Default CEF entries
By stretch | Thursday, June 5, 2008 at 5:19 a.m. UTC
It's interesting that even with an empty routing table, a modern Cisco router has several active CEF entries. Fire up an unconfigured router and issue the command
show ip cef to view the default entries. The following output was observed on a 3725 running IOS 12.4(9)T1:
Router# show ip cef Prefix Next Hop Interface 0.0.0.0/0 drop Null0 (default route handler entry) 0.0.0.0/32 receive 18.104.22.168/4 drop 22.214.171.124/24 receive 255.255.255.255/32 receive
I'm going to take a shot at identifying the purpose of each entry, in reverse order:
255.255.255.255/32- This is the local broadcast address for a subnet
126.96.36.199/24- The multicast block reserved for local network control traffic
188.8.131.52/4- All other multicast traffic (originally Class D space) should be dropped when multicast routing is disabled
0.0.0.0/32- Not sure, but assumed to be a reserved link-local address
0.0.0.0/0- Default entry matching all other addresses
About the Author
Jeremy Stretch is a network engineer living in the Raleigh-Durham, North Carolina area. He is known for his blog and cheat sheets here at Packet Life. You can reach him by email or follow him on Twitter.
Posted in Routing
June 5, 2008 at 1:05 p.m. UTC
I went to the IANA website and pulled this from RFC 3330
- Global and Other Specialized Address Blocks
0.0.0.0/8 - Addresses in this block refer to source hosts on "this" network. Address 0.0.0.0/32 may be used as a source address for this host on this network; other addresses within 0.0.0.0/8 may be used to refer to specified hosts on this network [RFC1700, page 4].
I hope this helps.
June 5, 2008 at 4:56 p.m. UTC
The 0.0.0.0/32 is the all-zero broadcast (I know, ancient, but obviously still supported).
April 30, 2013 at 12:34 p.m. UTC
Thanks for your blog.
Are subnet broadcast IP addresses just show up on CEF table?
February 27, 2014 at 3:13 a.m. UTC
Your blog is wild. This is the second or third times I found an answer to the questions I have, looking for an answer on different resources for a few days.
I am studying for the CCNA RS and working on IPv6 troubleshooting currently. I am debugging an IOS v15 to try to understand what is happening regarding addresses and routes.
Keep up the really good work!