I would like to expand a little bit more my previous message. As I mentioned before, the potential problems created by the introduction of Redundancy, are handled in different ways at the different Network Layers. At the Layer 3 level, Redundancy is handled very differently than STP. Routing Protocols simply select the Best Path and install it on the Routing Table. Thus, all the other paths remain in something like a "stand by mode" until the main route is no longer available. At this Layer you can also have a sort of "equivalent of Etherchannels", because you can configure "Maximum-Paths" and thus allow multiple paths from the same source to the same destination. Now, with regards to HSRP, GLBP and VRRP, they are NOT really Layer 3! IGP Routing Protocols are true Layer 3. HSRP, GLBP and VRRP are in a way "Applications" that sit on top of the Routing Protocols. They should be actually considered Layer 4 or Application Layer Protocols and thus, they can't be compared to Routing Protocols. They perform a totally different function and as mentioned before, we should not directly compare Protocols that operate at different Layers. Examine BGP for instance. That's not a Layer 3 Protocol either! (not an IGP)BGP also builds on top of the IGP Layer 3 Protocols and thus it is also an "Application" or Layer 4 Protocol. Of course, BGP and all the Redundancy protocols are totally different thingswhich indicates that even Protocols at the same Network Layer can't be compared directly either, unless they perform a similar function. Take care.
--- On Wed, 12/22/10, Carlos Valero <[email protected]> wrote: From: Carlos Valero <[email protected]> Subject: Re: [OSL | CCIE_RS] To: "khaled Saholy" <[email protected]>, "Uli" <[email protected]> Cc: [email protected] Date: Wednesday, December 22, 2010, 10:42 AM Hi, That's right. Layer 2 vs Layer 3 makes all the difference. In general, you ALWAYS need to make sure that you only compare Protocols in the same Layer. Loop prevention and Redundancy are two different things. In order to have Redundancy you need to have additional/alternative paths, correct? That's true regardless the Network Layer we are talking about. Now, this Redundancy could create problems. The solution to these potential problems is handled in different ways at the different Network Layers. At the Layer 2, the potential problem is the creation of a Layer 2 Loop! The solution to this potential problem is handled by "logically disabling" the alternative paths and leaving ONLY a SINGLE path. As you know, these disabled paths remain in "stand-by mode" and become active only if the active path goes down. This is what we call "Loop prevention" But this is NOT the only way! For instance, if the additional/alternative paths happen to be "Parallel links" between the same two Switches, then you can "bundle them together" thus creating an Etherchannel! This Etherchannel would be seen as a single Link, but if one of the Links in the bundle goes down, the others will take the load. Of course Etherchannels are not an "alternative solution" or a replacement to STP.You will actually have both in place; Etherchannel and STP at the same time, since STP will deal with the alternative paths that are not necessarily parallel links. Perhaps you know all this, but I still wanted to add my 2 cents. Merry Christmas! --- On Wed, 12/22/10, Uli <[email protected]> wrote: From: Uli <[email protected]> Subject: Re: [OSL | CCIE_RS] loop prevention and redundancy To: "khaled Saholy" <[email protected]> Cc: [email protected] Date: Wednesday, December 22, 2010, 8:57 AM Hi guys, I guess I missed those thing about layer 2 and 3. Thanks 2010/12/22 khaled Saholy <[email protected]> > > Uli, > > Loops in the switching LANs can happen when you add a second link between > two switches. So STP ,in short, works by keeping one path from the root > bridge (one of the elected switches in LAN) to the last switch in the > network and the redundant links will be blocked. You can say , STP is an > opponent to redundancy. > > You have to deferentiate , as Jason said, between layer 2 and layer 3 > networks. > > I hope it clarified part of the picture. > > Regards. > > Khaled Al-Saholy > > > > Date: Wed, 22 Dec 2010 04:49:25 -0800 > > From: [email protected] > > To: [email protected]; [email protected]; > [email protected] > > > Subject: Re: [OSL | CCIE_RS] loop prevention and redundancy > > > > Here is a link that may help > > > > > http://www.cisco.com/en/US/docs/switches/datacenter/sw/5_x/nx-os/high_availability/configuration/guide/ha_network.html#wp1089613 > > > > > > Jason Maynard > > > > > > > > > > ________________________________ > > From: Jason Maynard <[email protected]> > > To: Uli <[email protected]>; "<[email protected]>" > > <[email protected]> > > Sent: Wed, December 22, 2010 7:35:05 AM > > Subject: Re: [OSL | CCIE_RS] loop prevention and redundancy > > > > HSRP, GLBP and VRRP is for Layer 3 redundancy > > > > Here is my question to you > > > > What does STP accomplish (loop prevention), so...... why would loops > occur? > > > > What does STP allow you to do? > > Jason Maynard > > > > > > > > > > ________________________________ > > From: Uli <[email protected]> > > To: "<[email protected]>" <[email protected]> > > Sent: Wed, December 22, 2010 7:17:06 AM > > Subject: [OSL | CCIE_RS] loop prevention and redundancy > > > > Hi Expert, > > > > Since most of my time study cisco only in lab environment (in my > > university)and now I get into real world situation, I found something > that > > bother me a lot. In my understanding STP is for loop prevention and not > for > > redundancy, right ? but why my colleague and people I met they said it's > for > > redundancy (those guys are already in this business for at least 3 years > > experience).Thus, I always argue with them..hmm am I missed something > here ? > > because for redundancy we can use HSRP, GLBP and VRRP. > > > > > > Thanks > > _______________________________________________ > > For more information regarding industry leading CCIE Lab training, please > visit > > www.ipexpert.com > > _______________________________________________ > > For more information regarding industry leading CCIE Lab training, please > visit > > www.ipexpert.com > > _______________________________________________ > > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
