1. Home
  2. Cisco Systems
  3. Nu0 interface

Nu0 interface

Hi,

I should start out by saying that I am not a Cisco expert by any stretch so if I state the obvious in the post, please bear with me. I have set up a third party monitoring tool to monitor my Cisco 2960G switches - there are 3 - one core (4560) and 2 2960G's that are on the same VLAN. The software is showing a lot of traffic on the Nu0 interface on one of the switches. From what I have read this is a "blackhole" port where discarded packets go. The concern I have is that about every 2 hours or so the "Traffic Out" counter reads:

2979210990 bytes (this is the highest - the 24 hour average is 978751120. "The Traffic In" counter is 0. I have no idea what this means, possibly a "rouge" host plugged into the switch (i.e, bad NIC on a workstation) I've done a lot of googling on this and am still unsure as to how to interpret this. Any help is appreciated!
Reply to
charlescorps
Loading thread data ...

I am guessing Nu0 is Null0, which is used for a plethora of reasons, but one of the bigger reasons is to ensure there is an IGP route in the routing table to match an EGP summary-route being advertised by BGP. Or its used by EIGRP to ensure that loops are avoided by sending packets destined for IPs that may not be active to the bit-bucket. Please paste your routing table (sh ip route), and you may want to change any external IPs to a.b.c.d to avoid disclosing any specific information. Or paste your routing statement from show run. Either way, sounds like perfectly normal behavior, you probably have a summary (192.168.0.0/16 or 10.0.0.0/8 would be private examples) and a good chunk of that is probably not in use.....therefore null0 ensures your routers discard that traffic and don't keep trying to find a home that doesn't exist.

Reply to
Trendkill

Forgot the link:

formatting link

Reply to
Trendkill

Nu0 is in-fact the blackhole, it stands for Null0. Packets are never received here, they can only be sent there and anything sent to Null0 is discarded. This is a routing issue, check your routing table (show ip route), which will tell you what routes are being sent to Null0. If your default route points to Null0, this means you don't have one and any packet with a destination in the routing table will be discarded. You also don't specify what routing protocol (if any) is being used, which can also cause routes to be sent to Null0 depending on your configuration (i.e. "auto-summary"). Basically though, if everything seems to working fine I would not worry about it.

Reply to
Thrill5

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.