routing to next-hop vs routing to interface

concept learnt from IE’s Vol5.0 workbook for “IP Routing”

  • When routing to a next-hop value the router performs L2 to L3 resolution on the next-hop address. (e.g. ip route 150.1.4.4 255.255.255.255 155.1.146.4). So in the arp table, you’ll see the MAC for ip address: 155.1.146.4.
  • When routing to an INTERFACE, the router performs L2 to L3 resolution on the FINAL destination (not on the next hop). (e.g. ip route 150.1.6.6 255.255.255.255 fa0/0 configured on Router1). Let’s assume 150.1.6.6 is a Loopback interface on Router6 and Router 6 is connected to the LAN via Fa0/6. When we configure the ip route mentioned above on R1, on R1’s ARP table, you’ll see the MAC address of Fa0/6 interface for the loopback of R6 (i.e. 150.1.6.6). This is because, PROXY ARP is enabled by default on the routers. If we were to disable proxy arp on Fa0/6, you’d notice that you won’t be able to ping the loopback of R6 anymore, since the router does not know the correct l2 address to use when building the L2 frame. You’ll see “encapsulation failed” message in the debugs:

*Mar  5 02:18:49.733: IP ARP: creating incomplete entry for IP address: 150.1.6.6 interface FastEthernet0/0
*Mar  5 02:18:49.733: IP ARP: sent req src 155.1.146.1 000f.f756.6560,
                 dst 150.1.6.6 0000.0000.0000 FastEthernet0/0
*Mar  5 02:18:49.733: IP: s=155.1.146.1 (local), d=150.1.6.6 (FastEthernet0/0), len 100, encapsulation failed.

  • Resolution: 1) change the ip routing so it uses next hop rather than ARPing on Final destination. 2)statically configure the MAC address to use when sending packet to the loopback of R6 by using: router(config)”arp 150.1.6.6 <mac> arpa command.

Q-OSPF

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