Access switch (not all access switches. one access switch will still have two uplinks forwarding. but others will have one blocked)

Please advise.

Thank you.

Lou

Cisco Employee

GLBP really works well with STP?

Hi Lou,

This design (blocking the link between distribution switches) works and will keep both uplinks active if you do not span vlans over access switches.

If you have different vlans on access switches and layer 3 link between distribution switches and on distribution switches all vlans you will have the uplinks working.

Best regards,

Alex

Cisco Employee

GLBP really works well with STP?

Hi Lou,

I understand what you mean - did not catch that in the begining. Agree if you are spaning same VLANs acrosss multiple switches GLBP will block one more link between Distr and Access. That is a document bug indeed by the link you mentioned as they show single VLAN there. This one shows the correct topology which will work with GLBP:

http://www.cisco.com/en/US/partner/products/hw/switches/ps708/products_configuration_example09186a00807d2520.shtml#conf

So if GLBP is considered - access switches need to carry different VLANs.

P.S. I created internal DDTS to change documentation.

Nik

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