Lecture 1 - Basic Network and Routing Concepts

7/23/2019 Lecture 1 - Basic Network and Routing Concepts

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INFR 2411U – Advanced Networking I: CCNP ROUTE

CCNP ROUTE: Implementing IP Routing 300-101

Josh Lowe

September 2015




• Differentiating between Dynamic Routing Protocols

• Understanding Network Technologies (Traffic Types, Network Types, and ONetwork Technologies)

• Connecting Remote Locations with Headquarters – VPNs covered in tutor

• Routing and TCP/IP Operations

• Implementing RIPng – Covered in Lab and Tutorial




• Today’s enterprise networks can be incredibly complex

• For example, physical and logical topologies can look very differente.g.:

Physical Logical


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• Breaking the network down into smaller modules, each with its own purposthe analysis of the network



• Cisco recommends breaking the network up into two functional areas, theCampus and the Enterprise Edge



• Enterprise Campus

• Provides end users and devices with access to resources

• Single geographic area spanning a single floor, building,or several buildings in the same area

• Commonly designed using a hierarchical model (access,distribution, core)




• Enterprise Edge

• Goal is to provide remote users with the same access to

network services as local users• Aggregates private WAN links from service providers

• Provides VPN connectivity site-to-site and for remoteusers

• Provides Internet connectivity for Enterprise Campususers




• The basic goal of routing protocols is to exchange network layer reachabil(NLRI) between routers, and to adapt to changes in network topology




• Best practices say to only use one IP routing protocol throughout the enter

• Sometimes, this is not feasible/possible:• Your organization has acquired another organization which uses a different protocol

• Some devices in your network do not support one or more of the protocols in use

• Your organization is multi-homed to two or more ISPs

• In a multi-homed environment routes are typically exchanged with the IS(much more on that later!)

• Within the organization, OSPF or EIGRP is typically used

• In a single-homed environment (single ISP connection) static routes areshared between the organization and the ISP

• The customer often receives only a default route from the ISP in this situation




• Asymmetric routing (asymmetric traffic) is trafficthat uses one path for packets leaving and adifferent path for the return traffic

• Often occurs when you have multiple redundantpaths through the network

• Asymmetric routing is sometimes a desirable traitbecause it maximizes use of the availablebandwidth

• However, some technologies don’t work well withasymmetric routing, especially securitytechnologies

• E.g. Firewalls and VPNs often have issues withasymmetric traffic; NAT can also be problematic

• As we will see later, BGP is a great protocol forcontrolling path selection in redundant networks




• With so many different routing protocols available, how do you know which

•Each protocol has its own advantages and drawbacks

• In general, you need to consider the following when choosing a routing pro

1. Network size (some protocols scale better than others)

2. Multivendor support (does that new Juniper router you purchased support EIGRP?)

3. Knowledge level (how well do you understand the nuances of multi-area OSPF configu

4. Type of routing algorithm (link-state vs distance vector vs path vector, which is best for

5. Speed of convergence (how fast does the protocol recover from failures?)

6. Scalability (how much overhead is introduced by the protocol? Small networks don’t neprotocols)




• An autonomous system (AS) represents acollection of network devices under a commonadministrator

• Routing protocols can be divided based onwhether they exchange routes within an AS orbetween different ASes

• Interior Gateway Protocols (IGP) are used toexchange routes within an AS

• Exterior Gateway Protocols (EGP) are used toexchange routes between ASes




• Distance Vector Protocols:

• Uses a direction (vector) and a distance to determine the path to any link in the netwo

• Think of it like driving from A to B and relying only on the signs on the side of the road to get there

• Routers only know what their immediate neighbors tell them; lack details of the full net

• E.g. “You can get to network B through me, and the distance is 100”

• Example protocols include RIP and EIGRP

Network A: Distance 30

Network B: Distance 100

A

B




• Link-state Protocols:

• Uses the Shortest Path First (SPF) algorithm

to create an exact map of the entire topologyin an area

• Think of this like having a map of all theroads in the province and using that todetermine how to get from A to B

• A link-state map of the network allows routersto determine the best path to a destination ontheir own, instead of relying on their

neighbors to tell them

• Example protocols include OSPF and IS-IS

You Are Here




• Convergence describes the process of when routers notice changes in theexchange the information about the change, and perform necessary calcu

evaluate the best routes

• A converged network is one where all routers have the same view of the ntopology




• Convergence time describes how fast network devices can reach a state oafter a topology change (faster is obviously better)

• Each routing protocol has specific factors that influence how fast they concover them as we go over each protocol.




