source IP addr question

ebykm

Hi all, sorry if this have been asked earlier.

when pinging from a router (connected to console port) to another external IP, which IP is choosen by the router ?. the ip corresponding to the ping destination ?.

eg. if i ping to a host 200.1.3.35, does the host get ping from e0 ?

routerA
e0 200.1.3.x /26, e1 200.1.4.x /26
s0 200.1.1.x /24
s1 200.1.2.x /24

also when ospf & loopbacks are used, does it simply use router ID than the corresponding interface IP ?.

thanks.

BubbaJ

It uses the address of the interface. You can change this by using extended ping.

forbesl

BubbaJ wrote:

It uses the address of the interface. You can change this by using extended ping.

Or a source ping..

ping xxx.xxx.xxx.xxx source <interface name>

marlon23

yes e0.

It'll be IP of router's outgoing interface for that destination (depending on routing table which one).

dmaftei

ebykm wrote:

also when ospf & loopbacks are used, does it simply use router ID than the corresponding interface IP ?

OSPF and the router id have nothing to do with ping.

Puffy Elvis

Don't apologize, that's a great question!

Here are some follow-up questions...

1) Lets assume that you are NOT running any routing protocols and do not have any static routes configured. Are you only able to ping directly connected networks?

2) Lets assume that you run RIP and try to ping a non-directly connected network that has more than one interface that reaches the destination, which interface will the ICMP message travel from?

3) Lets assume you are running IGRP with equal cost load balancing and you try to ping a non-directly connected network that has 2 interfaces that reaches the destination AND the metric to reach the network is the same though both interfaces. Which interface will the ICMP message travel from?

My answers... see if everyone agrees:

1) You will only be able to ping directly connected networks. Not sure of the error you'll get back- probably destination unreachable?

2) The ping will travel from the interface with the lowest hop count to the destination, as illustrated by the routing table.

3) My best guess is that the ping will alternate going out both interfaces, one ping at a time.

EdTheLad

Puffy Elvis wrote:

3) Lets assume you are running IGRP with equal cost load balancing and you try to ping a non-directly connected network that has 2 interfaces that reaches the destination AND the metric to reach the network is the same though both interfaces. Which interface will the ICMP message travel from?

My answers... see if everyone agrees:


3) My best guess is that the ping will alternate going out both interfaces, one ping at a time.

By default the router will be setup for fast switching i.e. it will use the route cache.Therefore it will choose one interface.If you ping a different destination address on the same network it will use the second interface.So its not a packet for packet load balance rather its a per destination load balance.
If you require an equal load balance you need to configure process switching which means the router does a route table lookup for each packet recieved and hence is slower and more processor intensive.