Routers loopback addresses

[wilson.jr]

Hi,

I'm implementing OSPF on a network with 100+ router and wondering what loopback IP address to assign to routers. My enterprise IP subnet is 10.0.0.0/8

Should I pick some from this range e.g. 10.10.10.1, 2, 3, 4 .... /32 or something outside as 4.4.4.4.1, 2, 3, .../32. I know that I can pick up any address but wondering what other do

Thanks

[Peter Paluch · Accepted solution]

Hello,

Don't use public IP addresses that do not belong to you because that could cause reachability issues (consider that 4.2.2.2 or 8.8.8.8 are public DNS servers, for example). Always use IP addresses from the range you were assigned.

Personally I suggest using IP address from the upper end of your IP space, i.e. 10.255.255.x/32. This format makes them somewhat special, well distinguished from other IP addresses, and still perfectly usable.

Best regards,

Peter

[Jon Marshall · Accepted solution]

Just to add to Peter's post. If you have multiple sites then try to allocate a subnet per site for loopbacks, even better if you take the loopback addresses out of the range already assigned to each site.

Advertising /32s across our WAN just makes the routing tables messy and it is not needed.

Jon

[wilson.jr]

Forgot to mention that I would like to control which router is going to be DR and BDR (Router with highest loopback address (RID) will be assigned highest priority and elected as a DR and first lower as BDR). Having that already assigned range may not work here

[Peter Paluch · Accepted solution]

Hello,

That is not the most correct approach to select DR/BDR. These routers are elected based upon their interface priority that is configurable using th ip ospf priority command. Only if the priorities are identical, the RIDs are compared to determine which router is going to be the DR and which is going to be BDR. However, the RID is used only as a tiebreaker here, and it is not supposed to be assigned to routers in such a manner that it guarantees DR/BDR elections according to your wish.

Use the ip ospf priority interface command to influence DR/BDR elections, and assign the loopbacks in the way Jon suggested.

Best regards,

Peter

[wilson.jr]

I have multiple sites with router runing OSPF in failover configuration running HSRP

IP allocate for loopback interfaces for this site are 10.255.240.0/24. Is below IP assignment correct?

Active router

Interface loopback0

IP address 10.255.240.200 255.255.255.255

Passive router

Interface loopback0

IP address 10.255.240.199 255.255.255.255

Thank you