• You can influence convergence time by fine-tuning timers

• The faster a router notices a change, and notifies its neighbors of the change, the fast

can recalculate the best path

• You can also influence convergence time by configuring summarization anprotocol-specific features




• Route summarization helps to improve stabilityand scalability in a network by reducing theamount of routing information that is maintained

and exchanged between routers

• The result is smaller routing tables and fasterconvergence times

• You can summarize routes by squeezing severalsubnets into on aggregate entry that includes allof them

• This makes the routing tables much smaller, whichis especially important on routers on the Internet,where they have over 500,000 routes!




• Summarization also reduces the number of updatesthat need to be exchanged between routers

• For example, Router B is receiving the summaryroute 10.12.0.0/21 from Router A

• If the 10.12.4.0/24 network goes down, does RouterB need to know about that? Does it need to do anyroute recalculations?

• The result of summarization is faster convergencetime

• Different protocols support different routesummarization options (e.g. auto-summary, weirdOSPF summary rules)

• Keep in mind that in order to implement routesummarization efficiently, IP addresses must behierarchically assigned in contiguous blocks acrossthe network!




• Larger networks are at increased risk of routing protocol instability or long times

• Scalability describes the ability of a routing protocol to support further netw

• Hierarchical addressing, structured address assignment, and route summimprove the overall scalability regardless of routing protocol.

• Some protocols also have specific features to improve their scalability

• For example, OSPF supports multiple “areas” that help reduce the complexity of the n

to scale very well• EIGRP supports “stub routers” which limits the amount of routing information that has

through certain parts of the network




• Unicast (one-to-one)

• Traffic is exchanged between one sender and one receiver

• Source IP addresses are always unicast




• Broadcast (one-to-all)

• Used to send traffic to all devices in a subnet

• 255.255.255.255 is a local broadcast address, which reaches all devices in the local sforwarded by routers)

• A directed broadcast allows you to reach all devices in a remote subnet (e.g. 10.1.1.25

• IPv6 does not have broadcast addresses




• Multicast (one-to-some, one-to-group)

• Traffic is sent to multiple (but not necessarily all) destinations at the same time (called

• An interface may belong to any number of multicast groups, and will pick up traffic for

• In IPv4, the class D address range 224.0.0.0-239.255.255.255 is the multicast addres

• In IPv6, any address that belongs to the FF00::/8 subnet is a multicast address




• Anycast (one-to-closest)• The same address is assigned to multiple

devices• Routers will route the packet to whichever

device is closest to the source (based onrouting protocol metric)

• For example, the same DNS server can behosted in locations all over the world, usingthe same address

• When you need to resolve a hostname youuse that same address, and the router willsend you to the closest server




• Early routing protocols used broadcasts to exchange routing information, winefficient and generated extra traffic.

• Most modern routing protocols use multicast to communicate




• ICMPv6 is much more robust protocol than its IPv4 counterpart as it includNeighbor Discovery Protocol

• ICMPv6 Neighbor Discovery is used for automatic address allocation, addand duplicate address detection (and it also replaces ARP!)

• There are five messages:

• Router Solicitation (RS) – Sent by a device to request that a neighboring router send a router amessage

• Router Advertisement (RA) – Sent by routers periodically, or in response to an RS message. In

information needed by hosts to automatically configure their global addresses (e.g. network prefi• Neighbor Solicitation (NS) – Replaces an ARP request. Sent to the Solicited Nodes multicast o

device asking it to reply with its MAC address

• Neighbor Advertisement (NA) – Unicast reply to a NS message, containing the requested MAC

• Redirect – Tells a sending device that they should use a different next-hop router to get to the denot the droids you are looking for…)




• Not all Layer 2 network topologiessupport all traffic types

• Because unsupported traffic typesinfluence the operation of routingprotocols, it is important to be awareof the limitations of specific networktopologies

• There are three general networktypes:

• Point-to-point

• Broadcast

• NBMA




• Point-to-point

• A network that connects a single pair of routers

• A packet sent by one device is received by exactly one recipient on the other end

• Typical Layer 2 protocols that run over P2P networks include HDLC and PPP




• Broadcast

• A network that can connect many devices on a single segment (usually via a Layer 2 s

