Brainteaser? or not . . .

A friend asked me a question

if a router receives a packet for 2 or more ip addresses that are identical, how does it process the packet?

I asked about ARP, he said no
I asked about static routes, he said no
he said it's a dynamic routing protocol
I then suggested default route, he said no
I suggested administrative distance, he said no
then i suggested parent or child routes
he said no

it feels like he's making this **** up

anyone have an answer?

he said the answer is in the routing table and show ip route

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Comments

notgoing2fail

Selfmade wrote: »

A friend asked me a question

if a router receives a packet for 2 or more ip addresses that are identical, how does it process the packet?

I asked about ARP, he said no
I asked about static routes, he said no
he said it's a dynamic routing protocol
I then suggested default route, he said no
I suggested administrative distance, he said no
then i suggested parent or child routes
he said no

it feels like he's making this **** up

anyone have an answer?

he said the answer is in the routing table and show ip route

Well the question to me at least seems a little confusing. What does he mean by 2 or more identical IP addresses? Is it coming from the same host? Different hosts? Same interface? Different interfaces?

Anyways,

All a router does is examine the incoming packet, compare it against its routing table and route accordingly....

Selfmade

exactly, he's just asking how it selects the best route

the answer turned out to be that it looks for the longest binary match.

notgoing2fail

Selfmade wrote: »

exactly, he's just asking how it selects the best route

the answer turned out to be that it looks for the longest binary match.

You can do a sh ip route "ip address" to find out where the router is going to send that particular IP address...

Selfmade

notgoing2fail wrote: »

You can do a sh ip route "ip address" to find out where the router is going to send that particular IP address...

i know that, my friend just was looking for a specific answer

the longest binary match is how a router decides what route to use to send a packet.

chmorin

It actually uses the most specific route. Which is exactly what you said, the route that has the largest subnet mask.

xenodamus

This sounds like a very simple, yet very badly worded question...

Selfmade

xenodamus wrote: »

This sounds like a very simple, yet very badly worded question...

trust me, i wanted to brain my friend after he texted me that

I popped that same question on my other friend who is much smarter than me, it has him stumped even after I gave him the laundry list of things to rule out.

Ok, better phrased

A router receives a packet, the packet has an IP address, this IP address is identical to several IP addresses on the router's routing table. A dynamic routing protocol is enabled. How does a router decide which route to use as the preferred route?

Rule out the following as an answer,

static route
default route
administrative distance (or metric)
ARP
parent route
child route

there is an answer, you only need to know why and how it selects a PREFERRED route

Monkerz

If he was asking you how the router would route the packet, if the router had multiple matches in its routing table...

Why would you suggest ARP or Default Route?

Selfmade

he didn't phrase the question that way at first, at first he just asked if a packet went to a router and yet had 2 IP address matches, i asked if it would use ARP, he said no, then went on to rephrase the question a little better.

chmorin

If we wanted to get real evil you could be a smart ass and tell him equal cost load balancing would take place between two identical routes should that be the case.