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Solution with ISIS cost value because of the traffic congestion

Publication Date:  2012-07-27 Views:  110 Downloads:  3

Issue Description

We had a problem about traffic congestion between CE and PE router. As the topology, four CE routers connected to a CE router and the CE router was connected to PE router or level_3 router. Topology and traffic values is in the attachment [Before_isis_cost.pdf]

Alarm Information

We have only 1gb ports between PE and CE or CE and CE. For that city some complaints were coming from ipdslam side for the xdsl subscribers’ speed. Especially vdsl subscribers’ speeds were very slow in those days which connected to four CEs. After controlling, on CE and PE link the congestion was occurred with 990mb.  

Handling Process

With isis cost value, the traffic can be separated other links. We don’t want to get the traffic on the links and if the links’ cost value is increased the link, isis route can be learned over other links.

<67_eregli_h4_1>display isis peer
                          peer information for isis(1)
  system id     interface          circuit id       state holdtime type     pri
67_alapli_h4_1  ge1/0/1            0000000002        up   295s     l2       --
67_devrek_h4_1  ge1/1/0            0000000002        up   299s     l2       --
67_zonguldak_t3 ge1/1/1            0000000005        up   21s      l2       --
67_kepez_h4_1   ge1/1/11           0000000002        up   295s     l2       --
67_kozlu_h4_1   ge2/0/0            0000000003        up   297s     l2       --
total peer(s): 5

The router should not be traffic from 67_kepez_h4_1 and 67_kozlu_h4_1. for that reasons, we increase the isis cost value on these links.

[67_eregli_h4_1]interface gigabitethernet 1/1/11
[67_eregli_h4_1-gigabitethernet1/1/11]isis cost 200
[67_eregli_h4_1]interface gigabitethernet 2/0/0
[67_eregli_h4_1-gigabitethernet2/0/0]isis cost 200

After that process, the traffic decreased between CE and PE and the traffic congestion was prevented. After commands, traffic values were like in the attachment.[ after_isis_cost.pdf]

Root Cause

Because the topology, the link need to be 10gbit or the traffic need to be separated to other links.


The priority of the command is lower than that of the isis cost command in the interface.  and by default, the link cost of is-is interfaces is 10. If you are not able to change the topology, you can change the isis cost value for the traffic congestion.