Hi folks, I'm studying for my CCNA and have came accross something which I can't understand, it seems I have a routing loop and I thought you can't get routing loops with OSPF. I'm using a Boson simulator, here's my network... Router1 \ Switch1 / \ Router2 Router3 \ / Switch2 / Router4 so here Router 1, Router2 and Router3 connects to switch1, Router2,Router3 and Router4 connect to switch2. I was thinking of using the 2 routers in the middle of the switches for redundancy. PROBLEM: Ping from Router1 cannot see Router4. All interfaces are up. If I shutdown the interfaces on Router3 then it works. This is me just studying and I'm curious to what commands are available to stop this from happening. What can I do? thanks Dave
1. do a trace, to ident where the loop is. 2. connect to this router and verify the OSPF settings and statics etc. could be that some router are not right in OSPF, hence the packets hits the 0.0.0.0 route instead. HTH Martin
Hi Martin, done a trace and it displays something along the lines of... 1. 192.168.1.2 2. 192.168.2.3 3. 192.168.1.2 4. 192.168.2.3 * * * there was more lines after 4 but displayed the same thing. The only routes inside the router are the OSPF learned routes and the directly connected routes. I think this is an issue with the Boson simulator, I've came accross other strange things re OSPF. thanks Dave
ok, so this IP routes back to the previous one. eigther #1 forwards to wrong hext hop, or #2 does - This is what i means. There are several commands that you need to get familiar with inorder to determine the choise of route in the RTM. first is "show ip route" This will show you the complete route table. Use this to find the match for your destination packet. Then you have if its from static or OSPF, and from which neigbour its been seen/learned. Drill down this road and you should find the answer. If everythin looks fine, then the posibility of simulator software errors/"Works as designed"-features exsits. HTH Martin
hi Martin, thanks for your thoughts. It does seem as though the software is at fault, 1 router only shows 2 other routers in it's routing table, 1 router shows 1 router and the another router shows all routes in it's routing table. I'll contact Boson and see what they say about it. Does this mean that a routing loop shouldn't have occurred here? thanks again Dave
Hi, It is kinda hard to say, without any config's posted. if you could post the running config for all routers, aswell as the output of sho ip route for all router. Regards Martin
its miight be alil advanced and im not sure if the sim will do it but configure one of the links from roter 2 or 3 to router 1 or 4 as a floating static, meaning set the prioiry of ospf link higher than that of the primary and it will only use the one link but if it goes down the network will re converge with the other link automatically
