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Name:MageeJoin In:Feb 21, 2019
Difference between next hop and outbound interface in CCNA ...

Post by Magee » Dec 30, 2018

We know that when configuring a static route, you can use the next hop address or the outbound interface. Using the next hop IP address and using the outbound interface will affect the router's processing.
Figure 1
DEVICE ID IP-ADDRESS INTERFACE NEIGHBOR
R1 12.12.12.1 /30 FASTETHERNET 0/0 R2
R2 12.12.12.2/30 FASTETHERNET 0/0 R1
23.23.23.1/30 FASTETHERNET 1/0 R3
R3 23.23.23.2/30 FASTETHERNET 0/0 R2
Requirement: R1 can ping the IP address of the R3 interface.
1. In Ethernet, communication between two interfaces requires the use of a MAC address. According to the MAC address, the data is encapsulated into data frames and transmitted to the network, which is then transmitted to the other party through the physical line. Obtaining the MAC address of the other party is done through the ARP process.
a. When an outbound interface is used in a static route, the router considers the target network and interface to be in a "directly connected network."
R1(config)#ip route 23.23.23.0 255.255.255.252 f0/0 View: At this time, R1 thinks that 23.23.23.0/30 is directly connected to itself.

Figure II

In the Ethernet direct-connected network, communication between devices is obtained through ARP broadcast to obtain the MAC address of the target host. Ie when R1 wants to access
23.23.23.3 When this IP address is used, R1 considers that the target network is its own direct connection network (this time is still a static route), so R1 will send an ARP request broadcast to the network at F0/0 to find the corresponding 23.23.23.3. MAC address.
At this time, if R2 has the ARP proxy enabled, R2 will acknowledge the ARP request by R3, that is, R2 tells R1 that the MAC address corresponding to 23.23.23.3 is the MAC of the F0/0 interface of R2. If R2's ARP proxy feature is turned off, R1 will not be able to ping 23.23.23.3.
Assume that R3 is followed by some PCs. When R1 wants to access these PCs, it will generate an ARP entry cache corresponding to the PC and MAC address. If the number of PCs is large, the cache is also large. Causes R1 to run out of memory for maintenance.
b. The next hop is used in the static route, and the IP of the next hop is displayed in the routing table. R1(config)#ip route 23.23.23.0 255.255.255.252 12.12.12.2
View:

Figure III


At this time, the network segment going to 23.23.23.0/30 will only maintain an ARP cache, that is, 12.12.12.2 corresponds to the F0/0 interface address of R2. Even if R3 is followed by multiple hosts, only one ARP cache record needs to be maintained.
2. In a point-to-point network environment, whether it is to specify the next hop address or the outbound interface, the effect is the same. Layer 2 encapsulation is performed using protocols such as HDLC and PPP, and ARP resolution is not required.
in conclusion:
When you configure a static route, you can specify the outbound interface or the next hop of the bit. Which method is used depends on the actual situation. In a point-to-point network environment, the effect is the same whether you specify the next hop address or the outgoing interface.
However, in a broadcast network environment, specifying the next hop address and specifying the outgoing interface will achieve different effects. If specified as the outbound interface, then regardless of whether the destination address of the packet is valid, an ARP request will be triggered each time the packet arrives, and because the ARP proxy function is enabled by default in the IOS environment, this means that the router needs Equipped with a large number of ARP caches. If it is specified as the next hop address, the ARP request will only be triggered when the first packet destined for the destination network arrives.
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