AXISHK
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Default IP routing
From CCNP Troubleshooting,
ip route 0.0.0.0 0.0.0.0 fastethernet 0/1
Here, instead of ARPing for MAC address of next-hop IP address, you ARP for the MAC address of the destination IP address in each packet. That will result in an excessive number of ARP requests, which will cause strain on the CPU.
I don't understand why there is a lot of ARP request with fasthernet0/1 (instead of next-hop IP address). Router should cache the ARP for fastethernet0/1 and don't need to do arp request each time. Do I misunderstanding anything ?
Thx
ip route 0.0.0.0 0.0.0.0 fastethernet 0/1
Here, instead of ARPing for MAC address of next-hop IP address, you ARP for the MAC address of the destination IP address in each packet. That will result in an excessive number of ARP requests, which will cause strain on the CPU.
I don't understand why there is a lot of ARP request with fasthernet0/1 (instead of next-hop IP address). Router should cache the ARP for fastethernet0/1 and don't need to do arp request each time. Do I misunderstanding anything ?
Thx
ip cef
Problem in this case is that router can be on large network shared segment.
For example, my home network WAN ip address range is /22.
If you just configure interface as next hop in default route - How should receive packet?
Potentially there are 1021 hosts (3 addresses can be excluded right away -network address, broadcast address and router's WAN IP address) that can receive packet.
Since next hop IP address is unknown (destination IP is known) network frame will be configured with MAC address ffff.ffff.ffff as next hop and everyone in the network segment should receive that frame...
Everything goes around one question, what should be criteria for router to decide what is next hop IP address so it can send ARP request only for one host?
For example, my home network WAN ip address range is /22.
If you just configure interface as next hop in default route - How should receive packet?
Potentially there are 1021 hosts (3 addresses can be excluded right away -network address, broadcast address and router's WAN IP address) that can receive packet.
Since next hop IP address is unknown (destination IP is known) network frame will be configured with MAC address ffff.ffff.ffff as next hop and everyone in the network segment should receive that frame...
Everything goes around one question, what should be criteria for router to decide what is next hop IP address so it can send ARP request only for one host?
Hi There,
If the network setup has redundant links to reach the next-hop, the best practice is to mention the next-hop along with the exit interface for the floating static routes to work efficiently.
Hope the below link helps you in understanding the ARP request query that you had.
https://chasingmyccie.wordpress.com/2012/06/05/ip-routing-static-routes-to-an-interface-instead-of-next-hop-addresses/
If the network setup has redundant links to reach the next-hop, the best practice is to mention the next-hop along with the exit interface for the floating static routes to work efficiently.
Hope the below link helps you in understanding the ARP request query that you had.
https://chasingmyccie.wordpress.com/2012/06/05/ip-routing-static-routes-to-an-interface-instead-of-next-hop-addresses/
ASKER
Thx.
"Since next hop IP address is unknown (destination IP is known) network frame will be configured with MAC address ffff.ffff.ffff as next hop and everyone in the network segment should receive that frame"
For configuring "ip route 0.0.0.0 0.0.0.0 fastethernet 0/1" , fastethernet 0/1 is local interface and destination MAC doesn't know, network frame with MAC address ffff.fff.fff will be created.
What will be the case in case next hop IP address 10.0.0.2 (router WAN interface of remote side) be configured ? Does it use ARP request to find the MAC of remote interface and subsequent request will be used the cached destination MAC address ?
ip route 0.0.0.0 0.0.0.0 10.0.0.2
"Since next hop IP address is unknown (destination IP is known) network frame will be configured with MAC address ffff.ffff.ffff as next hop and everyone in the network segment should receive that frame"
For configuring "ip route 0.0.0.0 0.0.0.0 fastethernet 0/1" , fastethernet 0/1 is local interface and destination MAC doesn't know, network frame with MAC address ffff.fff.fff will be created.
What will be the case in case next hop IP address 10.0.0.2 (router WAN interface of remote side) be configured ? Does it use ARP request to find the MAC of remote interface and subsequent request will be used the cached destination MAC address ?
ip route 0.0.0.0 0.0.0.0 10.0.0.2
If default route is configured
ip route 0.0.0.0 0.0.0.0 10.0.0.2
router will use ARP to resolve MAC address of 10.0.0.2.
ARP message in Wireshark looks like:
Who has 10.0.0.2 tell 10.0.0.1?
(10.0.0.1 is WAN IP address) packet is sent to ffff.ffff.ffff MAC address with Ethernet protocol type value - 0x806.
Since destination MAC address is broadcast every device in network segment will receive it. If 10.0.0.2 is present it will respond with 10.0.0.2 is 00AF.128.3456. (00AF.128.3456 is MAC address of next hop device) Response is in this case unicast packet, so only 10.0.0.1 receive it.
After 10.0.0.1 learns MAC address of 10.0.0.2, network frames sent to next hop will be created with that specific MAC address and only 10.0.0.2 will receive those frames (not whole network segment).
ip route 0.0.0.0 0.0.0.0 10.0.0.2
router will use ARP to resolve MAC address of 10.0.0.2.
ARP message in Wireshark looks like:
Who has 10.0.0.2 tell 10.0.0.1?
(10.0.0.1 is WAN IP address) packet is sent to ffff.ffff.ffff MAC address with Ethernet protocol type value - 0x806.
Since destination MAC address is broadcast every device in network segment will receive it. If 10.0.0.2 is present it will respond with 10.0.0.2 is 00AF.128.3456. (00AF.128.3456 is MAC address of next hop device) Response is in this case unicast packet, so only 10.0.0.1 receive it.
After 10.0.0.1 learns MAC address of 10.0.0.2, network frames sent to next hop will be created with that specific MAC address and only 10.0.0.2 will receive those frames (not whole network segment).
Good one Mr. Predrag.
ASKER
#!When configuring interface as next hop :
ARP request will received by each node and each node will response
#2When configuring IP as next hop :
ARP request will received by each node and only the target device will response.
Is it what you mean ? In #1, how does the router know that the remote WAN IP is the next hop ?
Thx
ARP request will received by each node and each node will response
#2When configuring IP as next hop :
ARP request will received by each node and only the target device will response.
Is it what you mean ? In #1, how does the router know that the remote WAN IP is the next hop ?
Thx
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ASKER
Thx
The router will do arp for every packet going out of this interface. That arp request will be done against opposite interface, proxy arp will be used.