Chapter 8THE ROUTING TABLE: A CLOSER LOOK Reporters: Norly A. Estopa Namerto C. Medura Andy D. Ando Junrey F. Layao
THE ROUTING TABLE: A CLOSER LOOKObjectives: - Describe the various route types found in the routing table - Describe the routing table look-up process - Describe the routing behavior in routed networks - Determine the parent route and child route
This chapter analyzes the lookup process of the routingtable. Discuss classful routing behavior, as well as classless routingbehavior, which uses the no ip classless and ip classless commands. Also, will take a closer look at the routing table. This chapterfocuses on the structure of Ciscos IP routing table. Examine theformat of the routing table and learn about level 1 and level 2 routes.
Routing Table EntriesThe sample routing table in thefigure consists of route entries fromthe following sources:Directly connected networksStatic routesDynamic routing protocolsThe source of the route does notaffect the structure of the routingtable. The figure shows a samplerouting table with directlyconnected, static, and dynamicroutes. Notice that the Note: The routing table hierarchy in Cisco IOS was172.16.0.0/24 subnets have a originally implemented with the classful routingcombination of all three types of scheme. Although the routing table incorporates bothrouting sources. classful and classless addressing, the overall structure is still built around this classful scheme.
Level 1 Routes Routers R1 and R3 already have their interfaces configured with the appropriateIP addresses and subnet masks. We will now configure the interfaces for R2 and use debugip routing to view the routing table process that is used to add these entries. The figure shows what happens as the Serial 0/0/1 interface for R2 is configuredwith the 192.168.1.1/24 address. As soon as no shutdown is entered, the output fromdebug ip routing shows that this route has been added to the routing table.
Level 1 RoutesA level 1 route can function as a:Default route - A default route is a static route with the address 0.0.0.0/0.Supernet route - A supernet route is a network address with a mask less than the classful mask.Network route - A network route is a route that has a subnet mask equal to that of the classful mask. A network route can also be a parent route. Parent C 192.168.1.0/24 is directly connected, routes will be discussed in the Serial0/0/1 next section.
Ultimate RouteThe level 1 route192.168.1.0/24 can be furtherdefined as an ultimate route.An ultimate route is a routethat includes: - either a next-hop IP address (another path) - and/or an exit interfaceThe directly connectednetwork 192.168.1.0/24 is alevel 1 network route becauseit has a subnet mask that is the C 192.168.1.0/24 is directly connected,same as its classful mask. This Serial0/0/1same route is also an ultimateroute because it contains the We will see in the next topic that level 2exit interface Serial 0/0/1. routes are also ultimate routes.
Parent and Child Routes : Classful NetworksWhen the 172.16.3.0 subnet was added to the routing table, another route,172.16.0.0, was also added. The first entry, 172.16.0.0/24, does not contain any next-hop IP address or exit interface information. This route is known as a level 1 parentroute.
A level 1 parent route is anetwork route that does notcontain a next-hop IP addressor exit interface for anynetwork.172.16.0.0/24 is subnetted, 1 subnetsA level 2 route is a route that is asubnet of a classful networkaddressC 172.16.3.0 is directly connected, FastEthernet0/0
Level 1 Parent RouteThis parent route contains the followinginformation:172.16.0.0 - The classful network address forour subnet. Remember, the Cisco IP routingtable is structured in a classful manner./24 - The subnet mask for all of the childroutes. If the child routes have variable lengthsubnet masks (VLSM), the subnet mask will beexcluded from the parent route and includedwith the individual child routes. This will beshown in a later section.is subnetted, 1 subnet - This part of the routespecifies that this is a parent route and in thiscase has one child route, that is, 1 subnet.
Level 2 Child RouteThe second entry, 172.16.3.0, is the actualroute for our directly connected network. Thisis a level 2 route, also known as a child route,and contains the following information:C - The route code for directly connectednetwork.172.16.3.0 - The specific route entry.is directly connected - Along with the routecode of C, this specifies that this is a directlyconnected network with an administrativedistance of 0.FastEthernet0/0 - The exit interface forforwarding packets that match this specificroute entry.
