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Colby
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Posts by Colby
%C4K_EBM-4-HOSTFLAPPING
Aug 26th
This means loop!
A coworker came to me with an issue today. He asked if I’d ever seen this before and showed me:
.Aug 25 22:44:29 UTC: %C4K_EBM-4-HOSTFLAPPING: Host 00:00:85:DE:15:61 in vlan 54 is flapping between port Gi2/4 and port Gi2/2 .Aug 25 22:44:59 UTC: %C4K_EBM-4-HOSTFLAPPING: Host 00:00:85:DE:15:61 in vlan 54 is flapping between port Gi2/4 and port Gi2/2 .Aug 25 22:47:42 UTC: %C4K_EBM-4-HOSTFLAPPING: Host 00:00:85:DE:15:61 in vlan 54 is flapping between port Gi2/4 and port Gi2/2 |
I said “it means there’s a loop, give me the switch IP”. Then I began the mission of tracking down the loop. This was a pretty large site, but luckily I only had to go through a couple switches. Unfortunately this happened a couple hours ago and I didn’t save my work so we won’t be able to go through the real steps.
To track down a loop, you start with the “show mac-address-table address [flapping mac]” command:
SW1#sh mac-add add 0000.85de.1561
Unicast Entries
vlan mac address type protocols port
-------+---------------+--------+---------------------+--------------------
1 0000.85de.1561 dynamic ip GigabitEthernet2/2
54 0000.85de.1561 dynamic ip GigabitEthernet2/4 |
We see that the MAC is coming in on port gi2/2 and gi2/4. One port will lead us to where that MAC is plugged in and the other will lead us to the loop. Pick a port and start working through. This is where CDP comes in handy:
HSRP Tutorial
Aug 21st
Today’s topic is HSRP (Hot Standby Routing Protocol). HSRP is a Cisco proprietary “First Hop Redundancy Protocol”. It is typically used for redundancy at the first hop from a client segment. It is used with two or more routers in a group who share a virtual IP address. One router is active at a given time and will reply to ARP requests. In this example, we have R1 and R2 in standby group 100 with a virtual IP of 192.168.100.1. This IP will be the default gateway for all hosts in VLAN 100. Here is the topology:
This is a basic topology, both R1 and R2 have connections to the internet. They are running HSRP on their FastEthernet 0/0 interfaces. Here’s the basic HSRP config:
R1(config)#interface fa0/0 R1(config-if)#ip address 192.168.100.2 255.255.255.0 R1(config-if)#standby 100 ip 192.168.100.1 R2(config)#interface fa0/0 R2(config-if)#ip address 192.168.100.3 255.255.255.0 R2(config-if)#standby 100 ip 192.168.100.1 |
Very simple so far. We use the “standby [0-255] ip [virtual ip address]” command.
Let’s verify the config:
Passed ITILv3!!!
Aug 18th
It’s finally over, haha. Now I can get back to Cisco stuff! That was a pretty tough test. The material itself wasn’t easy for me to get into, but eventually I was able to push through it and things started clicking. I feel like I learned some valuable stuff here and there, but all in all, I’m glad it’s over.
Hopefully I’ll come up with some good topics to post about soon.
IOS Macros
Aug 16th
Here’s another short (but hopefully useful) post. We’ll be going through IOS Macros.
I’ve never used IOS Macros before, but I was asked about a problem today, and a macro seems to be an ideal solution. A friend of mine is an engineer for a service provider with a very large network. He has been tasked with implementing passive interfaces as the default for OSPF across the network. Most of the devices which will be modified rely on OSPF for management connectivity. When he runs the “passive-interface default” command, he will lose connectivity before he is able to run “no passive-interface [interface]” to restore connectivity. Macros tell the router to run the predetermined commands for us, which will save us from getting locked out.
The topology is simple and not worth a diagram. R1 and R2 are connected via their FastEthernet0/0 interfaces. They are running OSPF on this interface.
Start Your Own Blog
Aug 2nd
I had an idea last night and decided to run with it. I’m starting a site to host tech blogs, like wordpress or blogger, but strictly for tech stuff. Not sure if people will be into it, but figured why not. It’s all WordPress, and will likely stay that way. So far all I have is the name and a barebones setup. I’d like you guys to tell me what you think and maybe test it all out.
It’s free, I get nothing from it and probably never will. I will be tinkering all day, so themes will change, plugins will be added, etc. Also, it’s hosted from my house, so don’t expect it to be super fast or five nines.
Also, please tell me any features that should be added or any other thoughts. I’m trying to think of a new name as well, something less cert-centric, but everything I come up with is taken. Let me know what you guys think.
Secondary IP Addresses
Jul 25th
This is going to be a short post. I feel guilty for neglecting my blog (stupid ITIL), so I’m throwing this one together real quick.
There was a thread on Tech Exams recently, the poster was trying to figure out how to connect two subnets to a single Ethernet interface on a router. This was due to changing the address scheme within his company. Here’s what it looked like:
Very simple topology. The PC is part of the new address scheme and the printer is using a static IP from the old scheme. One of the easiest solutions here is to use a secondary IP on the Fa0/0 interface of R1. Here’s how it works:
EDGE(config)#int fa0/0 EDGE(config-if)#ip add 192.168.10.1 255.255.255.0 EDGE(config-if)#ip add 10.1.1.1 255.255.255.0 secondary |
Very simple commands. Let’s verify:
interface FastEthernet0/0 ip address 10.1.1.1 255.255.255.0 secondary ip address 192.168.10.1 255.255.255.0 |
It worked, the interface now has two IPs. Now we would just configure the IPs on the hosts (or DHCP server) using the proper subnet and gateway. Once the hosts are configured, everything will work as desired.
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