Press J to jump to the feed. Press question mark to learn the rest of the keyboard shortcuts
0
Posted byA+ | Network+ | CCNA-RS6 months ago
Archived

Static routing

So I just learnt something new from Keith Bogart's INE videos. While going over static routing, we all know you can use the next hop address or the outgoing interface. But he said it's wrong to use the outing interface for Ethernet lines. Why is this so? Is it because of CSMA?

5 comments
50% Upvoted
This thread is archived
New comments cannot be posted and votes cannot be cast
level 1
CCNA R&S9 points · 6 months ago

Using the interface as the next-hop address in a static route is not considered to be a best practice because your device will attempt to ARP for every IP address that is to be routed for that interface.

For example, if I configure ip route 192.168.0.0 0.0.0.255 Gi0/1, then my device forwards a packet for 192.168.0.10. The device will ARP for the owner of 192.168.0.10. If it needs to forward a packet for 192.168.0.15, it will also ARP for that. This is perfectly normal behavior if the 192.168.0.0/24 network is directly connected to Gi0/1 - however, if Gi0/1 is a point-to-point link between two routers, and 192.168.0.0/24 is actually directly connected to another interface on the opposite router, then we're needlessly ARPing for a wide range of IP addresses when all of them need to be forwarded to only a single IP.

This problem gets much, much worse with default static routes, especially when connected to the internet. If I configure ip route 0.0.0.0 0.0.0.0 Gi0/1, then my device will ARP for every single unknown IP address out of my Gi0/1 WAN interface and store the results in the ARP cache. In even the smallest networks, this can result in a massively-bloated ARP cache and cause delays on the router's data plane, increasing the amount of time it takes for the router to forward a packet.

Let me know if you have more questions!

level 2

nice break down there sir.

level 2
Just 'cause it ain't in my flair doesn't mean I don't have certs1 point · 6 months ago

Also this will fail if you're trying to go more than one hop away and proxy-arp isn't enabled.

level 2
A+ | Network+ | CCNA-RSOriginal Poster1 point · 6 months ago

Thanks for the very detailed response. Keith was indeed talking about ARP for the network on the connected line.

level 3
CCNA R&S2 points · 6 months ago

You're welcome!

Community Details

34.0k

Subscribers

112

Online

Create Post
r/ccna Rules
1.
No posting of illegal materials
2.
No posting of braindumps
3.
Be courteous and helpful
4.
Don't ask others to complete your labs
Cookies help us deliver our Services. By using our Services or clicking I agree, you agree to our use of cookies. Learn More.