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MLPPP fragmentation in bundle with an odd number of member links

2 Cisco 3640's running IOS 12.2.(5b) connected via 3 T-1 circuits. All 3 circuits configured into a single multilink bundle. "ppp multilink fragment-delay" not configured. Frames larger than 82 bytes are fragmented for sending across the multilink bundle, but only into two fragments and not three.

Is this considered proper behavior, as I can find no documentation that addresses anything close to this situation? (RFC 1990 definitely does not limit the number of fragments to a multiple or power of 2.)

If this is "proper" behavior, should using "ppp multilink fragment-delay" (with an appropriate delay value which results in at least three fragments per frame) cause fragments to be sent on each of the three links?

If a fourth T-1 link were added to the MLPPP bundle between the sites, would the IOS software fragment individual frames to use all four of the links of the bundle or would only two fragments still be used, assuming no configuration of "ppp multilink fragment-delay"?

What is the minimum fragment size that IOS will support? If there is a difference between configurations with "ppp multilink fragment-delay" configured and without that command configured, please specify the difference.

1 REPLY

Re: MLPPP fragmentation in bundle with an odd number of member l

By default, MLPPP has no fragment size constraint and packets are divided into a number of fragments whose number is based on the number of links in the bundle. The size of any fragment is unconstrained, but the maximum number of fragments is constrained by the number of links. Default delay is 30 milliseconds if the fragment delay has not been configured.

The ppp multilink fragment delay command is useful when packets are interleaved and traffic characteristics such as delay, jitter, and load balancing must be tightly controlled.

You can specify the max number of fragments a packet can be broken into (up to 16). Was this configured (ppp multilink frag max) on the router to set to it only 2 as you should be getting 3 fragments?

As per this doc (http://www.cisco.com/warp/public/788/voice-qos/designing-mlp-over-fr-atm.html#frag), it says in Cisco IOS, the fragmentation size is not configured directly, instead the desired delay is configured using the ppp multilink fragment-delay command. Cisco IOS then calculates the corresponding fragment size using the following formula:

fragment size = delay x bandwidth/8

Hope it helps.

Steve

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