Chapter 8

Unknown

Default AD = 255 (this router will never be used)

step 1

Internet Control Message Protocol (ICMP) creates an echo request payload (which is just the alphabet in the data field).

step 30

The router checks the ARP cache to determine whether the hardware address for 172.16.10.2 has already been resolved.

step 14

The router packet-switches the packet to the Ethernet 1 buffer.

step 26

The router's Ethernet 1 interface receives the bits and builds a frame. The CRC is run, and the FCS field is checked to make sure the answers match.

step 12

The routing table must have an entry for the network 172.16.20.0 or the packet will be discarded immediately and an ICMP message will be sent back to the originating device with a destination network unreachable message.

exitinterface

Used in place of the next-hop address if you want, and shows up as a directly connected route.

router must know minimum to route packets

destination address neighbor routers from which it can learn about remote networks possible routes to all remote networks the best route to each remote network how to maintain and verify routing info

ICMP

internet control message protocol

static routing

someone must hand type all of the network locations into the routing table

routing

taking a packet from one device and sending it through the network to another device on a different network.

network command

tells the routing protocol which classful network to advertise.

next-hop_address

the address of the next-hop router that will receive the packet and forward it to the remote network. this is the IP address of a router interface that's on a directly connected network. You must be able to ping the router interface before you can successfully add the route. If you type in the wrong next-hop address or the interface to that router is down, the static router will show up in the router's configuration but not in the routing table.

ip route

the command used to create the static route

destination_network

the network you're placing in the routing table

mask

the subnet mask being used on the network

Route update timer

Route update timer Sets the interval (typically 30 seconds) between periodic routing updates in which the router sends a complete copy of its routing table out to all neighbors.

counting to infinity

The routing loop problem just described can create an issue called counting to infinity, and it's caused by gossip (broadcasts) and wrong information being communicated and propagated throughout the internetwork. Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router.

classful routing

means that all devices in the network must use the same subnet mask.

step 32

The Data Link layer builds a frame with the destination hardware address and source hardware address and then puts IP in the Ether- Type field. A CRC is run on the frame and the result is placed in the FCS field.

step 16

The Data Link layer creates a frame with the destination and source hardware address, Ether-Type field, and FCS field at the end. The frame is handed to the Physical layer to be sent out on the physical medium one bit at a time.

step 24

The Data Link layer frames the packet of information and includes the following in the header: The destination and source hardware addresses The Ether-Type field with 0x0800 (IP) in it The FCS field with the CRC result in tow

step 10

The packet is pulled from the frame, and what is left of the frame is discarded. The packet is handed to the protocol listed in the Ether- Type field—it's given to IP.

step 19

The payload is handed to ICMP, which understands that this is an echo request. ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply.

step 28

The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address. Since the IP destination address doesn't match any of the router's interfaces, the routing table is checked to see whether it has a route to 172.16.10.0. If it doesn't have a route over to the destination network, the packet will be discarded immediately. (This is the source point of confusion for a lot of administrators—when a ping fails, most people think the packet never reached the destination host. But as we see here, that's not always the case. All it takes is for just one of the remote routers to be lacking a route back to the originating host's network and—poof!—the packet is dropped on the return trip, not on its way to the host.)

External EIGRP

default AD = 170

EIGRP

default AD = 90

step 20

A packet is then created including the source and destination addresses, Protocol field, and payload. The destination device is now Host_A.

step 18

At the Network layer, IP receives the packet and runs a CRC on the IP header. If that passes, IP then checks the destination address. Since there's finally a match made, the Protocol field is checked to find out to whom the payload should be given.

administrative_distance

By default, static routes have an administrative distance of 1 (or even 0 if you use an exit interface instead of a next-hop address). You can change the default value by adding an administrative weight at the end of the command.

step 9

Every device in the collision domain receives these bits and builds the frame. They each run a CRC and check the answer in the FCS field. If the answers don't match, the frame is discarded. If the CRC matches, then the hardware destination address is checked to see if it matches too (which, in this example, is the router's interface Ethernet 0). If it's a match, then the Ether-Type field is checked to find the protocol used at the Network layer.

step 17

Host_B receives the frame and immediately runs a CRC. If the result matches what's in the FCS field, the hardware destination address is then checked. If the host finds a match, the Ether-Type field is then checked to determine the protocol that the packet should be handed to at the Network layer—IP in this example.

