Joseph W. Doherty Thu, 04/03/2008 - 17:02
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One issue of seen is some brand "X" switches couldn't find their defined gateway after converting to GLBP, although connected hosts appeared to work correctly.


PS:

Remember GLBP uses different virtual MACs then HSRP, so when you make the change, some hosts might be unable to find the gateway until they update their ARP cache.

lamav Thu, 04/03/2008 - 19:17
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Nasheer, remember that GLBP uses asymmetric routing, so if you are doing any sort of server load balancing (IOS-SLB) or are using a CSM, dont use GLBP. You pretty much must use HSRP.


Thanks


Victor

Joseph W. Doherty Fri, 04/04/2008 - 05:34
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". . . remember that GLBP uses asymmetric routing . . ."


Strictly speaking, HSRP, alone, doesn't guarantee symmetrical routing, but Victor is correct that you can not guarantee exactly the same end-to-end path using GLBP as you normally would. Often asymmetric routing isn't a problem, but as Victor notes, it can be. Stateful NBAR classification on the GLBP router would be another example.

lamav Fri, 04/04/2008 - 06:43
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Joseph:


HSRP guarantees symmetrical routing if you make the same switch the HSRP primary for all vlans or utilize some other form of route tuning. That would be the added caveat if you're implementing IOS-SLB.


But I wasn't trying to give every nuance and caveat to the poster regarding the detailed inner workings of HSRP; I was just giving him something else to consider if he is doing server load balancing on his network and is thinking of migrating to a GLBP environment from HSRP.


Given the inherent leveraging of asymmetric routing in GLBP, it would be counter prodctive to use it with server load balancing.

Joseph W. Doherty Fri, 04/04/2008 - 08:33
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"HSRP guarantees symmetrical routing if you make the same switch the HSRP primary for all vlans or utilize some other form of route tuning."


Victor,


I take no exception to your posts beyond believing both might mislead about HSRP, alone, guaranteeing symmetrical routing. HSRP provides a virtual gateway, but assuming we have more than one physical gateway, it doesn't control returning traffic exiting via the same gateway (unless you have in mind routing to HSRP addresses). I consider symmetric routing, routing where packets flow forward and reverse down the same path.

lamav Fri, 04/04/2008 - 09:37
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Joseph:


I take no exception to your posts either. They can be very informative and helpful. But sometimes, as in this case, they can get unnecessarily tedious, obsessive and hair-splitting.


Once again, the poster is ALREADY using HSRP, and IF he is also doing some SLB (that part we dont know), then two things are true:


1.) He doesnt need me or anyone else to elaborate on how to deploy HSRP in an SLP environment, BECAUSE IT ALREADY EXISTS and obviously must be working. That is why I didnt discuss the design requirements for HSRP with SLB.


2.) He shouldn't migrate to GLBP because it inherently incorporates asymmetric routing, period, which will cause problems with SLB. In fact, Cisco specifically recommends NOT using GLBP with IOS-SLB or any sort of server load balancing. Moreover, when explaining how to configure IOS-SLB, the Cisco website specifically explains that one of the tasks that must be performed is to configure HSRP BECAUSE HSRP DOES NOT USE ASYTMMETRIC ROUTING BY DESIGN, AS GLBP DOES.


So, as far as your statement is concerned that "HSRP provides a virtual gateway, but assuming we have more than one physical gateway, it doesn't control returning traffic exiting via the same gateway," you are the one who is being inaccurate and misleading because you CAN control which physical gateway does the forwarding by making the same switch the HSRP primary for all vlans and "doing some other form of route tuning," (from my first post) if need be.


Please, move on, my friend. As I suggested to you once before: take a dep breath and enjoy life. :-)

lamav Fri, 04/04/2008 - 09:51
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Nasheer:


Sorry for the "back and forth" nonsense.


The bottom line:


GLBP has its advanatages, first and foremost is that it does network load balancing, as you know already.


So, the quick point I wanted to make to you is that IF you are doing some sort of server load balancing, follow the Cisco recommendations and guidelines and stick with the HSRP you are already utilizing because GLBP can break your slb environment.


From Cisco's website:


"Server Farm Design


GLBP can be used to share outbound network traffic from simple server farms. GLBP is best used in environments that do not require additional server load balancing features and function such as that provided by Cisco IOS Server Load Balancing (IOS-SLB), Cisco Content Switching


Module (CSM), and the Cisco Content Switching Services (CSS) family of products.


Thanks


Please feel free to rate the post if I have helped you.


Victor

Joseph W. Doherty Fri, 04/04/2008 - 13:05
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"I take no exception to your posts either."


You might have misunderstood my exception was not to any of your posts in general, but one point contained within your posts within this thread which I thought needed clarification; which seemed to me the implication that HSRP (alone) guarantees no asymmetrical routing.


"But sometimes, as in this case, they can get unnecessarily tedious, obsessive and hair-splitting."


Perhaps, but I'm sure you don't want anyone, including those reading these posts to be mislead. Sometimes hair splitting is crucial, for instance you now have ". . . HSRP primary for all vlans and "doing some other form of route tuning," (from my first post) if need be.", but your second post had "or", not "and". "And" I agree with, but not "or".


You believe my statement about return traffic is inaccurate. I'm always willing to learn. Look at the attachment. If Rtr1 is the HSRP gateway on the common Ethernet segment, how do you configure HSRP (alone) to keep return traffic from Rtr2?

Attachment: 
lamav Fri, 04/04/2008 - 16:17
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Joseph:


This is getting boring. Log off and go have a beer dude "and/or" a drink. LOL


Spending 15 hours a day on this thing can drive you batty. Take a breather. Have some fun.


On that note, I will be meeting my chica chula and trust me, the last thing on my mind will be you and GLBP. :-)


Jesus loves you. :-)

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