CCNP Recertification

Posted on Friday, 2nd May 2008 by sean

The other day I ran into some problems with a default route, which prompted a discussion with co-workers, which led me to look up the behavior of redistributing a static default route into a dynamic routing protocol. Take, for example, the following ! default route ip route 0.0.0.0 0.0.0.0 1.1.1.1 ! pick your routing protocol [...]

Tags: bgp, eigrp, isis, ospf
Posted in Routing | Comments (1)

Posted on Friday, 24th August 2007 by sean

When an EIGRP enabled router loses a neighbour, all routes through that neighbour need to be re-evaluated. Any feasible successors are immediately promoted to successors, and any other routes go active. “go active” means that the EIGRP router asks all its neighbours if they have a route to the destination. Each router that is queried [...]

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Posted on Thursday, 21st June 2007 by sean

Earlier I looked at stub areas. One problem we found was that you can’t have an ASBR in a stub area — no “redistribute static” on any external links. What a pity! Not so stubby areas get around this by allowing the ASBR to exist and propagate LSAs. The problem is that stub areas can’t [...]

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Posted on Wednesday, 13th June 2007 by sean

Earlier I gave a description of OSPF special areas (stubby, not so stubby, totally stubby). Here’s some examples to back it up. I use dynagen to simulate my environment, it’s so good I ended up selling my routers! Here’s the config: autostart = false [localhost] [[7200]] image = ..\images\c7200-jk9s-mz.122-40.bin npe = npe-400 ram = 192 [...]

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Posted on Saturday, 19th May 2007 by sean

I’ve been doing some prep for BSCI lately and am back into OSPF. One thing I ran into was keeping the various types of areas straight, and remembering all the restrictions that come with them. It wasn’t helped by an unclear description I read in the book, which caused me to really get into the [...]

Posted in Routing | Comments (4)

Posted on Tuesday, 11th July 2006 by sean

Implementation of multicast, at least for the purposes of the CCNP BCMSN exam, is pretty simple. Cisco has the Multicast Quick-Start Configuration Guide which goes over many different ways of doing it. The exam seems to only care about Protocol Independent Multicast (PIM), which uses the router’s routing table to determine whether or not a [...]

Posted in Routing, Switching | Comments (1)

Posted on Monday, 26th December 2005 by sean

Routing protocols, especially EIGRP, are very good at finding ways around network faults. I remember one time where we had a LAN failure, but a router on each side of the break happened to be connecting to the same site over two different WAN links, and EIGRP shunted all the LAN traffic through that office [...]

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Posted on Tuesday, 22nd November 2005 by sean

I’ve read and heard that there are some idiosyncrasities about connected routes and IS-IS when redistributing. Most recently while reading “Optimal Routing Design” and also when talking with a friend who was studying for the BCSI exam. When redistributing out of IS-IS, some routes are missing, notably connected routes. Let’s investigate. A simple three-router network: [...]

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Posted on Monday, 14th November 2005 by sean

A simple one to get the posts flowing… BGP peer groups simplify the configuration of BGP neighbors by moving the template into a configuration. For example, look at R3: router bgp 1 no synchronization bgp log-neighbor-changes network 10.3.3.0 mask 255.255.255.0 network 10.50.0.0 mask 255.255.0.0 neighbor 10.50.0.1 remote-as 1 neighbor 10.50.0.2 remote-as 1 neighbor 10.50.0.2 route-reflector-client [...]

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Posted on Thursday, 13th October 2005 by sean

Continuing on with route reflectors, here is an example. The specifics of the topology don’t matter too much. R0, R1, R3, and R6 are all in AS1 and happen to be sitting on the same LAN segment. R1 connects to R2 over a serial line, and R2 is in AS2. R3 is the route reflector, [...]

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