Path selection

Unanswered Question
Sep 28th, 2009
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We had a eigrp route go down and come back up. Since then the traffic seems to pref the gig link rather than than the 100meg link. When the gig link is shut traffic resumes back to the 100 meg link. When I enable the gig link the above happens again. This has only began since we lost this 100 meg link

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Peter Paluch Mon, 09/28/2009 - 05:38
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Hello,


Are you complaining that the GigE link is preferred in EIGRP? Well, it is normal for EIGRP to prefer a faster link. Please try be more specific about the issue and how would you like to see it resolved.


Best regards,

Peter


jalomenu Wed, 09/30/2009 - 02:13
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Hi Thanks for getting back to me.


We seem to have resolved the issue.


Traffic from Fenchurch to Docklands was via 2 x 100 meg links from Fen SW1 to Sun SW1 and to Lloyds via a 100 meg link with all systems working. Please see attached diagram. A 1 gig link was introduced between Fen SW2 and Sun SW1 with no apparent problems. However a fault was reported stating no traffic from Fenchurch to Lloyds although users could ping over the link. The 1 gig link was shutdown and traffic from Fenchurch to Lloyds was restored but it was noted that traffic from Fenchurch to Docklands was then an issue for some users but not others.


This was due to the default gateway of the devices in Docklands being Sun SW2 and this router having 4 equal cost routes. The traffic which is sent to the Loy R2 will fail as it is sent to the firewall without it passing through it in the forward direction. If the source was in Docklands and the destination in Fenchurch then the packet would either be sent to Loy R2 through the firewall but returned through Sun SW1 so have a different sequence number and be dropped or sent Sun SW2 - Sun SW1 - Fen SW1 to PC then return Fen SW1 - Sun SW1 to PC which works.


The resolution was the 1 gig link was shut down and this enabled traffic from Fenchurch to Lloyds. For the traffic between Fenchurch and Docklands the HSRP priority was set so that Sun SW1 was active for the networks flowing to Fenchurch. This has avoided the partial flow through the firewall which was causing traffic to fail for some users.


Best regards


John




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Giuseppe Larosa Mon, 09/28/2009 - 05:42
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Hello John,

check with


sh ip eigrp interface type x/y


EIGRP uses the BW parameters and delay parameters of interface receiving updates.


EIGRP metric is cumulative in delay and inverse proportional to lowest BW in the path.


probably the GE interface has a lower delay associated to it.


In other words the GE link should have been preferred also in the past.


Hope to help

Giuseppe

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