There is no form of 'loop avoidance' with OSPF like the distance/patch vector protocols (EIGRP/BGP).
OSPF's algorithm (and all link state routing protocols) calculates all the links of all the routers in the 'Area' similar to a 'tree' fashion with each router being the 'root' of it's calculated tree.
Doing this means that all routers in the same 'Area' have a complete topology of that 'Area' and then use this information when routing traffic, traffic will then traverse the 'Area'/network without creating loops and will garentee the best (loop-free) route.
Take a look at the following link for more detail how the SPF algorithm works and how it populates the routing table with the best 'loop-free' routes:
Like Richard and Liam said OSPF is a link state protocol and exchanges LSAs within an area to build a SPF tree ensuring that there are no loops within the area.
Between areas Area Border Routers (ABRs) send summary LSAs hiding the internal topology of the area. This is actually distance vector behavior. Due to this there needs to be some loop prevention mechanism.
ABR must have interface in area 0 and it must not be in DOWN state. Only ABR may generate summary LSA.
ABRs expect summary LSAs over area 0 only. Summary LSAs may not be sent over other areas.
ABR will accept and use summary LSA over non area 0 if if it does not have full adjacency in area 0. Doing so is safe because it can't flood them back to area 0 to create a routing loop.
Read this excellent paper by Petr Lapukhov for the full explanation:
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.