Mutual Re-distribution and tagging

[morph.andy]

Hi all

firstly apologies if I'm not clear on this - part of the reason for this post is to confirm that!  Attached config

I'm doing redistribution as below on R1 and R3 - with RIP having the higher AD, I know I need to tag the traffic as it go's into OSPF so I can then deny it being re-distributed back into RIP to avoid loops. The bit I'm wondering about is the following:

With OSPF having a lower AD than RIP, then it wont cause a loop if its re-distributed back into OSPF (I think I'm right on that). But it would be best practice to tag the routes anyway, and stop them being re-distributed back into OSPF anyway ?

[Thomas Schmitt]

You are almost right, but one thing to consider:

OSPF would prefer internal routes over redistributed routes in any case. But, in case OSPF installs an E2 route into routing table and you re-redistribute this route from RIP, then OSPF process has 2 E2 routes - the first one is "the real one" and the second is the "reredistributed real one".

From OSPF point of view, 2 different ASBRs advertise the same subnet and possible loop

[MHM Cisco World]

Loop happened sure 
R5 advertise the prefix to R3, R3 redistribute the route into OSPF 
OSPF will carry prefix to R1 as OSPF AD 110 
R1 now have same prefix from RIP (AD =120) and OSPF (AD=110)
R1 will select the OSPF lower AD 

LOOP start here, R1 will advertise the prefix with next-hop R2

and what make LOOP happened is R3 will prefer path to R1 instead of R5 ? why assume that the R5 have high metric than the link between R3+R1.

so it safe to cut the redistribute in one router.