CCIE – STP 802.1D – Fundamentals

INTRODUCTION For resiliency purposes, Layer2 topologies are designed and deployed in a physical loop. Unlike the IP protocol which uses the TTL field as a built-in mechanism for preventing packets from looping endlessly through the network, the Ethernet protocol does not provide such facility and therefore, unless we can prevent a layer2 loop, the following problems would occur: Broadcast storms – endless broadcast of frames through the entire L2 domain Hosts end-up receiving duplicates frames from different switches Unstable mac-address tables due to continuous host mac address learning on different ports The STP (Spanning Tree Protocol) was developed to avoid…

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Admin Distance Bug?

Granted, this is a duplicate post as the one posted by Jeremy Stretch here. When I first read his post, I just couldn’t believe it so I thought I’d lab it myself. However, do notice that my lab here is just slightly different: I am advertising two networks (different from Jeremy’s lab) The two routing protocols have been set with identical administrative distances (Jeremy does this for the advertised route instead) I am assuming that the IOS version running on my routers is more recent (potentially different from Jeremy’s lab) Since I’m using Gig interfaces, I’ve updated the OSPF auto-cost ref. bandwidth…

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The Discard Route

When we summarise a route, Cisco IOS will inject a discard route. It is all nice to understand what Cisco IOS does. However, we ought to ask the question: Why? This blog answers this question.

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