Jumbo MTU on Nexus 5000

Setting a per interface MTU (maximum transmission unit) is not supported on the Nexus 5000/2000 series switches.
If a Jumbo packet is required to traverse a Nexus 5000 series switch , the jumbo MTU must be set in a policy-map and applied to the ‘Sytem QOS’.

Configuration:

Configuration, PRE NX-OS 4.1:
policy-map JUMBO
 class class-default
  mtu 9216
system qos
 service-policy JUMBO

Configuration with POST NX-OS 4.1:
policy-map type network-qos JUMBO
 class type network-qos class-default
  mtu 9216
system qos
 service-policy type network-qos JUMBO

Continue reading “Jumbo MTU on Nexus 5000”

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Nexus defaults to PAP authentication

Ever configured a Nexus switch to use AAA to query a Tacacs+ server? Had some troubles applying standard IOS config to NX-OS?

Possibly if your Tacacs+ server is configured to only allow PAM (Password Authentication Manager) authentication for the users. See when a NX-OS switch sends a AAA authentication packet, by default it is encapsulated using PAP encoding. This is in contrast to normal IOS devices, that use PAM encoding by default.

To illustrate I used the following config:

ip tacacs source-interface mgmt0
tacacs-server host 10.5.0.82 key password
!
aaa group server tacacs+ TAC
server 10.5.0.82
use-vrf management
source-interface mgmt0
!
aaa authentication login default group TAC
aaa authorization config-commands default group TAC
aaa authorization commands default group TAC
aaa accounting default group TAC

Continue reading “Nexus defaults to PAP authentication”

Troubleshooting random Nexus reboots

November last year, a pair of Cisco Nexus 5010 switches, suddenly started rebooting randomly without user intervention.  Since these boxes were a front to a VM environment, stability were of urgent concern. But in order to stabilize the environment, the root cause of the reboots had to be isolated, and quickly.

The Cisco Nexus platform might not be as mature as many would like, but it is quickly becoming a very needed switch in Next-Generation datacenters. Of the things I like most about the Nexus boxes are the readily available local reporting and intuitive system checks.  Obviously there are many other features which is making the platform so popular. I’ll cover some of these in time.

Coming back to the rebooting issue. Unlike IOS devices that looses all local logging info, unless a crash dump was saved to NVRAM, the Nexus writes most of its log information to disk. Thus even after the reboot, you have all the information.
Continue reading “Troubleshooting random Nexus reboots”