auto-cost reference-bandwidth

Unanswered Question
Mar 17th, 2012

Hello Netpro

i have OSPF Running in my enterprise as IGP i have a gig interface on router  ,but i do not configured the auto-cost reference-bandwidth ,what is cons if this command isn't configured

for 1 gig  is it 10,000?

thanks

I have this problem too.
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John Blakley Sat, 03/17/2012 - 08:01

Ibrahim,

Autocost is used so that ospf can calculate it's own interface cost to use to get to a route. The formula is ref_bw/interface bw. For instance, if I changed the auto-cost to 10000, my FE interfaces now change to a cost of 100.

router ospf 1

log-adjacency-changes

auto-cost reference-bandwidth 10000

Router(config-router)#do sh ip ospf inter               

FastEthernet0/0 is up, line protocol is up

  Internet Address 10.45.0.4/24, Area 0

  Process ID 1, Router ID 192.168.45.4, Network Type BROADCAST, Cost: 100

  Transmit Delay is 1 sec, State BDR, Priority 1

Change to 100000:

Router(config-router)#auto-cost reference-bandwidth 100000

% OSPF: Reference bandwidth is changed.

        Please ensure reference bandwidth is consistent across all routers.

Router(config-router)#do sh ip ospf inter

FastEthernet0/0 is up, line protocol is up

  Internet Address 10.45.0.4/24, Area 0

  Process ID 1, Router ID 192.168.45.4, Network Type BROADCAST, Cost: 1000

  Transmit Delay is 1 sec, State BDR, Priority 1

So Auto-cost / IF BW = (100000 / 100 = 1000 (FE0/0 cost to get to route))

If you want to increase your cost on your other links, but leave the gig interface alone, you should be safe to use:

auto-cost refer 1000

(I'm not great at math, but I'm getting 1 for gig links, 10 for FE links, and 647 for serial at 1544)

The con is that it affects all of your interface costs. You could just change your costs manually with the ip ospf cost command on the interface as well and that will give you more granularity. If you use the auto-cost command, it's recommended that you change it everywhere.

John

JosephDoherty Sat, 03/17/2012 - 17:37

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Posting

i have OSPF Running in my enterprise as IGP i have a gig interface on router  ,but i do not configured the auto-cost reference-bandwidth ,what is cons if this command isn't configured

for 1 gig  is it 10,000?

Normally, if you haven't reset auto-cost, it should default a reference bandwidth of 100 Mbps.  100 Mbps or "faster" will have a cost of 1.  The consequences are, FastEthernet and GigEtherent will be considered the same.

Marwan ALshawi Sat, 03/17/2012 - 17:59

Just to add

Because 100 m and higher will be considered the same then foe etherchannel interface that has more than one gig interface in one logical link in the case of one physical link failure ospf can not calculate the best oath based on links bandwidth/cost as they all will be seen by ospf as 1

However if you logical links of multiple 1G you should consider th highest number ad your ospf reference cost/bandwidth

Hope this helps

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