Jul 012008

VLAN Interfaces are required in network scenarios where you have different VLANs and need Inter-VLAN switching on Layer3 (Routing capable) switches. Every VLAN that needs to be routed should have a VLAN interface.

Let's say we have VLAN 10 which hosts the subnet subnet, VLAN hosts subnet and VLAN 30 hosts subnet. For Inter-VLAN routing to work, we need to have a VLAN interface setup for each of these VLANs and configured with an IP address from the same subnet which will be the default Gateway for that subnet. Lets say,, are the IP addresses for VLAN Interfaces of VLAn 10,20,30 respectively.

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Jun 192008

Cisco Routers and Switches with L3 routing functions are seen to have problems with High CPU usage when SNMP is enabled. This can range anything from 15% to 40%. According to Cisco, these are low priority processes and other priority processes requiring CPU cycles are given priority over these processes and this level of CPU utilisation can be is normal. However, it is always better to be safe than sorry and get the CPU utilisation caused by SNMP to bare minimum so as to ensure the Routers function smoothly.

The reason behind the High CPU usage can be caused by the Network Management Server (SNMP Server) like HP Openview querying for the Routing Tables and ARP tables to learn about other networks  or querying for certain MIBs which can be resource intensive.

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May 072008

When troubleshooting a problem with Access Control lists, one of the things you would want to do is to clear the counters on the ACL matches.

In Cisco IOS, you can clear the ACL Matches counters as follows:

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Apr 252008

Another beginner tip that can be useful!

When you work on the Cisco Router or Catalyst Switch console, it would be annoying to have the console or terminal (telnet/ssh) logs to pop in between your commands. This can be even more irritating when it is busy switch or a router spitting messages continuously.

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Apr 182008

With Cisco IOS version 11.2, Cisco introduced the Named ACLs. Named ACLs are Standard or Extended ACLs which are give names instead of a ACL number. Technically, other than giving a name to the ACL there isn't any other difference when it comes to the functionality as in Standard or Extended ACL.

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Apr 182008

Extended ACLs are advanced than the Standard ACLs. Unlike the Standard Access Lists where it checks only the Source IP Address to control the flow of the packets, Extended ACLs can check the

Source & Destination Address

Protocols (IP,ICMP,TCP,UDP)

Source & Destination ports

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Apr 162008

While we saw here how to setup a Cisco Router as a Caching/Forwarding DNS Server. We can now look at how to make your Cisco Router as an Authoritative DNS server. When configured as an authoritative name server for its own local host table, the router listens on port 53 for DNS queries and then answers DNS queries using the permanent and cached entries in its own host table.

Careful consideration has to be given as this can consume considerable amount of resources like CPU cycles on the Cisco Router. If you are a small network and realise your Cisco ROuter is under utilised then there is a good business case to turn your router into a DNS server.

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