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New Member

Bandwidth command

Hi, I would like to know more about bandwidth command, if I have 155Mb ATM leased line but I configure as bandwidth 2Mb (to make this path is less desirable), will the performance go down ? I do this because I want to influence the routing path selection to the destination. Thanks.

5 REPLIES
New Member

Re: Bandwidth command

Bandwidth command has different implications in different IGP protocol environments.

The interface bandwidth command has following implications:

1. In OSPF environment cost of the outgoing interface in the path is affected with the interface command with the formula: outgoing interface cost: 100 Mbps/(interface bandwidth in bps). where 100 Mbps is the reference cost. As your leased line bandwidth is more than 100 Mbps, you need to change this reference bandwidth with following ospf command: auto-cost reference-bandwidth on all the routers.

Select reference bandwidth more than the highest access line speed in your OSPF A.S.

For OSPF you can refer following document:

http://www.cisco.com/warp/public/104/1.html

2. In EIGRP environment the amount of EIGRP traffic flowing on the interface is set by default to 50% of the configured bandwidth on the interface via bandwidth command.

cost of the path in eigrp is calculated with following formula:

Path cost = [(10Mbps/min. bandwidth in kbps of any link through which traffic flows to reach the destination) + cumulative delay across the path to the destination)]*256

Setting the interface bandwidth with bandwidth command in EIGRP A.S. purely depend upon the network topology e.g. Pure Multipoint Configuration (No Subinterfaces), Pure Point-to-Point Configuration (Each VC on a Separate Subinterface) etc.

For EIGRP you can refer following documents:

http://www.cisco.com/warp/public/103/12.html

http://www.cisco.com/warp/public/103/14.shtml

I hope this will solve your query.

New Member

Re: Bandwidth command

Hi Abah, thanks for your response.

My case is Router B have 2 path to destination "A" , the first path is 155Mb leased line ("B to C") + 2Mb leased Line ("C to A").

The second path is direct to destination ("B to A") but the bandwidth is 2Mb.

IGP Protocol that i use is OSPF, OSPF will choose the path by bandwidth and delay. So for this case router B will choose 155mb leased line with 2 hops to go to router A. I would like to influence the path that router choose so that mean choose the direct connection to A (1 hop) instead using router C (2 hops). I am thinking to put bandwidth command on all the leased line even on 155Mb leased line (bandwidth 2000) so make all have same bandwidth and the OSPF will choose direct connection to A. But is there any implication of bandwidth command that I put ? Is bandwidth command would affect the bandwidth that router use ?

Thanks.

New Member

Re: Bandwidth command

With regards to IGP's ... the bw command is purely for routing and wouldnt affect performance in the slightest. You could configure the bw as 2mb and still get 155mb out of the atm pipe.

HTH

New Member

Re: Bandwidth command

Hi,

As you are using OSPF (default administrative distance of 110) instead of changing the bandwidth you can add static routes (default administrative distance 0 or 1) therefore you can add a default route for 2 Mbps leased line and redistribute it into OSPF via redistribute static router ospf command.

Changing the interface bandwidth with bandwidth command will only change the route selection procedure and it in no way modifies the access line speed i.e. even if you mention bandwidth as 2 Mbps of ATM link, user data still can use 155 Mbps.

Anuj Bahl

New Member

Re: Bandwidth command

Thank you for HTH and Anuj. Regarding Anuj's suggestion I was wondering if I add static route on router B and redistribute the static route to another router (router A and C). Would router C will pass to router B first to go to router A ? Thanks.

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