DesigRtrPriority=255???

[gillymonst]

I have a single 8600 as a core router with a single static route to get all traffic out to the internet. All the other routes are dynamically created as each vlan is created for internal connectivity between vlans.
My question is under "VLAN/IP/OSPF" each vlan has the Designated Router Priority set to "255" instead of the default of "1". This was set by the previous admin and I am now building a new 8600 and question this setting. Can anyone tell me why I should keep this setting or if they would leave it at the default setting of "1".

Thanks.

 

[Jynxx]

Depends. If this 8600 is the "core" of your network, leaving it at 255 will not harm anything.

Router priorities are used during the OSPF election process to determine which router(s) is the Designated Router (DR), and which router(s) is the Backup Designated Router (BDR).

The router with the highest priority costs on an interface/VLAN that it is forming adjacency with becomes the DR(if there are no routers off from an interface/VLAN, then the 8600 would be default be the "DR" for that segment).

If there is a tie, it uses the highest router-id to determine which router is the DR.

So in general, it is a good idea when designing an OSPF network to configure which routers are DR's and which are BDR's. Cuts down on OSPF hello messages because after the DR/BDR relationship is figured out, then all routers on that segment will only communicate to the DR and BDR directly. The DR and BDR then, in turn, inform the other routers about changes, etc. So all routers do not need to send OSPF hello messages to every other router. In large networks this is important.

However if the network is fairly small, it will not matter too much.

With all set to 255 or all set to 1 (since they are all the same), makes no difference...though one would have to analyze the rest of your network configs to be certain, in relation to how the other routers are set for ospf priorities.

 

[Jynxx]

Forgot to mention, in regards to which routers are DR and which are BDR, it you have a fairly large network it is generally a good idea to make the least busy routers attached to a segment the DR and BDR, as they will have the most OSPF exchange to do.

 

[gillymonst]

Thanks.

No other routers off of the inside private vlans...just the one outside static route connection to get to the internet with an ospf priority of "1".

So I should leave the internal stuff at 255 I think.

 

[Jynxx]

Yeah, if the 8600 for the most part IS your network, then it will not matter what you set them to...so no harm in leaving them at 255, no.