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Arp and Proxy Arp Understandings(Who can solve ??)
faisal saleem
Member Posts: 2 ■□□□□□□□□□
in CCNP
Hi Guyz,
I have a simplest network ie 2 PC are connected with a router, here I used R1 as PC1 and R2 as PC2.I simply used GNS3 so no real routers. Now we want to ping from PC1 to PC2 and we have to answers 6 questions regarding ARP and Proxy Arp understanding.
PC1 <
> Router <
>PC2
R1 (f0/0) <
10.1.1.0/24
>(f0/0) R0 (f1/0) <
11.1.1.0/24
> (f0/0)R2
R1-f0/0 IP = 10.1.1.1/24
R1-f0/0 Mac = ca05.1e0c.0000
R0-f0/0 IP = 10.1.1.2/24
R0-f0/0 Mac = ca04.1e0c.0000
R0-f1/0 IP = 11.1.1.2/24
R0-f1/0 Mac = ca04.1e0c.001c
R2-f0/0 IP = 11.1.1.1/24
R2-f0/0 Mac = ca06.1e0c.0000
Note that these are like brand new routers having factory default settings and I do nothing but only set the ip addresses as mentioned above and then I go to R1 and put a route for R2 as well as I go to R2 and put a route for R1.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip route 11.1.1.0 255.255.255.0 10.1.1.2Now I go to R2 and put a route for R1
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#ip route 10.1.1.1 255.255.255.255 11.1.1.2
Okaz over routing is 100% done and both routers R1 and R2 can ping to each other easily. Now before we try to ping to R2 we will first check the arp table at R1
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
========================================================
Qustion#1:- Please explain what are the meanings of Age and Interface? Why this entry is in arp table however we do not try any ping or trace yet?
=========================================================
Now I try to ping to R2
R1#ping 11.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.1.1.1, timeout is 2 seconds:
. !!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 40/88/112 ms
Arp and Proxy arp story (behind the scene):-
Please note that the first dot in the ping out put clearly shows the ”arp broadcast” means R1 simply wants to send a packet (having source L3 = 10.1.1.1 and destination L3 = 11.1.1.1) but he only knows the L3 (means IP) info of the destination so he just send a broadcast on his f0/0 segment and asked to every body that ”hey any 1 knows the L2 ( means Mac) info of 11.1.1.1” so R0 is the only device on this segment hence R0 quickly reply and this is technically called proxy arp.Most of people say it an arp reply but it is not feasible name in the language of professional networking.
So in response of R1’s arp broadcast the R0 send an proxy arp towards R1 in which he is saying that ”hey R1 don’t worry dude I also don’t know the L2( means Mac) info of 11.1.1.1 but I can handle your packets trust me ,you just send me the packets and I will try to deliver to the final destination ie R2.and please use my Mac address as a L2 info. So R1 built the remaining ping packets and he use L3 destination address = 11.1.1.1 and L2 destination mac = f0/0 of R0
Okz we again come to the point where we left , R1 successfully ping to R2 so what should we wana see in the arp cache of R1, we except for a new entry in which L3 address should be 11.1.1.1 and L2 address should be the f0/0 of R0, am I right? Please open your books/videos or Google it and I am sure you will be agree with me what I am saying. so lets check it by show arp
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 5 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Ooops it is showing the new entry having L3 = 10.1.1.2 and L2 = ca04.1e0c.0000 it is the mac of f0/0 of R0.
========================================================
Question#2.What is meant by Age = 5 of first entry and age = ”–” in the second entry? Why both fields are different?
========================================================
Question#3. Why the new arp entry is not as what we studied and except I mean why the destination IP is 10.1.1.2 instead of 11.1.1.1 however we successfully ping from R1 to R2?========================================================
Okz now I wana clear the arp cache and according to this official doc ==>www.cisco.com/en/US/docs/ios/12_0s/feature/guide/arpoptim.html
I should use clear arp interface command so
R1#clear arp interface f0/0
And then again check
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 5 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Seems it is not working,now I use another command ie clear arp-cache , might be it will work
R1#clear arp-cache
And then again check
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 6 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Still un successful .Looks like router is mad or I
or might be the vendor(cisco), we all know better that if router will reboot then surely the fresh table will again buildup but we do not want to reboot so now I put only 2 very very simple commands on the router and after that I check again..
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Wow its worked well LOL u can see only 1 old entry is now in arp table ,,,,,
==========================================================
Qustion#4:- which 2 commands I used? hint = = > we mostly use it as a first aid while troubleshooting the links.
==========================================================
Ok at last I also again put 2 very simple/common commands on R1 and then I try ping to R2
R1#ping 11.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.1.1.1, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 20/53/96 ms
So now I again check the arp table
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 11.1.1.1 1 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Yahooooo
, Now I am satisfied and relax because it is 100% as what I read in books/docs, you can also see that the new entry which is clearly showing L3 destination = 11.1.1.1
======================================================
Question#5:- Which 2 commands I used in the last ?
======================================================
Question#6:- Why in the previous stage the new arp entry was showing destination L3 10.1.1.2 and now after applying 2 command it is showing 11.1.1.1 , what is the logic behind this? Its time to really blow your mind….!
=====================================================
Who can solve ????
Regards,
Faisal.