step 36

ICMP acknowledges that it has received the reply by sending an exclamation point (!) to the user interface. ICMP then attempts to send four more echo requests to the destination host.

step 2

ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a minimum, this packet contains an IP source address, an IP destination address, and a Protocol field with 01h. (Remember that Cisco likes to use 0x in front of hex characters, so this could look like 0x01.) All that tells the receiving host to whom it should hand the payload when the destination is reached—in this example, ICMP.

examples of routed protocols

IP and IPv6

step 35

IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the Protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.

step 11

IP receives the packet and checks the IP destination address. Since the packet's destination address doesn't match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table.

step 21

IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network. Since the destination device is on a remote network, the packet needs to be sent to the default gateway.

permanent

If the interface is shut down or the router can't communicate to the next-hop router, the route will automatically be discarded from the routing table by default. Choosing the permanent option keeps the entry in the routing table no matter what happens.

step 13

If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1. The output below displays the Lab_A router's routing table. The C means "directly connected." No routing protocols are needed in this network since all networks (all two of them) are directly connected.

step 29

In this case, the router does know how to get to network 172.16.10.0—the exit interface is Ethernet 0—so the packet is switched to interface Ethernet 0.

step 6

Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the IP address of the default gateway has already been resolved to a hardware address: If it has, the packet is then free to be handed to the Data Link layer for framing. (The hardware destination address is also handed down with that packet.) To view the ARP cache on your host, use the following command:arp -a If the hardware address isn't already in the ARP cache of the host, an ARP broadcast is sent out onto the local network to search for the hardware address of 172.16.10.1. The router responds to the request and provides the hardware address of Ethernet 0, and the host caches this address.

step 27

Once the CRC is found to be okay, the hardware destination address is checked. Since the router's interface is a match, the packet is pulled from the frame and the Ether-Type field is checked to see what protocol at the Network layer the packet should be delivered to.

step 8

Once the frame is completed, it's handed down to the Physical layer to be put on the physical medium (in this example, twisted-pair wire) one bit at a time.

step 23

Once the hardware address of the default gateway is found, the packet and destination hardware addresses are handed down to the Data Link layer for framing.

step 7

Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). A frame is then generated, encapsulating the packet with control information. Within that frame are the hardware destination and source addresses plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP. At the end of the frame is something called a Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC). The frame would look something like what I've detailed in Figure 8-3. It contains Host_A's hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host's MAC address—remember that!

step 3

Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.

examples of routing protocols

RIP, RIPv2, EIGRP, OSPF

step 31

Since the hardware address to 172.16.10.2 is already cached from the originating trip to Host_B, the hardware address and packet are handed to the Data Link layer.

step 4

Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so it can be routed to the remote network. The Registry in Windows is parsed to find the configured default gateway.

step 15

The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache. If the hardware address of Host_B has already been resolved and is in the router's ARP cache, then the packet and the hardware address are handed down to the Data Link layer to be framed. Let's take a look at the ARP cache on the Lab_A router by using the show ip arp command: The dash (-) means that this is the physical interface on the router. From the output above, we can see that the router knows the 172.16.10.2 (Host_A) and 172.16.20.2 (Host_B) hardware addresses. Cisco routers will keep an entry in the ARP table for 4 hours. If the hardware address has not already been resolved, the router sends an ARP request out E1 looking for the hardware address of 172.16.20.2. Host_B responds with its hardware address, and the packet and destination hardware addresses are both sent to the Data Link layer for framing.

step 22

The default gateway IP address is found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address.

step 5

The default gateway of host 172.16.10.2 (Host_A) is configured to 172.16.10.1. For this packet to be sent to the default gateway, the hardware address of the router's interface Ethernet 0 (configured with the IP address of 172.16.10.1) must be known. Why? So the packet can be handed down to the Data Link layer, framed, and sent to the router's interface that's connected to the 172.16.10.0 network. Because hosts only communicate via hardware addresses on the local LAN, it's important to recognize that for Host_A to communicate to Host_B, it has to send packets to the Media Access Control (MAC) address of the default gateway on the local network.

step 34

The destination host receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out to whom to hand the packet.

