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Community Member

ISL best practice

Hello All,

I have 2 MDS 9509 sitting across two sites on a same fabric with Intersite DWDN link = 1 Gbps

I have 48 port blades and 12 port blades.

Since the ISL max theoritical bandwidth is 1 Gbps, i think of locking the switch ISL ports at 1 Gbps (End to End )at both end


Doubt : 1

Is it a good idea to utlise the 48 port blades for this ISL


Use the costly 12 Port blades for this ISLs


Doubt : 2

I want 16 Gbps across max speed

is it a good idea to go with

2Gbps X 8 ISLs or 1 Gbps X 16 ISLs


Doubt : 3

These ISL are primarily used for replication traffic (True copy)

IS it a good idea to Bundle the links for

1. load balancing

2. Line card failure / replacement


Doubt : 4

If i go with Port channel

is it a good idea to go with Exchange based loadblancing?


Doubt : 5

Since the link is used for replicaton

is it a good idea to switch the IOD(In order delivery ) : OFF


Please help .....

Thanks in Advance :):)



Cisco Employee

Re: ISL best practice


1. Use the 48 port blades and dedicate the bandwidth for 1gbs ISL. The 12 port blade dedicates 4gbs by default. Since you're using only 1gbs with dwdm, it's not ideal.

2. Go with 1gbs x 16. There is no way of using 2gbs links since your dwdm is only 1gbs.

3/4. Use portchannel, you can always add/remove members non-disruptively. Use exchanged-based loadbalancing if you can. Use multiple linecards and multiple port groups for hardware diversity.

5. IOD shouldn't be required. Only needed for FICON.


Community Member

Re: ISL best practice

Hey thanks for this quick response ...


Some more doubts in mind ... :)

Doubt : 6

What shoulb be the B2B credit set for the port for a distance of 130 Kms, do i need to increase the B2B or leave it defalut.


Doubt : 7

I am planning to test various frame sizes across ISL.. What impact will it have on the latency


Doubt : 8

Since End to End speed is only 1 Gbps, is it a good idea to lock even the Storage replication ports to 1 Gbps End to End


Community Member

Re: ISL best practice

130kms is going to add some serious latency so you have to be doing asynchronous TC. Don't ever consider doing sync TC.

I believe B2B credits on a 9509 are allocated as required. You could keep an eye on what is happening and then set them statically if necessary. I think that by default, 250 B2B credits are available per port and thats enough for that distance. The rule of thumb is 1 B2B credit for a 2 Gpbs port so 130 km will only need 65 B2B credits but it never hurts to have more.

I tend to leave everything as is unless advised by the vendor. Get HDS in on this to stop any heartache when it goes pearshaped.


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