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how can Router to Switch communicate without setting a L3 switch to routing or Router on a Stick..

EARTH-SW #sh cdp neighbor gi0/21 detail
Entry address(es):
  IP address:
Platform: Cisco 2821,  Capabilities: Router Switch IGMP
Interface: GigabitEthernet0/21,  Port ID (outgoing port): GigabitEthernet0/1/0
Holdtime : 124 sec

Version :
Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version 12.4(15)T10, RELEASE SOFTWARE (fc3)
Technical Support:
Copyright (c) 1986-2009 by Cisco Systems, Inc.
Compiled Mon 14-Sep-09 14:01 by prod_rel_team

advertisement version: 2
VTP Management Domain: ''
Duplex: full
Management address(es):


EARTH-SW #sh run int gi0/21
Building configuration...

Current configuration : 301 bytes
interface GigabitEthernet0/21
 description MARS-ROUTER
 switchport mode trunk
 switchport voice vlan 100
 media-type sfp
 srr-queue bandwidth share 10 10 60 20
 queue-set 2
 priority-queue out
 mls qos trust dscp
 auto qos voip trust
 spanning-tree portfast
 service-policy input Police-Phone

mainsw1#sh inv

NAME: "GigabitEthernet0/21", DESCR: "1000BaseZX SFP"
PID:                     , VID:     , SN: FNS13330B9L



EARTH-SW #sh int gi0/21
GigabitEthernet0/21 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is f4ac.c195.5495 (bia f4ac.c195.5495)
  Description: MARS-ROUTER
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not set
  Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseZX SFP
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:04, output 00:00:00, output hang never
  Last clearing of "show interface" counters 1y10w
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 3402000 bits/sec, 697 packets/sec
  5 minute output rate 2212000 bits/sec, 365 packets/sec
     11401675839 packets input, 9903411453201 bytes, 0 no buffer
     Received 12207653 broadcasts (12207504 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 12207504 multicast, 10512 pause input
     0 input packets with dribble condition detected
     6534791519 packets output, 1196188409875 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out


site to site   fiber--  Site to site..





MARS-ROUTER #sh cdp neighbors gi0/1/0 detail
Entry address(es):
  IP address:
Platform: cisco WS-C2960G-24TC-L,  Capabilities: Switch IGMP
Interface: GigabitEthernet0/1/0,  Port ID (outgoing port): GigabitEthernet0/21
Holdtime : 167 sec


MARS-ROUTER  #sh run int gi0/1/0
Building configuration...

Current configuration : 169 bytes
interface GigabitEthernet0/1/0
 ip address
 ip ospf message-digest-key 1 md5 ***************
 negotiation auto
 service-policy output LLQ


NAME: "High Speed WAN Interface Card - 1 Port Gigabit Ethernet on Slot 0 SubSlot 1", DESCR: "High Speed WAN Interface Card - 1 Port Gigabit Ethernet"
PID: HWIC-1GE-SFP      , VID: V01 , SN: FOC133819JC

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