How can router understand protocols such as SSH or telnet

[engahmedsaied]

How can router understand protocols such as SSH or telnet 
and router layer 3 device 

second matter I found this attachment from CCNA security book Keith Barker 
wrote there router look at application layer information how ?!!!

Thank you in advance.

[aydinnmu1 · Accepted solution]

Hi,

i think you confused routing process.

Router ; route the packets by using their layer 3 address.

this is not mean router cannot understand upper layer protocols. it just forwarding by layer 3 addressess.

for example: we can define Access-list for layer 4 tcp and udp packets. router can decide permit or deny although these lists filter by look up packet's layer 4 section.

In SSH or Telnet  session router's role is end device.

In routing process router's role is intermediate device.

Best regards.