IPSEC over GRE tunnel bandwidth

[alan-wong]

We have office sites connected with IPSEC over GRE tunnel and running EIGRP.  Now I know we can use "delay number" to control the less attractive path.  However, what is the bandwidth use for?  I read some Cisco page about bandwidth calcuation, but I do not understand.  Please advise.

What is below bandwidth function? and how to calculate it?  Please help.

For example:

interface Tunnel1
bandwidth 1544
ip address 172.31.0.1 255.255.255.0
tunnel source FastEthernet0/1
tunnel destination 111.111.111.111
interface Tunnel2
ip address 172.61.0.1 255.255.255.0
bandwidth 10000
tunnel source FastEthernet0/1
tunnel destination 222.222.222.222

[Peter Paluch · Accepted solution]

Hello Alan,

EIGRP uses the bandwidth parameter for two separate purposes. First, the bandwidth is one of interface metrics used by EIGRP to compute the total metric of a route. When EIGRP computes a metric towards a destination, it takes the least bandwidth along the path to the destination (i.e. it takes the bottleneck bandwidth into account), and uses that in its metric formula (in the form of 10^7/bottleneck bandwidth so that the smaller the bandwidth is, the higher the resulting metric is). To know the bottleneck bandwidth of a route, each EIGRP router receiving an update about a destination over a particular interface takes the smaller value from the bandwidth in the update and the bandwidth of the receiving interface (meaning that either the bottleneck is somewhere down the route, or this router's interface is the bottleneck). This also means that manipulating the bandwidth may not have the results you expect - if your interface's bandwidth is higher than the bottleneck bandwidth, increasing the parameter further will have no impact on the resulting metric.

Second, EIGRP uses the bandwidth parameter to estimate the maximum allowable bandwidth it is entitled to consume when sending its packets. By default, EIGRP is allowed to eat up to 50% of the interface's configured bandwidth. Note, again, that if the bandwidth is set too low, it will impede EIGRP's capability to send its messages, and if it is set too high, even 50% of it may overload the physical bandwidth of the underlying interface.

Therefore, the bandwidth should always be set to a realistic value approximating the bandwidth of the underlying physical interface, and under no circumstances shall it be used to influence EIGRP's choice of best paths. If tweaking EIGRP metrics is necessary, it is recommended to use the delay parameter I have mentioned in my previous response.

Best regards,

Peter

[alan-wong]

Hi Peter

If that is the case and I am NOT consider as a metric calcuation.  I am only consider to set the bandwidth to control and prevent EIGRP eat all of my actually bandwidth.

bandwidth 1544  <<-- is that mean 1544 x 0.5 = EIGRP occuiped bandwidth?

[alan-wong]

Also, for example.  If my internet line is 10Mbps. can I still use "bandwidth 1544" or I need to put less or more?

Thank Peter

[Peter Paluch · Accepted solution]

Hi Alan,

If that is the case and I am NOT consider as a metric calcuation.  I  am only consider to set the bandwidth to control and prevent EIGRP eat  all of my actually bandwidth.
bandwidth 1544  <<-- is that mean 1544 x 0.5 = EIGRP occuiped bandwidth?

Yes, with bandwidth 1544, EIGRP has the right to consume up to 722 Kbps from the interface should it need that much bandwidth to transfer its data. Keep in mind that if EIGRP does not use the bandwidth, it is available for normal data streams. Also, the percentage of how many percent EIGRP can consume can be configured using the ip bandwidth-percent eigrp command on a per-interface basis.

Also, for example.  If my internet line is 10Mbps. can I still use "bandwidth 1544" or I need to put less or more?

You can safely use bandwidth 1544 here. Although the declared bandwidth of the tunnel is lower than the tunnel itself is capable of achieving, EIGRP still has plenty of room for its packets if needed (mostly in periods of intense topology changes).

Best regards,

Peter