Examining RIP-2 Terms and Applying Those Terms
Think you’re ready for CCNA? Could you, from memory, do the exercises listed in last week’s post? Could you do them and come up with the same key points as my sample answers in today’s post? Today I’ll look at the answers to the exercise posted last week. And while I’m sure you have used your own wording, did your answers include the same substance as my answers below? Also, for further review, I’ll give examples, and pose some questions back to you.
Note that last week’s exercise had two parts: terminology review and configuration review. This week, we’ll focus on reviewing the terms and the concepts behind them.
Terminology Review and Application
As a brief review, the first task was to review some terms. The goal was to determine whether you could both recall and apply the key facts about each term, as applied to RIP-2, given the context of a sample network diagram. The terms were:
Distance Vector, RIP Update, Split Horizon, Poison Route, Periodic Update, Routing Table, metric, passive interface, auto summary
The figure was:
I’ll hit the key points in the format definition here, but focus on examples. I’ve also added some questions – post your ideas and answers, and ask questions back to me if you like.
Split Horizon:
This is a behavior of a routing protocol that can limit what subnets the router advertises in its advertisements on an interface-by-interface basis. In particular, if the router’s route lists interface X as the outgoing interface, that router doesn’t advertise a route for that subnet out interface X.
For example, R3 is connected to network 192.168.1.0. The figure doesn’t list the subnet mask, so let’s assume it’s /24. R3 advertises 192.168.1.0/24 to R2, so R2 adds a route to its routing table, destination network 192.168.1.0/24, outgoing interface S0/0/0, next hop IP address 172.16.6.2. Split Horizon logic on R2 prevents R2 from advertising about 192.168.1.0/24 is updates R2 sends out S0/0/0 back towards R3.
Food for thought: does R3 advertise about 192.168.1.0, in updates it sends out it’s own F0/0 interface?
RIP Update
A message defined by RIP that lists routing information, including the subnet number, subnet mask, and metric.
Using the same example as above, R3 sends a RIP Update message to R2. That message lists all the subnets that R3 decides to advertise to R2. R3 lists 192.168.1.0, mask /24, metric 1.
Food for through: what will be in the RIP Update sent by R3 to R2, assuming all is up and working? Feel free to make and state any assumptions you make, eg, like my assumption of a /24 mask on R3’s F0/0 interface.
Distance Vector
One of several routing protocol categories, into which RIP happens to fall, with the term being based on some of the core functions of these protocols. The update info includes the “distance” of each route, commonly called the metric. Each advertised route also includes a vector, either implied or listed in the update – in other words, that next hop info about the route, and the outgoing interface (direction).
In this case, again assuming mask /24 on R3’s F0/0 interface, with R3’s advertisement of 192.168.1.0/24 to R2, R3 listed a distance (metric) of 1. R2 can assume a vector towards this network of R2’s S0/0/0 interface and R3 as the next hop router.
Food for thought: when you thought about the above description, did any other facts about RIP-2 Updates come to mind? (It’s a minor point.)
Poison Route
A route listed in a routing update that the routing protocol purposefully assigns a metric value considered to be infinite, meaning that the route has failed.
Say R3’s F0/0 interface is shutdown as an experiment. R3 then sends a poison route to R2, meaning R3 sends an update message, listing 192.168.1.0/24, with an infinite metric.
Food for thought: how big is infinite to RIP-2? In other words, what is that metric listed in the poison route?
Periodic Update
A routing protocol behavior in which the routing protocol sends Update messages on a regular interval (the period).
RIP sends periodic Updates every 30 seconds or so. In this case, that Update from R3 to R2 that I’ve mentioned above so many times… R3 sends that every 30 seconds. With RIP, it’s a full Update – in other words, it lists all the routes, every message, rather than just those that have changed.
Food for thought: Compare this one to EIGRP: Does it have periodic Updates? Would you consider OSPF to have periodic updates?
Routing Table
A list kept by routers that lists the best IP routes known to the router. Each entry lists a subnet or network, the associated mask, possibly the outgoing interface and/or IP address of the next-hop router, and other related info.
Example: R2 will list a route for 10.0.0.0/8, outgoing interface S0/0/1, next-hop router 172.16.5.1. IT also lists a route for 192.168.1.0/24, outgoing interface S0/0/0, next-hop route 172.16.6.2. R2 also lists three connected routes. The connected routes list the local interface type/number, but no next-hop router IP addresses.
Food for thought: Without looking at a sample of “show ip route”, what else do you remember seeing in entries of the IP routing table on a Cisco router?
Metric
A numeric value that a routing protocol uses to imply how good a particular route is, so that when a router learns more than 1 route for the same network/subnet, the router can choose the best route as the route with the lowest metric.
Example: RIP uses hop count as metric. If R3 continues to advertise 192.168.1.0/24 to R2, R2 lists a metric of 1 for this route. If R2 then advertises 192.168.1.0/24 to R1, R1’s route for 192.168.1.0/24 will have metric 2. If we put a direct link between R1 and R3, and R3 advertised 192.168.1.0/24 directly to R1, that route would be a metric 1 route from R1’s perspective. As a result, R1 would instead use this new metric 1 route instead of the old metric 2 route.
Food for thought: What show command identifies the metric for a route learned using RIP?
Passive Interface
An interface local to a router, for which the routing protocol does not send routing protocol messages out that interface.
Example: Assuming the figure shows the entire topology, each different LAN has only 1 router attached. None of the routers needs to send routing updates out their LAN interfaces, because no other router exists on those LANs. For instance, R3 could make its F0/0 interface passive, and not send RIP Update messages out that interface.
Food for thought: What happens in this network if R3 makes its S0/0/1 interface passive instead of making F0/0 passive?
Auto Summary
Saving the most misunderstood of these terms for last… The process by which a routing protocol chooses to advertise a route for the entire classful network (the class A, B, or C network), instead of individual subnets of that network, under certain conditions. Specifically, the condition is that when advertising out an interface whose IP address is in network X, automatically summarize routes from networks other than network X.
For example, consider R3 again, but now let’s assume that R3’s F0/0 interface uses a mask of /27. That means R3 has a connected route for subnet 192.168.1.0/27. With auto summary enabled, R3 will not advertise a route for 192.168.1.0/27, but instead advertise a route for 192.168.1.0/24 – the class C network in which subnet 192.168.1.0/27 resides. With auto summary disabled, R3 would instead advertise subnet 192.168.1.0/27.
Food for thought: Assuming auto summary is enabled on all three routers, what auto summary actions will R1 take? R2?
Related Posts:CCENT and CCNA Self-Assessment Wonderings (Plus an Exercise)
Why is Passing CCNA so Difficult?
Index Page: Why CCNA Seems Difficult
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