Layer 3 etherchannel problem

Unanswered Question
Dec 28th, 2009

Dear Concern,
I have two sites and configured Layer 3 ether channel. But facing a problem like that  when microwave link goes down site A is disconnected for 1.5 min and after  1.5 min it comes up. But when fiber link goes down then site A does not disconnected, it works normally.
Please find attachment for configuration and diagram. Urgent issue

Thanks in advance.

I have this problem too.
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Giuseppe Larosa Mon, 12/28/2009 - 12:06

Hello,

given the different nature of the two links, I would use each of them as an indipendent layer 3 link with a routing protocol running over both.

in this way the routing protocol can detect the failure of a link and can adapt to this by using only the other link.

for example you could use EIGRP or OSPF

Hope to help

Giuseppe

skmdimran Mon, 12/28/2009 - 12:25

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Thanks for your quick response.   What do u mean by nature of the links ? please clarify.

Can you help me about pagp times ? how can I define it ?

Giuseppe Larosa Mon, 12/28/2009 - 12:59

Hello,

I try to repeat I would use the two links as two different L3 links

example

default interface gi1/47

default interface gi1/48

int gi1/47

no switchport

ip address 192.168.x1.y1 255.255.255.252

int gi1/48

no switchport

ip address 192.168.x2.y2 255.255.255.252

router eigrp100

network 192.168.x1.0

network 192.168.x2.0

no auto-summary

! other network commands for nets to be advertised

note: the other end must use the same EIGRP AS number (100 in my example)

if one link is fiiber and the other one is based on a RF microwave link  they have different nature.

For this reason I would use each with its own IP subnet.

With default timers EIGRP detects a failure in 15 seconds

Hope to help

Giuseppe

skmdimran Wed, 12/30/2009 - 02:23

Thanks for the update. I have implemented OSPF and it takes 30 seconds for link failure issue. Since OSPF is running in existing system thats why client dont agree to use EIGRP. Actually huge voice and data traffic will be passed through the links so there is absolately no tollarance of delay , it should be zero delay.

Is there any way to overcome this delay issue ?


Please update .

thanks a lot !

Giuseppe Larosa Wed, 12/30/2009 - 03:19

Hello Skmdimram,

I've used EIGRP just to make an example.

With default settings OSPF can take up to 40 seconds to detect a dead neighbor.

Depending on what your devices support you can use:

sub-second hello interval

see in interface config mode you can use:

ip ospf dead-interval {seconds | minimal hello-multiplier multiplier}

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_osp1.html#wp1014268

minimal means 1 second of dead interval.

better results with BFD bidirectional forwarding detection that allows to implement IP fast convergence. BFD is a lightweight protocol that checks ip connectivity and provides its services to routing protocols including OSPF.

You can enable BFD for a subset of the interfaces for which OSPF is routing by using the ip ospf bfd command in interface configuration mode.

see

http://www.cisco.com/en/US/docs/ios/iproute/configuration/guide/irp_bfd_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1054785

you need to verify what features are supported on your devices. if they are multilayer switches and they are not C6500 I'm afraid BFD is not supported.

You can use www.cisco.com/go/fn

to check with feature navigator

I've tried and I get only C3750 Metro ethernet or ME 3400 among multilayer switches

Hope to help

Giuseppe

skmdimran Sat, 01/02/2010 - 11:24

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Thanks a lot for sharing a nice document about BFD, its really help me .

Would you please suggest me how I will find out that OSPF load balancing are happening between two links ? Actually I want to know , is there any command that will show load balanced traffic .

Many thanks.

Regards

Imran

Giuseppe Larosa Sat, 01/02/2010 - 11:30

Hello Imram,

sh ip cef exact-route

can tell you on what link given a source and destination address of the flow the packet is sent.

Other more platform specific CEF show commands may exist for your devices.

example

sh ip cef exact-route 1.1.1.1 ?
  A.B.C.D  Destination address of session

BFD should be fine with your 7609 devices.

Hope to help

Giuseppe

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