step 25

The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time.

step 33

The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time

default routing

We use default routing to send packets with a remote destination network not in the routing table to the next-hop router. You should only use default routing on stub networks—those with only one exit path out of the network, although there are exceptions to this statement, and default routing is configured on a case-by-case basis when a network is designed. This is a rule of thumb to keep in mind.

autonomous system (AS)

a collection of networks under a common administrative domain, which basically means that all routers are sharing the same routing table information are in the same AS.

dynamic routing

a protocol on one router communicates with the same protocol running on neighboring routers

connected interface

default AD = 0

static router

default AD = 1

IGRP

default AD = 100

OSPF

default AD = 110

RIP

default AD = 120

routing table

map of internetwork - describes how to find remote networks

S

means that the route is a static entry.

c in routing table output

means the networks listed are directly connected

routing protocol

used by routers to dynamically find all of the networks in the internetwork and to ensure that all routers have the same routing table.

exterior gateway protocols (EGP)

used to communicate between ASes. and example is border patrol gateway (BGP)

Interior gateway protocols (IGP)

used to exchange routing information with routers in the same autonomous system (AS).

administrative distance (AD)

used to rate the trustworthisness or routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

routed protocol

used to send user data through the established enterprise.

dynamic routing

when protocols are used to find networks and update routing tables on routers.

route flush timer

Route flush timer Sets the time between a route becoming invalid and its removal from the routing table (240 seconds). Before it's removed from the table, the router notifies its neighbors of that route's impending demise. The value of the route invalid timer must be less than that of the route flush timer. This gives the router enough time to tell its neighbors about the invalid route before the local routing table is updated.

Route invalid timer

Route invalid timer Determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid. It will come to this conclusion if it hasn't heard any updates about a particular route for that period. When that happens, the router will send out updates to all its neighbors letting them know that the route is invalid.

holddown

A holddowX prevents regular update messages from reinstating a route that is going up and down (called flapping). Typically, this happens on a serial link that's losing connectivity and then coming back up. If there wasn't a way to stabilize this, the network would never converge and that one flapping interface could bring the entire network down! Holddowns prevent routes from changing too rapidly by allowing time for either the downed route to come back up or the network to stabilize somewhat before changing to the next best route. These also tell routers to restrict, for a specific time period, changes that might affect recently removed routes. This prevents inoperative routes from being prematurely restored to other routers' tables.

split horizon

Another solution to the routing loop problem is called split horizon. This reduces incorrect routing information and routing overhead in a distance- vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received. In other words, the routing protocol differentiates which interface a network route was learned on, and once this is determined, it won't advertise the route back out that same interface. This would have prevented RouterA from sending the update information it received from RouterB back to RouterB.

route poisoning

Another way to avoid problems caused by inconsistent updates and stop network loops is route poisoning. For example, when Network 5 goes down, RouterE initiates route poisoning by advertising Network 5 with a hop count of 16, or unreachable (sometimes referred to as infinite). This poisoning of the route to Network 5 keeps RouterC from being susceptible to incorrect updates about the route to Network 5. When RouterC receives a route poisoning from RouterE, it sends an update, called a poison reverse, back to RouterE. This ensures that all routers on the segment have received the poisoned route information.

distance vector

Distance vector The distance-vector protocolI in use today find the best path to a remote network by judging distance. For example, in the case of RIP routing, each time a packet goes through a router, that's called a hop. The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols. They periodically send the entire routing table to directly connected neighbors.

holddown timer

Holddown timer This sets the amount of time during which routing information is suppressed. Routes will enter into the holddown state when an update packet is received that indicates the route is unreachable. This continues either until an update packet is received with a better metric, the original route comes back up, or the holddown timer expires. The default is 180 seconds.

hybrid

Hybrid protocolI use aspects of both distance vector and link state—for example, EIGRP.

link state

Link state In link-state protocols, also called shortest-path-first protocols, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Link- state routers know more about the internetwork than any distance-vector routing protocol. OSPF is an IP routing protocol that is completely link state. Link-state protocols send updates containing the state of their own links to all other directly connected routers on the network, which is then propagated to their neighbors.

maximum hop count

One way of solving this problem is to define a maximum hop count. RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops, Network 5 will be considered down. Thus, the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable.