3.2 Interpret the components of a routing table

Let’s bring up the routing table of R1:

 

Screenshot_2018-03-26_11-57-04.png

Screenshot_2018-03-26_11-39-28

3.2.a Prefix

The prefix refers to the network prefix. In  R1’s routing table there are 3 prefixes listed:

/8, /24, /28

The prefix refers to the number of binary 1’s in the binary network mask:

/8   = 11111111.00000000.00000000.00000000

/24 = 11111111.11111111.11111111.00000000

/28 = 11111111.11111111.11111111.11110000

3.2.b Network mask

The network mask is the prefix but in Dotted Decimal Notation:

/8   = 11111111.00000000.00000000.00000000

/8   = 255.0.0.0

/28 = 11111111.11111111.11111111.11110000

/28 = 255.255.255.240

A network mask informs us which bit in the IP address are network bits or host bits.

3.2.c Next hop

The next hop is the address where the router will forward packets to. For example the hops from R1 => R4 are:

R1 => R3 => R4

So the next hop for R4 from R1 is R3.

3.2.d Routing protocol code

The routing protocol codes are listed at the top of the output:

C = Connected

O = ospf

D = EIGRP

R = RIP

B = BGP

This code us from which source this routes where taken from before being loaded into the FIB.

3.2.e Administrative distance

Administrative distance is the way Cisco routers chose between routes with the same prefix. Cisco favours some types of routes and routing protocols over others.

Refer to line 3 of the routing table above:

O     10.234.132.0/24 [110/2] via 10.200.20.46, 00:34:57, GigabitEthernet5/0

 

The square brackets hold the Administrative Distance and Metric (110 and 2 respectively). The O at the beginning at the line corresponds to OSPF, if you look below you will see OSPF has an Administrative Distance of 110.

Here is list of Administrative Distances:

0     => Connected
1     => Static
20    => BGP (external routes)
90    => EIGRP (internal routes)
100   => IGRP
110   => OSPF
115   => IS-IS
120   => RIP
170   => EIGRP (external routes)
200   => BGP (internal routes)
254   => DHCP default route
255   => Unusable

3.2.f Metric

The metric is the last number inside the [] brackets. Line 3 refers to the route to 10.234.132.0/24 via 10.200.20.46 which has a Administrative Distance of 110 and a OSPF metric of 2.

Metric is the routing protocols internal measure of the route. Each routing protocol measures this differently. In each case however a lower Metric indicates what the routing protocol determines to be a better route.

3.2.g Gateway of last resort

The gateway of last resort is similar to the “Default Gateway” or “Default Router” but it differs in that this route will only be used if no other entry exists in the routing table.

In the excerpt above it states that “Gateway of last resort is not set”.

In this case any packet destined for a subnet not in the routing table would be discarded.

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