CCIE Candidate

Holddown timer - when a new metric is received for a route already in the routing table, the holddown timer starts counting down, 180 seconds by default. The new metric won’t get installed until the holddown timer expires.
Invalid timer - Starts counting down every time RIP hears a route advertised. If the route [...]

The rest of this chapter hits the highlights of configuring RIP. It isn’t brain surgery, but I was a little surprised at just how much flexibility there is in RIP. Even though using hop-count as a metric is hokey, and even though either of my Grandmas (who don’t know IP and are both [...]

When choosing a routing protocol for a network, understanding how routes converge and what routers do to prevent a routing loop is important. RIP is not exactly an all-star when it comes to convergence. Not only is RIP kind of dumb in how it computes the best route, it’s also slow to converge [...]

A few days of dealing with life, and back at the CCIE study. I built a 2621XM and threw it on the network here for something to do. It needed new IOS, put 12.4.12 Advanced Security on it. Not sure it’s the most appropriate feature set, but it’ll do for now. [...]

Last push for tonight, then I’m off to sleepy-land. Tomorrow, we’re getting a Nor’easter here in NH, meaning I have A Long Day coming.
policy routing - configuring the router to make a forwarding decision on some criteria other than the destination address.
process switching - the slow process whereby the router performs several steps to [...]

Policy routing is what you do when you need to be absolutely sure that your network is difficult to troubleshoot. Uh…I mean to say…policy routing is what you do when you want the router to make a forwarding decision based on something other than the destination IP address. And, from lots of practical [...]

Just before we get to multilayer switching a couple of comments on router behavior when routing classfully versus classlessly.

In classless routing, if there’s a default route and there’s no specific prefix to match up, then the default route will be used to forward the packet.
In classful routing, if there’s a default route and there’s no [...]

This section of the chapter discusses how the router will build adjacency information, and also discusses details about frame-relay inverse ARP.
In order for the router to build his adjacency table, he uses ARP regularly. We know what ARP is all about, right? It’s the protocol used by a device that knows the IP [...]

Looking up the appropriate route in the routing table and figuring out where the packet needs to go next are the most challenging parts of this process. They use the most CPU. Cisco has created some special methods (switching paths) to make these processes efficient: fast switching and Cisco Express Forwarding (CEF). [...]

This chapter covers the basics of IP forwarding - what a router does when it gets a packet. How does the router make its fowarding decision?
“Forwarding” and “routing” are synonyms. Routers follow a generic logic flow to determine how to route a packet (assuming inbound on an ethernet interface):

The router receives an inbound [...]

It’s been a miserable couple of weeks.  I was miserably sick last week, and before that I was nutty with work projects needing me to be there in the middle of the night.  But I’m back.  Life has settled sort of back into the normal groove.  And I have 3 chapter’s worth of write-ups to [...]