Intro to the Routing Process

Posted on July 23, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , , |

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ICND1 and ICND2 break down

To route packets, a router needs:

  • The destination address
  • Source of learning about other networks
  • Possible routes to other networks
  • A way of maintaining routes
  • A method of selecting paths

A router stores the route to destination networks it knows in a routing table. Routing table contains routing information that helps a router in determining the routing path. To show the ip table, use command “show ip route”.

For example, the following is the result of the “show ip route” command on a network router running ospf routing protocol.

RouterB#show ip route
Codes: C – connected, S – static, I – IGRP, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, L1 – IS-IS level-1, L2 – IS-IS level-2, ia – IS-IS inter area
* – candidate default, U – per-user static route, o – ODR
P – periodic downloaded static route

Gateway of last resort is not set

O [110/65] via, 00:00:03, Serial0/0
C is directly connected, Serial0/0
C is directly connected, FastEthernet0/0

From the above, we can see, routing table provides an ordered list of known network addresses.

For each network address, there are metrics that are used to determine the desirability of the route.

Routing table associations tell a router that a particular destination is either directly connected to the router or that it can be reached via another router (the next-hop router) on the way to the final destination. For example, “C is directly connected, Serial0/0 ” means is directly connected network, while “O [110/65] via, 00:00:03, Serial0/0” means network can be reached via another router at address

In order to pass the CCENT and CCNA exam, we have to understand the following routing table entries:

  • Directly connected network (code: C – connected) — This entry comes from having interfaces attached to network segments. This entry is obviously the most certain; if the interface fails or is administratively shut down, the entry for that network will be removed from the routing table.
  • Default route (code: C – connected) — This is an optional entry that is used when no explicit path to a destination is found in the routing table. This entry can be manually inserted or be populated from a dynamic routing protocol.
  • Static routing (code: S – static) — These routes are entered manually by a system administrator directly into the configuration of a router.
  • Dynamic routing (code: I – IGRP, R – RIP, O – OSPF …) — These routes are learned by the router, and the information is responsive to changes in the network so that it is constantly being updated.

There are three ways for the router to know an ip address:

  1. Router knows by default the network addresses of the directly connect networks.
  2. System admin can statically configured some network addresses with command “ip route FastEthernet0/1“, where is the destination address, is the subnet mask, FastEthernet0/1 is the interface the packet will exit from the router.
  3. Router can learned about non-directly connected networks by communicating with its neighbors with various routing protocols.

Once router receives an packet, it will check the destination ip address. If the destination ip address is in the router’s routing table, the packet will be forwarded to the corresponding exiting interface, otherwise, the packet will be dropped after sending an Internet Control Message Protocol (ICMP) message to the source address of the packet.

The routing process have five steps:

  1. The router receives a packet on one of its interfaces.
  2. The router de-encapsulates the frame and uses the protocol information of the frame to determine that the network layer packet will pass to the IP process.
  3. The router checks the destination address in the IP header. Either the packet is destined for the router itself or it needs to be forwarded. If the packet needs to be forwarded, the router searches its routing table to determine where to send the packet.
  4. If the destination network is on a directly attached network, the router will use the ARP process to obtain the MAC address of the host and forward it to the network segment. If the network is reachable through another router, the router will use the MAC address of the next-hop router and forward the packet out the interface indicated in the routing table. ARP establishes correspondence between network addresses and LAN hardware addresses. A record of each correspondence is kept in the ARP cache. To display the ARP cache, use command “show ip arp” under EXEC mode.
  5. The outgoing interface process encapsulates the packet appropriately to the media and sends the packet onto the network segment.

Routing protocol vs Routed protocol

A Routed Protocol is a protocol by which actual network packets are routed by the routers. Examples of routed protocol are IP, IPX, and AppleTalk.

A Routing Protocol is only used between routers. Its purpose is to help routers exchange information about the network the routers know about or learned about, so that the routers can build and update routing tables. All IP interior gateway protocols must be specified with a list of associated networks before routing activities can begin. A routing process listens to updates from other routers on these networks and broadcasts its own routing information on those same networks. Examples of routing protocol are Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Internet Gateway Routing Protocol (IGRP), Enhanced Internet Gateway Routing Protocol (Enhanced IGRP or EIGRP), Intermediate System-to-Intermediate System (IS-IS).

There are three classes of routing protocols:

  • Distance VectorDistance is the cost of reaching a destination, usually based on the number of hosts the path passes through (hops). Distance vector protocols use a distance calculation plus an outgoing network interface (a vector) to choose the best path to a destination network. All the routers in the network send out periodic updates even if there are no changes in the network, every update contains the entire routing table. Upon receiving the routing table of the neighbor, a router can verify all the known routes and make changes to its local routing table based on updated information received from the neighbor router. This process is also known as “routing by rumor.” Examples of this protocol include Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).
  • Link State — In link-state routing protocols, every router tries to build its own internal map of the network topology. Each router sends messages into the network when it goes online, listing the routers to which it is directly connected and providing information about whether the link to each router is active. The other routers use this information to build a map of the network topology and then use the map to choose the best path to the destination network. Link State protocols track the status and connection type of each link and produces a calculated metric based on these factors. Link state protocols know whether a link is up or down and how fast it is and calculates a cost to ‘get there’. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops. Compared with Distance Vector routing rotocols, Link-state routing protocols are quieter — only send periodic updates (link-state refreshes) at long time intervals, approximately once every 30 minutes. When there’s change in the network, the routers only send information about the change itself, instead of the whole routing table, therefore Link-state routing protocols respond quickly to network changes. Examples of Link state protocols are Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).
  • hybrid — combination of disctance vector protocol and link state protocol, example is Enhanced Interior Gateway Routing Protocol (EIGRP).

Autonomous System is a group of routers under a autonomous administrative domain.

Interior Gateway Protocol (IGP) : the routing protocols used inside the autonomous system. Example: RIP, OSPF,  IGRP, EIGRP

Exterior Gatewary Protocol (EGP) or Border Gateway Protocol (BGP): the routing protocols used when one autonomous sytem need to communicate with another autonomous system.

The routing table holds only one entry per network. If there are more than one routing protocols provides route information to a particular destination network, the router needs to decide which source of information should be used in the routing table. Different routing protocols use different metrics to measure the quality of path to a destination network, in order to directly compare information provided by different routing protocols, the Cisco routing process assigns a weight, known as the administrative distantce, to each source of information.

Administrative distance: Routers use the administrative distance to choose between routes learned through two (or more) routing protocols. Lower administrative distance path will be chose.

Default Administrative distance

  • Unkown = 255
  • RIP = 120
  • IS-IS = 115
  • OSPF = 110
  • IGRP = 100
  • EIGRP = 90
  • Static route = 1
  • Connected route = 0

note: Static routes using an exit interface have an administrative distance of 0. For example: “ip Route Serial0/1”

To display which routing protocols are active on the router, use command “show ip protocols“.

For review of IP routing, watch this CCNA TV at cisco learning center.


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