Always Networks Blog

CCIE Written Blueprint: 1.1.d Explain IP operations

1.1.d (i) ICMP unreachable, redirect

ICMP Unreachable

Generated by a host or gateway to indicate that the packet was discarded as the destination is unreachable. It will not be generated for multicast traffic. It is sub-divided into 15 types as follows:

Code Value Message Subtype Description
0 Network Unreachable The datagram could not be delivered to the network specified in the network ID …
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CCIE Written Blueprint: 1.1.c Explain general network challenges

Wed 13 August 2014

1.1.c (i) Unicast flooding

One of the main causes is asymmetric routing. This is covered in 1.1.c(iii). Useful document here: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563-143.html

The primary impact of this is that all hosts connected in that VLAN receive the traffic. Suppose two 10gig servers are communicating, and asymmetric routing is taking …

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CCIE Written Blueprint: 1.1.b Identify Cisco express forwarding concepts

Sat 09 August 2014

Cisco Express Forwarding - that thing that "just works". You don't really need to know anything about it...until it stops just working. Here's my notes.

1.1.b Identify Cisco express forwarding concepts

CEF maintains its tables to facilitate the routing of packets purely in memory, with no CPU overhead.

If the CPU can’t handle the packet, it “punts” the packet to the software …

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CCIE Written Blueprint: 1.1.a Describe basic software architecture differences between IOS and IOS XE

Wed 06 August 2014

I have had an idea. I decided the best way to focus on the CCIE topics to study for the written exam (and try and reign myself in from just reading the "interesting" stuff!) is to aim for the exam topics...if I know all of them, how can I fail?

The exam topics are available here: Cisco CCIE Written Exam Topics

I have copied …

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Non-Transitive or Transitive? - BGP Path Attributes

Sun 22 June 2014

I was reading about BGP Path Attributes and came across this table (thanks to http://netcerts.net/bgp-path-attributes-and-the-decision-process/):

--------------------------------------------------------------------- ---------------------------------------------------------- LIST OF BGP PATH ATTRIBUTES Attribute Name Category / Class ORIGIN Well-Known Mandatory AS\_PATH Well-Known Mandatory NEXT\_HOP Well-Known Mandatory LOCAL\_PREF Well-Known Discretionary ATOMIC\_AGGREGATE Well-Known Discretionary AGGREGATOR Optional Transitive COMMUNITY Optional Transitive MULTI\_EXIT\_DISC (MED) Optional Non-Transitive ORIGINATOR\_ID Optional Non-Transitive CLUSTER LIST Optional …
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OSPF Network Types - A Neighbour Killer?

Yep, I spell neighbour the British way. No, I don't do it in IOS. But I should be able to. :)

While going over some OSPF stuff today, I found a statement saying that OSPF network types have to match for routers for form a neighbour relationship. Because I'm a nerd, I questioned this. My reason? I don't remember OSPF network type being a field in …

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IronPort Proxy Logs - Viewing something useful

One of the most frustrating things about working with the IronPort proxy servers is how difficult it is to view the log files. The logs are stored on the proxy server in text files - these are often multiple gigabytes in size - downloading takes an age, and opening them in Windows is next to impossible.

Fortunately, if you SSH to the management interface of the proxy …

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Checkpoint VPN Error: According to the policy the packet should not have been decrypted

Magnifying Glass

I encountered an issue recently while trying to allow access to a new subnet over an existing VPN. The far end device was a Cisco router, and had an access list matching an entire class A subnet which was applied to the crypto map. The traffic destined for the new subnet was arriving at our firewall, and showing in the logs as dropped, with the …

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Archiving and deleting IOS images

Thu 16 January 2014

I recently had the need to upgrade the IOS on a Cisco switch (think it was a 3750). As usual, the flash was too small so I needed to remove (and archive/save) the old IOS before putting the new one on. A lot of the images these days include HTML and other things, so are contained in folders rather than a single file. So …

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Bulk DNS Lookup in Windows Powershell – Better than NSLookup!

So firstly, I've just seen that's its been over 3 months since I last posted - for anyone that looks back here regularly, Sorry! Thanks for coming back though, I am going to try to be more active.

I started a new job at the end of August as a Network Engineer as part of the network team in a data centre supporting multiple European sites …

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