Routing Tables and Protocols

Routing Tables and Protocols

By Ruston Pennington

A host as a simple decision to make. (on the right)

How does it know?

It starts with a host

​Is the destination on the local subnet?

​Or is the destination on a remote subnet?

Match

Which of these are on the same local subnet?

• 192.168.1.223/24

• 172.16.3.3/16

• 172.31.3.3/24

• 10.15.33.153/8

• 10.15.32.110/16

192.168.1.3/24

172.16.2.223/16

172.31.3.54/24

10.225.225.1/8

10.15.33.2/16

• 192.168.1.3/24

• 172.16.2.223/16

• 172.31.3.54/24

• 10.225.225.1/8

• 10.15.33.2/16

The host as a simple decision to make. (on the right)

If the destination is in the ARP table it uses the corresponding MAC address in the destination field of the Ethernet Frame

If not, use the router

It checks the ARP table

​Is the destination on the local subnet?

​Or is the destination on a remote subnet?

Multiple Choice

If your IP address and subnet mask is 192.168.15.2 and 255.255.255.0,

is the IP address 192.168.1.15 in your local subnet?

• Yes

• No

What the router does

• When it receives an Ethernet frame, check if the FCS (Frame Check Sequence) is correct. If not, drop the frame. 

• Check if the destination address of the frame is:

  • destined to our MAC address 

  • destined to a broadcast address of the subnet our interface is in. 

  • destined to a multicast address that we listen to.

What the router does cont.

• De-encapsulate the IP packet from the frame, discard the Ethernet frame. 

• Look for a match in the routing table for the destination IP address, figure out what the outgoing interface is and optionally, the next hop IP address is. 

• Decrease the TTL (Time to Live) field in the IP header, recalculate the header checksum. 

• Encapsulate the IP packet in a new Ethernet frame. 

• Check the ARP table for the destination IP address or next hop IP address. 

• Transmit the frame. 

Above is an example of a router table.

Router table

And this is the corresponding network map.

This example is a good example of Static Routing, but what if something goes down or gets a new IP address?

Network Map

Dynamic Routing

​Routers rewrite their own tables on the fly. Protocols use metric values to decide the best routes. Metrics such as:

MTU – Maximum Transmission Units specifies the maximum transmission size of an interface. 

Bandwidth – How much information can be sent on a single connection. (56kb vs 10Gb for example) 

Cost – The line that is less expensive will be a better choice. 

Latency – How long does it take this route to react to the data being sent. (Satellite takes longer than cable) 

Convergence – Once all the routers in each network have learned the associated routing tables. This timing varies based on the protocol used. 

Above shows the relation of the 5 main protocols

Routing Information Protocol (RIPv1) – Uses hop count to determine metric. Fixed time interval for routing table comparisons. Maximum hop count = 15 

RIPv2 – Has security built in. 

​Distance Vector

IGRP and EIGRP use multiple metrics to calculate the lowest distance vector using:

Topological delay time

Bandwidth of the narrowest bandwidth segment of the path

Channel occupancy of the path

Reliability of the path

​Distance Vector

Open Shortest Path First (OSPF) – The moment OSPF routers are connected they start advertising their routing tables. However one router has to be configured as a “designated router”. Have to have an “Area ID” setup for OSPF so the routers know what area they belong to. If the link is up you can get there, if it’s down you can’t. This also takes in to account bandwidth on the links. 

Like OSPF routing, IS-IS uses hello packets that allow network convergence to occur quickly when network changes are detected. IS-IS uses the SPF algorithm to determine routes. Using SPF, IS-IS evaluates network topology changes and determines if a full or partial route calculation is required.

Link-State

EGP is used for communication between ISPs and there is only one to remember. Border Gateway Protocol(BGP) in this communication method you don’t really use IP addresses you use ASN or Autonomous System Numbers. This protocol uses a hybrid of Distance Vector and Link State to determine routes. 

Exterior Gateway Protocol (EGP

Multiple Choice

Automatic routing table update is a feature of:

• Static routing

• Route redistribution

• Dynamic routing

• route aggregation

Multiple Choice

Which of the following answer refers to a link-state routing protocol? 

• BGP

• EIGRP

• RIP

• OSPF

Multiple Select

Examples of distance-vector routing protocols include: 

• EGP

• OSPF

• RIP

• EIGRP

• BGP

Multiple Choice

Border Gateway Protocol (BGP) falls into the category of: 

• Hybrid routing protocols

• Link-state routing protocols

• Distance-vector protocols

• Static routing protocols

Multiple Choice

A manually configured routing table entry is known as: 

• Static route

• Dynamic route

• Hybrid route

• Default route

Multiple Choice

True or False:

 The term “Default route” refers to a “last resort” network path used by a router for forwarding all packets with destination addresses not listed in it routing table. 

• True

• False

Multiple Choice

A field in an IP datagram that specifies how many more hops a packet can travel before being discarded is called: 

• SPB

• TTL

• MTU

• VTC

Multiple Choice

Address Resolution Protocol (ARP) provides what kind of resolution? 

• IPv6 to IPv4

• MAC to IP

• Domain name to IP

• IP to MAC

Multiple Choice

Which of the following is an exterior gateway protocol? 

• OSPF

• BGP

• EIGRP

• RIP

Multiple Select

Which of the following refer(s) to (an) interior gateway protocol(s)? (Select all that apply) 

• RIP

• EIGRP

• BGP

• OSPF