• Supports broadcast messages, which go to every other device on the segment

• An example broadcast Layer 2 protocol is Ethernet




• Nonbroadcast Multaccess (NBMA)

• A network that can connect many devices to a single segment but does not have broa

• A sender needs to make multiple copies of the same packet if he needs to reach all desegment, and needs to know the address of each recipient

• Example protocols that run on NBMA networks include Frame Relay and ATM




• NBMA networks can use a variety of topologies, but most often are hub-anpartial mesh. Full mesh NBMA networks are expensive and don’t scale we

• Hub-and-spoke NBMA topologies can be especially problematic for routingCan you think of why? (there are multiple reasons)




• Split Horizon

• The split horizon rule is designed to prevent routing loops in distance vector protocols

• The rule states that an update received on an interface can’t be sent back out that sam• In a hub-and spoke network, this means that if the hub receives an update from a spo

it back out to the other spokes using the same physical interface!

• The solution is to either disable split horizon on the interface, or else modify the netwopoint-to-point subinterfaces (more later)




• Neighbor Discovery

• Most routing protocols multicast Hello packets in order to discover their neighbors auto

• Nonbroadcast networks don’t allow broadcasts (or multicasts) so routers are unable toeach other

• Instead, on these network types you must statically configure your neighbors

• Additionally, since NBMA is a multi-access network type, OSPF needs to elect a desig

• By definition, the DR needs to be able to talk directly to all other routers on the segme

• The hub is the only device with circuits to all other devices, so it must always be the D

DR




• As mentioned previously, you can use subinterfaces to circumvent some oassociated with NBMA networks

• Point-to-Point subinterface: Each subinterface uses its own subnet. Connectivity acare just a collection of point-to-point links, which means there is no issues with neighbsplit horizon

• Point-to-multipoint subinterfaces: One subnet is shared among all virtual circuits beRequires additional routing protocol configuration to support




• A static route can be used in the following circumstances:

• When it is undesirable to have dynamic routing updates forwarded across slow bandw

• When the administrator needs total control over the routes used by the router • When a backup to a dynamically route is necessary

• When it is necessary to reach a network accessible by only one path (a stub network,

• When a router needs to have only a default route pointing toward the ISP router

• When a router is underpowered and does not have the resources to handle a dynamic

B




• Configure a static route with the ip route command.

Router(config)#

ip route prefix mask address interface dhcp distance name next-hop-name

number tag tag

Parameter Description

prefix mask The IP network and subnet mask for the remote network to be entered into the IP routing table.

address The IP address of the next hop that can be used to reach the destination network.

interface The local router outbound interface to be used to reach the destination network.

dhcp (Optional) Enables a Dynamic Host Configuration Protocol (DHCP) server to assign a static route to a d

distance (Optional) The administrative distance to be assigned to this route.

name next-hop-name (Optional) Applies a name to the specified route.

permanent(Optional) Specifies that the route will not be removed from the routing table even if the interface assoc

down.

track number (Optional) Associates a track object with this route. Valid values for the number argument range from 1

tag tag (Optional) A value that can be used as a match value in route maps.




R1(config)# ip route 0.0.0.0 0.0.0.0 10.1.1.1

R1(config)# exitR1# show ip route

<output omitted>

Gateway of last resort is not set

C 172.16.1.0 is directly connected, FastEthernet0/0

C 10.1.1.0 is directly connected, Serial0/0/0

S* 0.0.0.0/0 [1/0] via 10.1.1.1

R1#

• R2 is configured with a static route to the R1 LAN and a default static route to the

• R1 is configured with a default static route.

R2(config)# ip route 172.16.1.0 255.255.255.0 S0/0/0R2(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1




• Point-to-Point Protocol (PPP) is a non-proprietary Layer 2 protocol for conendpoints together

• Through the use of Network Conrol Protocols (NCPs) it can support a variprotocols, such as IPv4 and IPv6

• Basic PPP configuration is very straightforward:

R1(config)# interface serial 0/0/0

R1(config-if)# encapsulation ppp




• PPP connections can be optionally authenticated using one of two protocoCHAP

• You can enable PPP authentication on an interface using the following com ppp authentication { chap | chap pap | pap chap | pap }

• PAP authentication requires the remote device to send a name and passwtext), which is checked against a database

