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Cisco Serial I/F Delay. Why is it lower on a single T-1 than multilinked T-1s?

Posted on 2003-10-30
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Last Modified: 2010-04-17
I configured two 7206s at a remote site, set them up with HSRP and then multilinked 3 T-1s to the hub site for router A and configured a single T-1 to the hub site on router B. Then I noticed that all of the remote site LAN traffic hopped from router A to router B and rode the single T-1 to the hub site . . .

Then I noticed that the delay on "show int" for the multilink was 100000, and for each single T-1 that was part of the multilink it was 20000. This was why EIGRP was saying that the preferred route was via router B, because as a single T-1 the delay was 20000, but showing 100000 over the multilink.

Anybody know what's going on here? Is "delay" a measured or calculated value?

Thanks,
Steve
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Question by:SteveJ
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NicBrey earned 63 total points
ID: 9656002
In stead of multilink, rather use CEF.  PPP multilink requires each packet to be process switched, CEF use fast switching. That is probably why you see more delay on the multilinked T1s.  
http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

Also not exatcly sure how the Delay is worked out. It's not specified in the EIGRP white paper either...
I think it is measured, but then some more calulations are made to get the final value that the router will use to calculate the metric of a route.
White paper:
http://www.cisco.com/en/US/tech/tk365/tk207/technologies_white_paper09186a0080094cb7.shtml#metrics
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by:epylko
epylko earned 62 total points
ID: 9700039
It sounds like your multilink is misconfigured. It shouldn't be possible to have traffic going over 1 T1 since you wouldn't have IP addresses on the individual T1 links. Post your config so we can take a look at it.

Also, PPP multilink is not and should not be process switched unless you are using a sufficiently old version of IOS. Some newer technologies like MLPPP over ATM (12.2-ish) has bugs where it process switches incoming MLPPP, but it looks like those bug fixes have been commited.

Finally, if you don't like the delay, change it with the "delay" interface command.  That's the best way to tweak EIGRP parameters, although you could use the bandwidth parameter too.

-Eric
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