Layer 2 loops are caused because switches by default flood traffic out all ports, so any time you connect switches in a circular path a loop would form if not for spanning-tree.
In layer 3 this is not the case. Since you have to specifically tell traffic where to route, making a circle does not make a loop. Routes make loops by referring back to a host that thinks its path is back to itself. So adding layer 3 interfaces is not the cause of the loops, incorrect routes are. Take this example R1. int g0/0 ip address 10.10.10.1 255.255.255.0 no shut ! ip route 6.6.6.6 255.255.255.255 10.10.10.2 R2. int g0/0 ip address 10.10.10.2 255.255.255.0 no shut ! ip route 6.6.6.6 255.255.255.255 10.10.10.1 Now try to traceroute to 6.6.6.6. R1 will send it to R2, R2 will send it to R1, R1 will send to R2, repeat until the TTL expires. This is a layer 3 loop. You will generally notice layer 3 loops by traffic not reaching its destination(and proved by traceroute), or by routes being repeatedly installed and removed from the routing table (debug ip routing). Layer 2 loops will generally either keep looping the traffic around so much that you lose access to the device and the whole damn network crashes, or if you are lucky you might not have it so bad and you will see a message in the logs that says something like host x.x.x.x is flapping between interface..... -Marc On Tue, Jul 12, 2011 at 3:37 PM, Alef <[email protected]> wrote: > No no redistribution. I tried to keep it as simple as possible. > Ok let me try my hand at some ascii art. > > But before i do that, is there some inherent stupid thing about putting vlan > interfaces in all the vlans on all the switches ( i have dot1q trunks in x > between all 4 switches). I can imagine in my network it would create a loop. > Just a hunch if i follow the paths. Would i not automatically introduce L3 > loops by doing that. I think ipexpert topology is setup similar ? > > ok scratch that, i uploaded a small jpg > http://tinypic.com/r/fkqfqr/7 > > All switches loopbacks are reachable by at least 3 paths (2 trough the FR > cloud, and 1 other) > > R2# > O 11.11.11.11 [110/66] via 100.100.100.6, 01:01:51, Serial1/1.256 > [110/66] via 100.100.100.5, 01:01:51, Serial1/1.256 > [110/66] via 100.100.24.4, 00:00:43, Serial1/0.24 > 44.0.0.0/32 is subnetted, 1 subnets > O 44.44.44.44 [110/66] via 100.100.100.6, 01:01:51, Serial1/1.256 > [110/66] via 100.100.100.5, 01:01:51, Serial1/1.256 > [110/66] via 100.100.24.4, 00:00:43, Serial1/0.24 > > On a probably unrelated side note, anyone ever got : > %OSPF-4-ERRRCV: Received invalid packet: Bad Checksum from 100.100.14.1, > Vlan14 > > Cisco website says change the port or cable, which i did, and cable, well > considering it's a dynamips sim i did that as best as i could :-) > > Can't get the message to go away though. > > On Jul 12, 2011, at 5:24 PM, Di Bias, Steve wrote: > >> >> What makes you think you have any kind of loop? What are the symptoms? Are >> you redistributing anywhere? >> >> -----Original Message----- >> From: [email protected] >> [mailto:[email protected]] On Behalf Of Alef >> Sent: Tuesday, July 12, 2011 8:27 AM >> To: [email protected] IE >> Subject: [OSL | CCIE_RS] loop in network? >> >> All, >> >> So i have this network, and i am thinking i have a loop. I'm just not sure >> where or how. >> And i think it started happening after i created vlan interfaces in all >> vlans on all switches. So Cat1 is having vlan14,vlan567 and vlan69 >> interfaces and all the other switches in similar. >> >> They are NM16ESW cards, but IEEE stp is running so a Layer2 loop should be >> out of the question. Could i still have a routing loop? >> I am running OSPF in one giant area 0. >> >> How would i go about solving it? I can't use split horizon as OSPF doesn't >> use that. Should i artificially set routes such that they get a lower metric >> and manipulate things like that? >> >> I attached a small picture to clarify (this is taken from the Security lab >> Day4 VoD btw). >> Mind you, it might be something different entirely although i don't think so. >> >> But it's probably going to be something very stupid. And yes, i probably >> don't understand switching very well yet :-) >> >> Alef >> >> >> UHS Confidentiality Notice: This e-mail message, including any attachments, >> is for the sole use of the intended recipient (s) and may contain >> confidential and privileged information. Any unauthorized review, use, >> disclosure or distribution of this information is prohibited. If this was >> sent to you in error, please notify the sender by reply e-mail and destroy >> all copies of the original message. > > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
