In OSPF Networks setting, should i specify each of the downstream subnet such as /29 or i could use /24?
Anyone could share the best practise please?
/24 will work and has sense if you want distribute OSPF through all downstream interfaces.
I have 2 EBGP router
Each EBGP router has 1 upstream which we would receive full routing table.
Both EBGP router are connected using IBGP and with client to client reflection enabled on both EBGP instances
Beneath these two EBGP routers have 5 Distribution routers R1 R2 R3 R4 R5
Each Router is connected to both EBGP router via OSPF
Redistribute Static Route is set as type1 on each of the R1 R2 R3 R4 R5
On both EBGP router instance, its OSPF setting is set Redistribute default route always as type 1
Each of the R1 R2 R3 R4 R5 has multiple /29 for example
R1 has 10.10.10.0/29 R2 has 10.10.10.128/29
In this case for R1 and R2 OSPF Networks setting, should i set it as R1 10.10.10.0/29 R2 10.10.10.128/29 or both 10.10.10.0/24?
OSPF Best Practices (there are many but here are a few)
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It is generally a best practice to use the smallest subnet possible (or even the /32 address of the other router) when setting OSPF networks to avoid a neighbor forming inadvertently.
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Use MD5 authentication between routers to avoid inadvertent neighbors
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Keep OSPF routing tables as small as possible…performance can start to degrade on OSPF when you get into thousands of routes because it takes a while for the SPF algorithm to run.
In answer to your question, it really depends on whether the OSPF routers that sit below the BGP border routers are all connected into the same LAN segment via a switch or if they are individually cabled into the BGP routers.