• CHAP authentication sends a challenge string to the remote device which

encrypted using the shared secret key and sent back• After receiving the encrypted key the local router uses its configured share

perform the same encryption on the challenge string

• If the strings match, both routers have the same shared secret, if it doesn’the secrets were not the same




• PPP is traditionally used on serial links, however ISPs also like to use it onbroadband networks (specifically, DSL)

• ISPs value PPP because of the authentication, accounting, and link manafeatures

• Customers tend to use Ethernet in their homes, and PPP is not natively suEthernet

• PPP over Ethernet (PPPoE) allows the sending of PPP frames encapsulatEthernet frames




• PPPoE creates a PPP tunnel over an Ethernet connection, allowing PPP fsent across the tunnel to the customer

• A modem typically converts the PPPoE messages in the plain PPP messaversa, by adding and removing Ethernet headers

• PPPoE allows ISPs to authenticate customers (CHAP), provide them with automatically, and get detailed accounts of their connectivity, while still lettEthernet networks




• Frame Relay provides several benefitsover traditional point-to-point leasedlines depending on the needs of theorganization

• Leased lines provide permanentdedicated capacity, but are expensiveand inflexible. You also require aseparate physical interface on the routerfor each connection

• Frame Relay, however, requires only asingle access circuit to the Frame Relayprovider to communicate with other sitesconnected to the same provider, and thecapacity can vary between sites




• Frame Relay uses virtual circuits (VCs) to connect sites together

• Frame Relay allows multiple logical VCs to be multiplexed over a single ph

interface (for point-to-multipoint configurations)

• These VCs are identified by the Layer 2 Frame Relay address, the Data-liIdentifier (DLCI)

• To provide IP layer connectivity, a mapping between IP addresses and DLdefined, either dynamically (using Inverse ARP) or statically

• By default, a Frame Relay network is an NBMA network, requiring manualof EIGRP and OSPF neighbors and route exchanges




• An IPv4 packet has a maximum size of 65,535 bytes.

• An IPv6 packet can support up to 4,294,967,295 bytes.

• However, most transmission links enforce a smaller maximum packet lengmaximum transmission unit (MTU).

• When a router receives an IPv4 packet larger than the MTU of the outgoinmust fragment the packet (unless the Don’t Fragment, DF, bit has been se

• Reassembly of the packet is the responsibility of the destination device




• Fragmentation causes several issues including the following:

• CPU and memory overhead in fragmentation of the packet

• CPU and memory overhead in destination devices during reassembly of packets• Retransmission of the entire packet when one fragment is dropped

• Firewalls that do Layer 4 through Layer 7 filtering may have trouble processing IPv4 fr




• To avoid fragmentation, the TCP Maximum Segment Size (MSS) defines tamount of data that the receiving device is able to accept in a single TCP

• The MSS is not negotiated between sender and receiver.

• The sending device is required to limit the size of the TCP segment equal the MSS reported by the receiving device.

• To avoid fragmentation of an IPv4 packet, the selection of the TCP MSS isbuffer size and MTU of the outgoing interface minus 40 bytes. Why 40 by

• Example:• The default Ethernet MTU is 1500 bytes.

• A TCP segment over IPv4 sent out an Ethernet interface will have a TCP MSS of 1460

• This is 1500 bytes for the Ethernet MTU, minus 20 bytes for the IPv4 header, and 20 bheader.




• The TCP MSS helps avoid fragmentation at the two ends of the TCP conndoes not prevent fragmentation due to a smaller MTU on a link along the p

• Path MTU Discovery (PMTUD) determines the MTU along a path from thesource to destination

• A host uses the full MSS determined by the outgoing interface and sets thso that packets cannot be fragmented

• If a router along the path needs to fragment the packet because of a lowerthe egress interface, it will drop the packet due to the DF bit being set and

Destination Unreachable message back to the originator of the packet

MTU

1500MTU

1500

MTU

1492R1 R2 R3 R4

Size 1500

DF XSize 1492

DF




• IPv6 routers do not fragment a packet unless it is the source of the packet

• If an IPv6 router receives a packet larger than the MTU of the outgoing int

drop the packet and send an ICMPv6 Packet Too Big message back to theincluding the smaller MTU.

• The PMTUD operations for IPv6 are similar to that of PMTUD for IPv4



Thank you.

Lecture 1 - Basic Network and Routing Concepts

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