Let’s imagine that we have a simple topology like below:
Configure subinterfaces on both AR routers for VLAN tag termination:
[R1]interface GigabitEthernet 0/0/0.100
[R1-GigabitEthernet0/0/0.100]ip address 10.0.0.1 24
[R1-GigabitEthernet0/0/0.100]dot1q termination vid 100
[R2]int GigabitEthernet 0/0/0.100
[R2-GigabitEthernet0/0/0.100]ip address 10.0.0.2 24
[R2-GigabitEthernet0/0/0.100]dot1q termination vid 100
Try to ping IP address of neighboring router:
[R1]ping 10.0.0.2
PING 10.0.0.2: 56 data bytes, press CTRL_C to break
Request time out
Request time out
Request time out
Request time out
Request time out
--- 10.0.0.2 ping statistics ---
5 packet(s) transmitted
0 packet(s) received
100.00% packet loss
Let’s troubleshoot this problem by checking ARP table:
[R1]display arp all
IP ADDRESS MAC ADDRESS EXPIRE(M) TYPE INTERFACE VPN-INSTANCE
VLAN/CEVLAN PVC
------------------------------------------------------------------------------
10.0.0.1 5489-98d1-3ea6 I - GE0/0/0.100
------------------------------------------------------------------------------
Total:1 Dynamic:0 Static:0 Interface:1
Why can’t we see neighboring router in ARP table?
Because subinterfaces for VLAN tag termination discard broadcast packets after receiving the packets.
How to deal with it?
Just enable ARP broadcast on subinterfaces and check again:
[R1-GigabitEthernet0/0/0.100]arp broadcast enable
[R1]ping 10.0.0.2
PING 10.0.0.2: 56 data bytes, press CTRL_C to break
Reply from 10.0.0.2: bytes=56 Sequence=1 ttl=255 time=110 ms
Reply from 10.0.0.2: bytes=56 Sequence=2 ttl=255 time=40 ms
Reply from 10.0.0.2: bytes=56 Sequence=3 ttl=255 time=10 ms
Reply from 10.0.0.2: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.0.0.2: bytes=56 Sequence=5 ttl=255 time=50 ms
--- 10.0.0.2 ping statistics ---
5 packet(s) transmitted
5 packet(s) received
0.00% packet loss
round-trip min/avg/max = 10/46/110 ms
[R1]dis arp all
IP ADDRESS MAC ADDRESS EXPIRE(M) TYPE INTERFACE VPN-INSTANCE
VLAN/CEVLAN PVC
------------------------------------------------------------------------------
10.0.0.1 5489-98d1-3ea6 I - GE0/0/0.100
10.0.0.2 5489-9898-4fc2 20 DF0 GE0/0/0.100
100/-
------------------------------------------------------------------------------
Total:2 Dynamic:1 Static:0 Interface:1
A packet can be forwarded, without ARP broadcast on the subinterface for VLAN tag termination, if the access device can send ARP packets.
If the access device cannot send ARP packets, the system discards the packet, when the arp broadcast enable command is not run on the subinterface for VLAN tag termination.
The system tags an ARP broadcast packet and forwards it through the subinterface for VLAN tag termination, when the arp broadcast enable command is run on the subinterface for VLAN tag termination.
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