Thursday, June 08, 2006

 

SNAP Encapsulation for ARP. (Problem #69)



Consider the simple network above with the two routers connected to each other over ethernet. After pinging R2 from R1, show arp on the routers shows

R1

R1#show arp
R1#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 12.1.1.1 - aabb.cc00.8300 ARPA Ethernet0
Internet 12.1.1.2 1 aabb.cc00.8400 ARPA Ethernet0

R2

R2#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 12.1.1.1 1 aabb.cc00.8300 ARPA Ethernet0
Internet 12.1.1.2 - aabb.cc00.8400 ARPA Ethernet0

Your task in this problem is to figure out what needs to be done so that the show arp output on both routers changes to

R1

R1#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 12.1.1.1 - aabb.cc00.8300 ARPA Ethernet0
Internet 12.1.1.2 12 aabb.cc00.8400 SNAP Ethernet0

R2

R2#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 12.1.1.1 13 aabb.cc00.8300 SNAP Ethernet0
Internet 12.1.1.2 - aabb.cc00.8400 ARPA Ethernet0

Note that now the "Type" field in the output of show arp indicates SNAP encapsulation for the peer's MAC address.

Comments:
Under the fastEthernet interface:

(config-if)#arp snap
(config-if)#no arp arpa

clear arp, generate some traffic, and the new arp entry will show up as "SNAP".
 
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