GRE Bandwidth

[lxcollin1]

Hi All,

On a GRE tunnel with a full T1, how much actual bandwidth can be used for tunnel data? It looks like the bandwidth is set for 9kb by default. Does this have any influence on actual bandwidth? Are there any settings to manipulate how much bandwidth GRE can use of the T1?

The reason I ask is that the "show interface" usually indicates that the RX or TX is at 255/255. I'm wondering if the tunnel is truely maxed out or if this is incorrect due to the 9kb bandwidth default.

Thanks!

- Lee

[spremkumar]

hi

The tunnel b/w which u r referring is the default one which comes automatically on once u enable interface tunnel in the routers.

You can change the same accordingly respective of ur primary link b/w over which its built.

in this case you can set it up to the full t1(1544K) under the tunnel interface using bandwidth command.

Again it matters when u have some routing protocol in place esp like ospf,eigrp which considers the b/w parameter to decide the routing path.

regds

[Richard Burts]

Edwin is correct that the 9k bandwidth is the default value. It is my understanding that Cisco defaults the bandwidth to a quite low value because they want the tunnel to not look too attractive. The tunnel may in fact cross multiple router hops to reach the tunnel destination but it will look like one hop. In setting up the tunnel the router has no idea what the real capacity is of the segments along the path to the destination. So Cisco chooses a small value for tunnel bandwidth.

The value of bandwidth does not constrain the actual transmission through the tunnel. The router will send as much data as fast as it can and is constrained by the physical media not by the configured bandwidth.

Your observation is correct that the calculation of TX percent and RX percent is based on what the router believes the bandwidth is. But the calculation of TX and RX percent does not affect the actual behavior of the tunnel. It will send as much traffic through the tunnel as it is physically capable of. And certain routing protocols may use the bandwidth in calculating the metric of routes. For that reason you may want to configure a bandwidth that is a closer match to the physical media.

HTH

Rick