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.

Layer 2 Protocols on Serial Links

Earlier today I was tasked with getting two serial interfaces to talk with each other. Frame-relay is far from my strong suit, so I question-marked my way through the FR commands, making up DLCIs, adding map statements, etc. After several minutes with no luck I looked at the solution. As it turns out, Frame-relay was entirely unnecessary! And so began my Layer 2 serial interface education.

Read more