Cisco ASRs and GigE Autonegotiation

I’ve never seen a GigE interface that wouldn’t let me manually specify the speed and duplex.  I’m not talking about the speed limitations of the Nexus 2148T (it can’t to 10/100), I’m talking about disabling Ethernet autonegotiation.  While working with a Cisco ASR1001 the other day, I’d finally met my match.

The ASR1001 has four built-in GigE interfaces which accept copper for fiber SFPs (though not the traditional GLC-T).  I was trying to disable autonegotiation on a single-mode fiber interface but the speed and duplex commands didn’t exist.

But, if autonegotiation was my only option, why did the IOS XE coders torment me by including it as a default part of the interface config?

Turns out, these built-in interfaces only support GigE, not 10Mbps or 100Mbps. More to the point though, only copper interfaces support manually defined speeds. Fiber interfaces operate in one fashion only, the only choice you have is whether or not to use autonegotiation. In my case I disabled it like this.

PPP Error Recovery: LAPB

I was asked to configure LAPB for error recovery on a PPP link the other day. I had two questions: 1) What’s LAPB? 2) How? The latter is pretty simple, the former much more complex.

First, in the interest of time, let’s talk about how this is done.

Router(config)#int s0/0/0
Router(config-if)#encapsulation ppp
Router(config-if)#ppp reliable-link

That’s it! Now, what exactly have we accomplished? By enabling LAPB we’ve inserted another layer of error-checking in our link that’s transparent to upper layers. NOTE: LAPB can only be used on unique PPP circuits, not multilinks. If you’re looking for a link quality solution for multilinks, check out LQM.

OSPF’s Maximum Metric

There are some situations where you just don’t want any traffic flowing through a particular router unless there are ABSOLUTELY NO other alternatives. How can we do that? Tell a particular router to advertise his/her LSAs with the maximum metric possible (0xFFFF).

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