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Posts tagged Tutorial
%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:
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.
OSPF Summarization
Jun 18th
This post is about OSPF Summarization. We’ll be using a familiar topology and going over two ways to summarize with OSPF.
There are two conventional ways to summarize networks in OSPF, we can use the “area range” command and the “summary-address” command. “Area range” is used on the ABR to summarize networks between areas. The “summary-address” command is used on the ASBR to summarize external networks.
Here’s the topology:
I’m not going through the basic OSPF config, so assume everything is configured as the diagram suggests. On R1 I’ve added Lo11-14 and used “ospf 100 area 0″ under the respective interfaces. On R2 I’ve added Lo15-18 and used “redistribute connected subnets”. Let’s look at the RIBs on a couple routers:
First we’ll check out “sh ip route” on R1:
R1#sh ip route
...
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O E2 2.2.2.2 [110/20] via 10.1.123.2, 01:13:32, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O E2 3.3.3.3 [110/20] via 10.1.123.3, 01:12:38, FastEthernet0/0
4.0.0.0/32 is subnetted, 1 subnets
O E2 4.4.4.4 [110/20] via 10.1.123.3, 01:03:47, FastEthernet0/0
172.30.0.0/24 is subnetted, 4 subnets
O E2 172.30.6.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2 172.30.7.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2 172.30.5.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2 172.30.8.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
C 192.168.4.0/24 is directly connected, Loopback14
10.0.0.0/24 is subnetted, 2 subnets
O IA 10.1.34.0 [110/74] via 10.1.123.3, 01:12:39, FastEthernet0/0
C 10.1.123.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Loopback11
C 192.168.2.0/24 is directly connected, Loopback12
C 192.168.3.0/24 is directly connected, Loopback13 |
Lots of routes here. The ones to note are the 172s showing as External Type 2, which are R2′s loopbacks. Also notice our connected loopbacks.
OSPF Area Types: Not So Totally Stubby
Jun 18th
This is the last post in a series about OSPF Area Types. Today we’ll go over Not So Totally Stubby Areas. We’ll be using the same topology as the NSSA post, but this time we will inject a specific route (40.40.40.0/24) from the ASBR (R4) instead of a default.
Quick refresher, OSPF Not So Totally Stubby Areas have intra-area routes (Type 2 LSAs) and also external routes in the form of Type 7 LSAs, which are converted to Type 5 LSAs by the ABR. No inter-area routes (Type 3 LSAs) are permitted in a Not So Totally Stubby Area and a default route will be injected by the ABR.
(For more detailed information on LSAs and Area Types, check out this post.)
Here’s the topology:
OSPF Area Types: NSSA
Jun 15th
Today we’ll go over Not So Stubby Areas (NSSA). We will be using a slightly different topology here, we will make R4 an ASBR with a connection to the internet.
Quick refresher, OSPF NSSAs have inter and intra-area routes (Type 2 and Type 3 LSAs) and also external routes in the form of Type 7 LSAs, which are converted to Type 5 LSAs by the ABR.
(For more detailed information on LSAs and Area Types, check out this post.)
Here’s the topology:
I’m not going through the basic OSPF config, so assume everything is configured as the diagram suggests. I’ve also redistributed loopbacks on each router (“redistribute connected subnets” under the OSPF process) to give us some external routes, and I added 34.34.34.34/32 to Area 34 so we have an intra-area route to look at. I’ve also added a static default route on R4 which I’m injecting into OSPF with the “default-information originate” command. Let’s look at some show commands BEFORE we make area 34 an NSSA:
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