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Category: CCNP

Tclsh script examples: how to generate router loop interfaces with IPv4 addresses

This example shows how to generate 254 loop interfaces with assigned ipv4 addresses up to The code is:

for {set i 0} {$i < 256} {incr i} {
ios_config "int loop $i" "ip address 172.16.$i.1"
ios_config "end"

and you may just simply copy and paste it into a Cisco router CLI. Therefore first run tclsh within of privileged EXEC mode

CCNP – Route redistribution – Simulating routing loops


Teaching CCNP ROUTE courses and describing the problematic of two-way routing redistribution I need an example describing the case which point out to the process of creating a routing loop. This example shows how such routing loop may arise using badly configured redistribution process andusing bad seed metric. The topology with two redisitributing routers creates routing feedback (two way, multipoint redistribution). The loop is created for packets routed from the ospf network to the destination network.

VTP (+DTP) security threat – myth or fact?

There is some confusion and misunderstanding regarding VTP (VLAN Trunking Protocol)+DTP (Dynamic Trunking Protocol) and this article will hopefully make it all clear. Most of the confusion comes from the statement that VTP is always a security threat to your network. There are even some CCIE material which I find to be wrong on this topic so my advice here is to test everything by yourself.