From: Kenneth Wygand (KWygand@customonline.com)
Date: Fri Apr 23 2004 - 15:49:10 GMT-3
It will send the packet out based on the longest match in the routing
table, regardless of whether or not the router knows about it as a
directly connected interface. See below output capture.
<snip>
Lab_A#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Lab_A(config)#exit
Lab_A#sh ip int brie
04:11:36: %SYS-5-CONFIG_I: Configured from console by consolef
Interface IP-Address OK? Method Status
Protocol
Ethernet0 192.168.0.1 YES manual up
up
Serial0 201.101.0.1 YES manual up
up
Serial1 200.100.0.1 YES manual administratively down
down
Lab_A#debug ip packet
IP packet debugging is on
Lab_A#
Lab_A#
Lab_A#
Lab_A#ping 192.168.0.10
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.10, timeout is 2 seconds:
04:11:47: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, sendin
g
04:11:47: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, encaps
ulation failed.
04:11:49: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, sendin
g
04:11:49: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, encaps
ulation failed.
04:11:51: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, sendin
g
04:11:51: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, encaps
ulation failed.
04:11:53: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, sendin
g
04:11:53: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, encaps
ulation failed.
04:11:55: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, sendin
g
04:11:55: IP: s=192.168.0.1 (local), d=192.168.0.10 (Ethernet0), len
100, encaps
ulation failed.
Success rate is 0 percent (0/5)
Lab_A#
Lab_A#
Lab_A#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Lab_A(config)#ip route 192.168.0.10 255.255.255.255 ser0
Lab_A(config)#exit
Lab_A#ping 192.168.0.10
04:12:15: %SYS-5-CONFIG_I: Configured from console by console
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.10, timeout is 2 seconds:
04:12:18: IP: s=201.101.0.1 (local), d=192.168.0.10 (Serial0), len 100,
sending.
04:12:20: IP: s=201.101.0.1 (local), d=192.168.0.10 (Serial0), len 100,
sending.
04:12:22: IP: s=201.101.0.1 (local), d=192.168.0.10 (Serial0), len 100,
sending.
04:12:24: IP: s=201.101.0.1 (local), d=192.168.0.10 (Serial0), len 100,
sending.
04:12:26: IP: s=201.101.0.1 (local), d=192.168.0.10 (Serial0), len 100,
sending.
Success rate is 0 percent (0/5)
Lab_A#
</snip>
Kenneth E. Wygand
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, ACSP, Cisco IPT Design Specialist, MCP, CNA,
Network+, A+
Custom Computer Specialists, Inc.
"The only unattainable goal is the one not attempted."
-Anonymous
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
BALAKRISHNAN Balaji
Sent: Friday, April 23, 2004 2:30 PM
To: ccielab@groupstudy.com
Subject: RFC 1812 " Requirements for IP Version 4 Routers" ??
Hi group,
I have a clarification on RFC1812 about route forwarding decision..
When a router received a packet, if the destination ip is part of its
directly connected interface subnet , it will do the arp request for
that destination ip out to that interface. Now, we can change that
behavior by configuring more specific route for that destination ( say
/32 host route) pointing to different gateway. On this case, router
forwards the packet to the next-hop defined. right ???
But my manager don't agree with that. He says if destination IP address
in the packet is on the same subnet as any Ethernet interface of the
router, then the router should not do any route lookup at all, and must
immediately forward the packet on that matching interface . Ie. more
specific route present in the router's routing table is then not
relevant. He pointing to the RFC 1812 section "5.2.4.2 Local/Remote
decision ". The section goes line this ..the following algorithm MUST be
used to determine if the Immediate Destination is directly accessible
quote
* isolate the network prefix used by the interface.
* Isolate the corresponding set of bits from the IP Destination
Address of the packet.
* Compare the resulting network prefixes. If they are equal to each
other, the packet can be transmitted through the corresponding network
interface.
* If the destination was not a member of a directly connected network
prefix, the IP Destination is accessible only through some other
router.
unquote
I believe it does not consider all the possibilities for a route
decision. Do anybody know better RFC reference that I can point to my
manager to prove that I am right ???
- bala.
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