cost for ospf routes on vlan interfaces

[carl_townshend]

Hi all

With OSPF is it possible to see what the total cost is for a path ?

also how does ospf cost calculate the cost when running it over a vlan interface? does it just add a cost of 1 for every vlan it passes to reach a destination?

also if I redistrubuted some static routes as e2, Am I right in saying the route would load balance if there were multiple paths and 1 was slower ?

cheers

Carl

[Giuseppe Larosa]

Hello Carl,

OSPF metric is the cumulative cost as calculated on local node to destination so yes you see the total cost in the IP routing table just using show ip route a.b.c.d

Yes, an  SVI L3 Vlan interface has an OSPF cost associated to it, with default settings that cost is 1. When changing the auto-reference bandwidth in OSPF we need to take care of the cost on SVIs that have associated a bandwdth that is not 1Gbps ( I think it is more 4 or 8 Gbps is my guess). You can use

ip ospf cost

to set the cost manually on SVI interfaces in interface configuration mode to reflect your choices of reference bandwidth.

O E2 can be load balanced over equal cost paths if  a path is slower it will have a greater metric and it will not be used

Hope to help

Giuseppe

[carl_townshend]

Hi

Regarding the e2 routes, I thought that the cost doesnt increase across the links? So if there were say 2 hops from the router and 3 over another route, it will still load balance as the cost is the same, or am I wrong ?

[Giuseppe Larosa]

Hello Carl,

routing to an O E2 route becomes routing to the ASBR node that injected the O E2 route in the OSPF domain, so even if the O E2 route metric does not increase over multiple router hops the internal path to the ASBR node does

The shortest internal path to ASBR node is used, equal cost multi path is possible,

Hope to help

Giuseppe

[carl_townshend]

Hi

Can you explain this? If that is the case then does e2 really make a difference than e1 ? Please explain

Cheers

[Giuseppe Larosa]

Hello Carl,

I'm sorry if I have been unclear.

I was meaning a single O E2 route with multiple internal paths to the ASBR node.

If there are multiple ASBR nodes, first the O E2 route with the lowest seed metric is preferred regardless of the internal path cost to reach the ASBR. This can lead to sub-optimal routes.

With O E1 the seed metric is directly added to the internal path cost to the ASBR node making a more precise route path choice.

If you have multiple exit points from OSPF domain ( multiple ASBR nodes) the use of O E1 would be a better choice.

Hope to help

Giuseppe

[carl_townshend]

Hi

So I gather e2 doesnt add the path cost from the asbr to the final destination, but you still will use the shortest path to the asbr, but the metric will still show as 20 , is this correct ?

[Giuseppe Larosa]

Hello Carl,

>> So I gather e2 doesnt add the path cost from the asbr to the final destination, but you still will use the shortest path to the asbr, but the metric will still show as 20 , is this correct ?

yes this is correct

Hope to help

Giuseppe