I have a simplest network ie 2 PC are connected with a router, here I used R1 as PC1 and R2 as PC2.I simply used GNS3 so no real routers. Now we want to ping from PC1 to PC2 and we have to answers 6 questions regarding ARP and Proxy Arp understanding.
PC1 <
> Router <
>PC2
R1 (f0/0) <
10.1.1.0/24
>(f0/0) R0 (f1/0) <
11.1.1.0/24
> (f0/0)R2
R1-f0/0 IP = 10.1.1.1/24
R1-f0/0 Mac = ca05.1e0c.0000
R0-f0/0 IP = 10.1.1.2/24
R0-f0/0 Mac = ca04.1e0c.0000
R0-f1/0 IP = 11.1.1.2/24
R0-f1/0 Mac = ca04.1e0c.001c
R2-f0/0 IP = 11.1.1.1/24
R2-f0/0 Mac = ca06.1e0c.0000
Note that these are like brand new routers having factory default settings and I do nothing but only set the ip addresses as mentioned above and then I go to R1 and put a route for R2 as well as I go to R2 and put a route for R1.
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#ip route 11.1.1.0 255.255.255.0 10.1.1.2Now I go to R2 and put a route for R1
R2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#ip route 10.1.1.1 255.255.255.255 11.1.1.2
Okaz over routing is 100% done and both routers R1 and R2 can ping to each other easily. Now before we try to ping to R2 we will first check the arp table at R1
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
========================================================
Qustion#1:- Please explain what are the meanings of Age and Interface? Why this entry is in arp table however we do not try any ping or trace yet?
=========================================================
Now I try to ping to R2
R1#ping 11.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.1.1.1, timeout is 2 seconds:
. !!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 40/88/112 ms
Arp and Proxy arp story (behind the scene):-
Please note that the first dot in the ping out put clearly shows the ”arp broadcast” means R1 simply wants to send a packet (having source L3 = 10.1.1.1 and destination L3 = 11.1.1.1) but he only knows the L3 (means IP) info of the destination so he just send a broadcast on his f0/0 segment and asked to every body that ”hey any 1 knows the L2 ( means Mac) info of 11.1.1.1” so R0 is the only device on this segment hence R0 quickly reply and this is technically called proxy arp.Most of people say it an arp reply but it is not feasible name in the language of professional networking.
So in response of R1’s arp broadcast the R0 send an proxy arp towards R1 in which he is saying that ”hey R1 don’t worry dude I also don’t know the L2( means Mac) info of 11.1.1.1 but I can handle your packets trust me ,you just send me the packets and I will try to deliver to the final destination ie R2.and please use my Mac address as a L2 info. So R1 built the remaining ping packets and he use L3 destination address = 11.1.1.1 and L2 destination mac = f0/0 of R0
Okz we again come to the point where we left , R1 successfully ping to R2 so what should we wana see in the arp cache of R1, we except for a new entry in which L3 address should be 11.1.1.1 and L2 address should be the f0/0 of R0, am I right? Please open your books/videos or Google it and I am sure you will be agree with me what I am saying. so lets check it by show arp
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 5 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Ooops it is showing the new entry having L3 = 10.1.1.2 and L2 = ca04.1e0c.0000 it is the mac of f0/0 of R0.
========================================================
Question#2.What is meant by Age = 5 of first entry and age = ”–” in the second entry? Why both fields are different?
========================================================
Question#3. Why the new arp entry is not as what we studied and except I mean why the destination IP is 10.1.1.2 instead of 11.1.1.1 however we successfully ping from R1 to R2?========================================================
Okz now I wana clear the arp cache and according to this official doc ==>www.cisco.com/en/US/docs/ios/12_0s/feature/guide/arpoptim.html
I should use clear arp interface command so
R1#clear arp interface f0/0
And then again check
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 5 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Seems it is not working,now I use another command ie clear arp-cache , might be it will work
R1#clear arp-cache
And then again check
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.2 6 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Still un successful .Looks like router is mad or I
or might be the vendor(cisco), we all know better that if router will reboot then surely the fresh table will again buildup but we do not want to reboot so now I put only 2 very very simple commands on the router and after that I check again..R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Wow its worked well LOL u can see only 1 old entry is now in arp table ,,,,,
==========================================================
Qustion#4:- which 2 commands I used? hint = = > we mostly use it as a first aid while troubleshooting the links.
==========================================================
Ok at last I also again put 2 very simple/common commands on R1 and then I try ping to R2
R1#ping 11.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.1.1.1, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 20/53/96 ms
So now I again check the arp table
R1#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 11.1.1.1 1 ca04.1e0c.0000 ARPA FastEthernet0/0
Internet 10.1.1.1 - ca05.1e0c.0000 ARPA FastEthernet0/0
Yahooooo
, Now I am satisfied and relax because it is 100% as what I read in books/docs, you can also see that the new entry which is clearly showing L3 destination = 11.1.1.1======================================================
Question#5:- Which 2 commands I used in the last ?
======================================================
Question#6:- Why in the previous stage the new arp entry was showing destination L3 10.1.1.2 and now after applying 2 command it is showing 11.1.1.1 , what is the logic behind this? Its time to really blow your mind….!
=====================================================
Who can solve
????Regards,
Faisal.