Tag Archives: Cisco

Cisco Nexus Considerations (28 of 30)

I found some documentation that I wrote a few years ago on the Nexus product line, a few things have changed since I wrote this, but it is largely true.

Dual Homed or Active/Active Fex Design
The Nexus 7K to 2K does not support dual homed connectivity. Cisco does support this in 5K to 2K design, but not in 7K to 2K. The single homed design is the only supportable solution. This has implications that if there is a single Nexus 7K failure, it will take down all of it’s down stream FEX’s.… Read more

Cisco Symmetrical Script (14 of 30)

A while ago I had written a script that would logon to Cisco devices, run a traceroute from IP-A to IP-B then do the same for IP-B to IP-A. It would then query each hop with snmp and see if they worked out to be the same path. If it wasn’t, it would show you the differences.

I have put it up on github.

Update the below with your personal configurations

my $snmp = ‘snmp_read’;

my $ios_username = ‘ios_username’;
my $ios_password = ‘ios_password’;
my $ios_enable_password = ‘enable_password’;

Then you can run the script via cli, or send an html form to the script.… Read more

MTU 1524 on GRE Tunnels (13 of 30)

When I first came across this configuration I was confused as to how it was handling either UDP packets of 1500 bytes or TCP packets where PMTUD failed, as either an ICMP issue, or an issue where TCP did not correctly address PMTUD issues.

interface Tunnel10
ip address
ip mtu 1524
ip tcp adjust-mss 1352
tunnel source Loopback1
tunnel destination

The traditional method for fixing this issue would be to clear the df bit, set the MSS as you would normally, and lower the MTU.… Read more