The level 2 child route is the specific routeentry for the 172,16.3.0/24 subnet. Noticethat the subnet mask is not included withthe subnet, the level 2 child route. Thesubnet mask for this child route (subnet) isthe /24 mask included in its parent route,172.16.0.0.Level 2 child routes contain the routesource and the network address of theroute.Level 2 child routes are also consideredultimate routes because they will containthe next-hop IP address and/or exitinterface.
Parent and Child Routes : Classful Networks The parent route contains the following information: 172.16.0.0 - The parent route, the classful network address associated with all child routes. /16 - The classful subnet mask of the parent route. variably subnetted - States that the 3 subnets, 2 masks - Indicates the number of child routes are variably subnetted subnets and the number of different subnet and that there are multiple masks masks for the child routes under this parent for this classful network. route.
Parent and Child Routes : Classful Networks Using one of the child routes as an example, we can see the following information: C - The route code for a directly connected network. 172.16.1.4 - The specific route entry. /30 - The subnet mask for this specific route. is directly connected - Along with the Serial0/0/0 - The exit interface for forwarding route code of C, specifies that this is a packets that match this specific route entry. directly connected network with an administrative distance of 0.
Steps in the Route Look up process The Route Lookup Process Follow these steps in the figure to see the route lookup process. Dont worry about fully understanding the steps right now. You will better understand this process when we examine a few examples in the following sections.
Steps in the Route Look up process Step 1. The router examines level 1 routes, including network routes and supernet routes, for the best match with the destination address of the IP packet.
Steps in the Route Look up process Step 1a. If the best match is a level 1 ultimate route - a classful network, supernet, or default route - this route is used to forward the packet.
Steps in the Route Look up process Step 1b. If the best match is a level 1 parent route, proceed to Step 2.
Steps in the Route Look up process Step 2. The router examines child routes (the subnet routes) of the parent route for a best match.
Steps in the Route Look up process Step 2a. If there is a match with a level 2 child route, that subnet will be used to forward the packet.
Steps in the Route Look up process Step 2b. If there is not a match with any of the level 2 child routes, proceed to Step 3.
Steps in the Route Look up process Step 3. Is the router implementing classful or classless routing behavior?
Steps in the Route Look up process Step 3a. Classful routing behavior: If classful routing behavior is in effect, terminate the lookup process and drop the packet.
Steps in the Route Look up process Step 3b. Classless routing behavior: If classless routing behavior is in effect, continue searching level 1 supernet routes in the routing table for a match, including the default route, if there is one.
Steps in the Route Look up process Step 4. If there is now a lesser match with a level 1 supernet or default routes, the router uses that route to forward the packet.
Steps in the Route Look up process Step 5. If there is not a match with any route in the routing table, the router drops the packet.
Steps in the Route Look up process Note: A route referencing only a next-hop IP address and not an exit interface must be resolved to a route with an exit interface. A recursive lookup is performed on the next-hop IP address until the route is resolved to an exit interface.
Longest Match :Level 1 Network RoutesThe best match orlongest match is theroute in the routingtable that has themost number of left-most matching bitswith the destinationIP address of the The route with the most number of equivalent left-mostpacket. bits, or the longest match, is always the preferred route.
Step 1-there is a match between thedestination IP address 192.168.1.2 andthe level 1 ultimate route of192.168.1.0/24.Step 1a - R 192.168.1.0/24 [120/1] via172.16.2.2, 00:00:25, Serial0/0/0
Level 1 Ultimate RouteThe subnet mask that is used todetermine the longest match is notalways obvious. Lets examine thisconcept in more detail, using severalexamples.
The 172.16.0.0/24 is a parent route of three subnets or child routes.Before a child route is examined for a match, there must be at least amatch between the destination IP address of the packet and the classfuladdress of the parent route, or 172.16.0.0/16.
