OSPF Loopback interfaces are treated as a stub host and will only have a 32 bit host route on the other devices no matter how the subnet mask is entered under the OSPF network statement.
We will explore how we can disable this default behaviour using a few methods.
As you can see, R4 is configured with IP address 192.168.1.4 and R5 with 192.168.1.5.
We have loopback interfaces added with /24 subnet masks on R4 and R5
R4#sh run int loop0
Building configuration...
Current configuration : 61 bytes
!
interface Loopback0
ip address 4.4.4.4 255.255.255.0
end
R4#
R5#sh run int loop 0
Building configuration...
Current configuration : 61 bytes
!
interface Loopback0
ip address 5.5.5.5 255.255.255.0
end
R5#
We are running basic OSPF Configs as follows…
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I was going through 6to4 tunnel configs and thought I’d post some info on converting IPv4 address into IPv6 Address. This is pretty straight forward and its obviously involves HEX conversion.
Here, I will convert the address 192.168.25.234
First we divide each octet by 16 and write down the remainder, primary school maths! 
192 ÷ 16 = 12 remainder 0
168 ÷ 16 = 10 remainder 8
25 ÷ 16 = 1 remainder 9
234 ÷ 16 = 14 remainder 10
We also know that HEX has the following Values
A = 10
B = 11
C = 12
D = 13
E = 14
F = 15
So we can write 192.168.25.234 into HEX like so… C0A8:19EA
Now we will change the HEX Address C0A8:19EA into regular IPv4
C0 = (12 x 16) + 0 = 192
A8 = (10 x 16) + 8 = 168
19 = (1 x 16) + 9 = 25
EA = (14 x 16) + 10 = 234
QED
I was going through some TCP windowing over a high speed WAN link, and thought it might be worth to post some info regarding this.
We do face a typical problem of having slow speed file transfer even if the dedicated pipe is capable of supporting it.
There are three factors which affects this…
- TCP Window Size.
- Round trip latency of the circuit.
- Bandwidth of the circuit.
Maximum throughput you can get from a line with 10ms latency and a TCP window of 32KB can be calculated with…
32KB –> 32 x 1024 x 8 = 262144 Bits
262144 ÷ 0.01 = 26214400 bps = 26.2144 Mbps
Let’s say we have an OC-3 line, which is at 155 Mbps and a round trip latency of 10ms, and we need to calculate the TCP window size to maximize the throughput…
155.52 x 106 = 155520000 bps
TCP_WINDOW = 155520000 x 0.01 = 1555200 Bits = 194400 Bytes = 189.84375 KB
Hope I got the calculation right
Multilink is a way of bundling more than one PPP WAN links and bundle them together into one logical interface. This is one of the method you can implement when you have a primary and secondary links in place and create a logical conduit where both links are present.
This method can be used to load balance data across the links and at the same time it will support redundancy when one one link fails.
I have tested this configuration on 2 of my 2500 Routers and I haven’t tested these on more recent IOS so some commands may vary. I will be checking them on a pair of 3845s/2691s on a later date and will add my findings to this post.
As you can see the Diagram, I am using R4 and R5 which are connected via Serial 0 and Serial 1 respectivly.
R4 Configuration
Multilink Interface
interface Multilink1
ip address 172.16.1.1 255.255.255.252
ppp multilink
ppp multilink links maximum 2
ppp multilink links minimum 1
ppp multilink group 1
Serial 0
interface Serial0
no ip address
encapsulation ppp
clock rate 64000
ppp multilink
ppp multilink group 1
Serial 1
interface Serial1
no ip address
encapsulation ppp
clock rate 64000
ppp multilink
ppp multilink group 1
As you can see, since this is a lab scenario, I have set the clock rate on the link. If you are setting this up on a WAN link, you don’t need to do this as the circuit provider would be setting this up for you.
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Frame-Relay is one of the core concept of networking and nowadays it is one of the under appreciated part on modern networking.
I am just going to go over how to configure it using 4 Routers and one will act as a Frame-Relay cloud.
Before I go ahead and explain the configuration, You can see the below diagram which represents the physical topology.
As you can see, FRS is connected via…
Serial 1/1 to R1 Serial 1/0
Serial 1/2 to R2 Serial 1/0
Serial 1/3 to R3 Serial 1/0
Now we have got the physical topology clear, we will move onto Frame-Relay configuration and the DLCI assignment.
As you can see the DLCI configuration…
On R1 : DLCI 122 is connected to R2 and DLCI 123 is connected to R3
On R2 : DLCI 221 is connected to R1
On R3: DLCI 321 is connected to R3.
If you are configuring the DLCI then I would suggest you follow this method of numbering, Because from the DLCI Number, I can simply distinguish where the DLCI is connecting to…
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If you want to jump between servers without any password authentication but you still need security here is what you have to do.
There are two ways of achieving this:
On Debian/Ubuntu you can just type:
$ cd $HOME
~$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key:
Press Enter each question and you will have a pair of keys ready to use.
You will be asked for a passphrase but if you do not want to insert anything just leave it blank.
This procedure will create one private and one public key.
$ ls .ssh/
id_rsa id_rsa.pub known_hosts
The private key must be secured on your box whereas the public key can be copied across
~$ ssh-copy-id -i .ssh/id_rsa.pub 192.168.1.30
At this time you will be asked for the password but once the key has been copied you will be able to ssh just perfectly.
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