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?

Router(config-if)#do sh run int g0/0/0
interface GigabitEthernet0/0/0
 ip address
 negotiation auto

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.

Router(config)#int g0/0/0
Router(config-if)#no negotiation auto
Router(config-if)#do sh run int g0/0/0
interface GigabitEthernet0/0/0
 ip address
 no negotiation auto

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).

Read more

Make OSPF EVEN MORE Efficient

If you’re looking for a ridiculously easy way to make your OSPF process more efficient, you’ve come to the right place. According to Cisco, many of the CPU cycles spent on evaluating Dijkstra’s algorithm are unnecessary. You can save your router a lot of work by performing incremental SPF calculations in many cases rather than full SPF calculations.

How do you do it? It’s easy:

R0(config)# router ospf 1
R0(config-router)# ispf

That’s it! Best of all, it doesn’t cause the adjacency to drop and it doesn’t need to be enabled on all routers!

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