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router process

Ibrahim Jamil
Frequent Contributor
Frequent Contributor

Hi Netpro

whicj of the below line the router process first?pls a bit explanation

ip route 1.2.3.4 255.255.255.255 4.4.4.4

ip route 1.2.3.0 255.255.255.0 4.4.4.4.4

thanks

8 Replies 8

Neeraj Arora
Participant
Participant

Jamil,

Router/Switch/FW or any layer 3 device will always try to match using the longest prefix first and then check the other less specific or summary routes.

for an eg. if you have the routes as below:

ip route 1.2.3.4 255.255.255.255 3.3.3.3

ip route 1.2.3.0 255.255.255.0 4.4.4.4

Any traffic going towards 1.2.3.4 will go towards 3.3.3.3 and any other traffic destined towards

1.2.3.0/24 except for the host 1.2.3.4, it will use the other link towards 4.4.4.4

this way you can utilize multiple outgoing links, if required

Neeraj

hi Neeraj

i made a mistake in question,must be like below:

ip route 1.2.3.4 255.255.255.255 3.3.3.3

ip route 1.2.3.0 255.255.255.0 3.3.3.3

still the answer would be the same

it will look for the longest prefix match and if you are sending traffic towards 1.2.3.4, then it will chose the first route in your config as it matches it perfectly

but if you send the traffic towards 1.2.3.5 or 1.2.3.6 and so on...it will chose the second route because the first route is not an exact match

Hi,

what's the use if they both point to the same next-hop ?

Regards.

Alain

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Hi Alain

its just a scenario, to see which of these process first?

Hi,

it's not a question of which is gonna be processed first but which of these 2 entries will be used to forward a packet to a certain destination and as these are 2 different prefixes( prefix-length) then the longest match will get used.

Regards.

Alain

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Hi

so

ip route 1.2.3.4 255.255.255.255 3.3.3.3  ----->>>>Wins because of the longest match /32 am i right???

ip route 1.2.3.0 255.255.255.0 3.3.3.3

Hi,

if the destination IP is 1.2.3.4  then yes otherwise the router will use the second static route

Regards.

Alain

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