Note: Remember that the route lookup process will need to do arecursive lookup on any route that references only a next-hop IPaddress and not an exit interface. For a review of recursive lookups,refer to Chapter 2, "Static Routing."
Routing Table Lookup ProcessAs shown in the figure, a parent route does not include a next-hop address or an exit interface but is only a "header" for its level 2 child routes, the subnets.The subnet mask for the child routes - /24 in the figure - is displayed in the parent route, 172.16.0.0, for subnets that use the same subnet mask.Before any level 2 child routes are examined for a match, there must first be a match between the classful address of the level 1 parent route and the destination IP address of the packet.
Routing Table Lookup Process Example: Level 1 Parent Route and Level 2 Child Routes• In the example in the figure, PC1 sends a ping to PC2 at 172.16.3.10. R1 receives the packet and begins to search the routing table for a route.
Routing Table Lookup ProcessThe first match that occurs is withthe level 1 parent route, 172.16.0.0.Remember, with non-VLSM subnetsthe classful mask of the parent isnow displayed. Before any childroutes (subnets) are examined for amatch, there must first be a matchwith the classful address of theparent route.Because the first route entry is alevel 1 parent route that matchesthe destination address (Step 1b ofthe route lookup process), the routelookup process moves to Step 2.
Routing Table Lookup ProcessBecause there is a match with theparent route, the level 2 child routeswill be examined for a match.However, this time the actual subnetmask of /24 is used for the minimumnumber of left-most bits that mustmatch.
Routing Table Lookup ProcessThe route lookup process searchesthe child routes for a match. In thiscase, there must be a minimum of 24bits that match.
Routing Table Lookup Process• After the match with parent route has been made Level 2 child routes will be examined for a match -Route lookup process searches for child routes with a match with destination IP
Routing Table Lookup ProcessFirst, the router examines theparent route for a match. Inthis example, the first 16 bitsof the IP address must matchthat of the parent route. Theleft-most 16 bits must matchbecause that is the classfulmask of the parent route, /16.If there is a match with theparent route, then the routerchecks the 172.16.1.0 route.Child routes are onlyexamined when there is amatch with the classful maskof the parent.
Routing Table Lookup ProcessChecking the first subnet,172.16.1.0, the 23rd bit doesnot match; therefore, thisroute is rejected because thefirst 24 bits do not match.
Routing Table Lookup ProcessNext, the router checks the172.16.2.0/24 route. Becausethe 24th bit does not match,this route is also rejected. All24 bits must match.
Routing Table Lookup ProcessThe router checks the last child route for 172.16.3.0/24 and finds a match. The first 24 bits domatch. The routing table process will use this route, 172.16.3.0/24, to forward the packet withthe destination IP address of 172.16.3.10 out the exit interface of Serial 0/0/0.R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0What happens if the router does not have a route? Then it discards the packet.
Routing Table Lookup Process• How a router finds a match with one of the level 2 child routes -First router examines parent routes for a match -If a match exists then: Child routes are examined Child route chosen is the one with the longest match
Routing Table Lookup ProcessExample: Route Lookup Process with VLSM• What about our RouterX topology, which is using a VLSM addressing scheme? How does this change the lookup process?• Using VSLM does not change the lookup process. With VLSM, the /16 classful mask is displayed with the level 1 parent route (172.16.0.0/16 in the figure).• As with non-VLSM networks, if there is a match between the packets destination IP address and the classful mask of the level 1 parent route, the level 2 child routes will be searched.• The only difference with VLSM is that child routes display their own specific subnet masks. These subnet masks are used to determine the number of left-most bits that must match the packets destination IP address. For example, for there to be a match with the 172.16.1.4 child route, a minimum of 30 left-most bits must match because the subnet mask is /30.
Routing Table Lookup Process• Example: Route Lookup Process with VLSM -The use of VLSM does not change the lookup process -If there is a match between destination IP address and the level 1 parent route then -Level 2 child routes will be searched