Quick question

hurricane1091

I'm not sure why I was thinking about this, but anyway here it goes. OSPF does not have split horizon. EIGRP does. Why is this the case exactly? I can only think that EIGRP hello/dead intervals can mismatch, where as OSPF cannot...but I might be going down the wrong path here. Thanks in advance, I should probably know this answer and am over thinking it.

EdTheLad

OSPF is a link state protocol, it creates a link state update and sends it to its neighbors. This update will describe all the local interfaces ip prefixes with associated costs. Why would split-horizon be required if a router is only advertising it's own networks? All ospf routers within an area will create a type 1 lsa which gets flooded unaltered throughout the area.
The only routing by rumor type behavior in ospf is at the ABR, to avoid implementing loop control mechanisms like split horizon between area's, ospf is designed with a backbone area, area's can only communicate via the backbone, which makes the area connectivity hub and spoke like.

Eigrp is distance vector which means it advertises it complete routing table to neighbors, split-horizon basically means, if i learned a route from you i wont advertise the route back to you i.e. i wont create a loop. Hello/Dead timers are used for neighbor communications.

hurricane1091

EdTheLad wrote: »

OSPF is a link state protocol, it creates a link state update and sends it to its neighbors. This update will describe all the local interfaces ip prefixes with associated costs. Why would split-horizon be required if a router is only advertising it's own networks? All ospf routers within an area will create a type 1 lsa which gets flooded unaltered throughout the area.
The only routing by rumor type behavior in ospf is at the ABR, to avoid implementing loop control mechanisms like split horizon between area's, ospf is designed with a backbone area, area's can only communicate via the backbone, which makes the area connectivity hub and spoke like.

Eigrp is distance vector which means it advertises it complete routing table to neighbors, split-horizon basically means, if i learned a route from you i wont advertise the route back to you i.e. i wont create a loop. Hello/Dead timers are used for neighbor communications.

Thanks for the quick reply. I've really forgotten about the fundamentals of OSPF it feels like. I haven't touched it in awhile, and I need to revisit it.

OfWolfAndMan

Like Ed said, it's really for the purpose of supporting the forwarding intelligence that EIGRP has based on it's "routing by rumor" behavior and preventing routing loops.

Imagine three connected routers, point to point, R1, R2, R3, left to right. R1 has attached network 10.1.1.0/24 and sends advertisement to R2. Assuming SH is not enable on anything in this topology, R2 will advertise the same route to R1, and R3. R3 advertises the route back to R2. If the link between R1 and R2 goes down, R2 will query 3, 3 says it has a route to that network, and R2 sends its packets that way. R3 then sends the packet back to R2 since he is receiving the route from R2, and then BOOM! routing loop.

hurricane1091

I get too deep into thought about these things lol. Thanks for the reply guys. I understood the EIGRP part but OSPF was confusing me. If R1 was a higher OSPF priority than R2, and R3 had a higher OSPR priority than R2, R2 would then have 2 DRs. Something about that was confusing me. I drew some diagrams out on my desk that I think helped.

gorebrush

There's really nothing else to add here - replies above have hit the nail on the head.

networker050184

hurricane1091 wrote: »

I get too deep into thought about these things lol. Thanks for the reply guys. I understood the EIGRP part but OSPF was confusing me. If R1 was a higher OSPF priority than R2, and R3 had a higher OSPR priority than R2, R2 would then have 2 DRs. Something about that was confusing me. I drew some diagrams out on my desk that I think helped.

I think you are confused on DRs and their roles. There won't be two (there can be a BDR) and regardless it has nothing to do with forwarding and split horizon.

hurricane1091

172.168.1.1 100 FULL/DR 00:00:32 172.168.1.1 GigabitEthernet0/0

192.168.1.1 1 FULL/DR 00:00:32 172.168.2.2 GigabitEthernet0/1

Router#



I had come up with something in packet tracer and it has R0-R1-R2, and R1 does see R0 and R2 as DRs. I'm making assumptions here that this is okay since R0 does not have an adjacency with R2.

R1 is seen as the BDR for R0 and R3.

networker050184

The DR/BDR is a link local concept. In that sense a router can have as many DRs as it does multiaccess links. There will not be more than one on a single link though.