router port question

[dkblee]

hi! I've a wan interface that's 1000mbps in router A and another router that's connected to router A with port 100mbps. Can i fix both interface with speed 100 and duplex full for both sides? Will there be any issue?

Thanks.

[ANTONIO DEUS]

Hi dkblee,

What kind the equipment do you have? It is a switch or a router? What kind ports are you talking about (Tx or Fx)?

ADAJio

[Zeeshan Siddiqui]

Set speed to 100 on both routers and duplex full.

[Somasundaram Jayaraman]

Hi,

If you hard code the speed & duplex to be the same on both sides, then it wont be an issue.

Cheers

Somu

[dkblee]

hi! So it;s recommended to fix the speed to 100 and full duplex for both end if they are connected with ports of different speed (eg. 100 and 1000mbps)? thx

[Zeeshan Siddiqui]

Hard code the speed / duplex (it is recommended) if not you can see collision etc

[dkblee]

hi! for my case, it was working fine for quite sometimes, i used to use auto for both connecting interfaces. The issue only happen recently, whereby the duplex settings become duplex. I can see that interface and line protocol is up at both end. These 2 interfaces are in /30 segment and i can't even ping the opposite interface.

We reported this to the telco, and we're advised to change it to Auto at one end and statically fix it on other end. Not too sure the logic behind as it was working fine previously, it might be something has been changed on the telco devices that causes the auto negotiation to fail. Even after making the changes, a restart of both routers are required. Shut/no shut the interfaces didn't help as well. Any idea or explanation on this?

the router interfaces come with no cdp enable. I've turned that on...do you forsee any issue on this as well? for me i think cdp is a gd feature to have.

Thx.

[johnlloyd_13]

Hi,

I've talked to our telco about this issue previously and was informed that our router's Metro-E circuit needs to be in sync with their MUX setting (which they say it's 100/full) for it to work.

I would say disable CDP on the router's WAN port to avoid any security vulnerability and enable it on the LAN unless you really needed to like troubleshooting or if you're running VoIP.

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[Peter Paluch]

Hello,

I do not see personally any added value to fixing the port speed and duplex. As the autonegotiation nowadays works nicely, I would personally abstain from touching the speed and duplex settings manually. Setting them statically has no practical advantage, and the failure to configure both the speed and duplex statically on both ends of the link may cause the link to not come up at all, or cause duplex mismatches.

Best regards,

Peter

[johnlloyd_13]

Hi Peter,

I would disagree. I've encountered several occasions wherein our CE router WAN link wouldn't go up unless we've manually hardcode the router's speed and duplex setting to match the telco's.

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[Peter Paluch · Accepted solution]

Hello John,

Thanks for voicing a different opinion!

Yes, you are correct. If the autonegotiation for whatever reason does not work reliably then obviously, specifying the speed and duplex is the only option of having the link work. I believe to have seen a similar problem with connecting a Cisco 2811 with a Foundry switch.

That being said, if it has been verified that the autonegotiation works satisfactorily, I would personally suggest against setting the duplex/speed manually. We have discussed quite many times here on CSC that setting these values manually results in quite unpredictable behavior, with some devices deactivating autonegotiation, other devices advertising only the selected speed duplex, some devices even deactivating the auto-MDIX function - there are too many issues what could go wrong with it...

So, I shall correct myself a little: I see no added value in setting the duplex/speed manually unless it is proven that the link won't work at all without specifying these parameters statically. What's your view on this?

Best regards,

Peter

[johnlloyd_13 · Accepted solution]

Hi Peter,

Yes, this time I agree.

Personally, I make it a habit of manually changing our router's speed/duplex settings and fallback to auto if it doesn't work. This works for me most of the time.

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[Peter Paluch]

Hi John,

Thank you!

Best regards,

Peter