From: paul cosgrove (paul.cosgrove@gmail.com)
Date: Wed Mar 04 2009 - 18:48:21 ARST
Actually Khurram has an interesting question there. On ethernet you can
disable ip routing and still ping IPs within your vlan (or off it if you
have a default gateway etc.), so why should you not be able to do something
similar on frame relay?
Ethernet hosts can:
- select an outgoing interface to use by comparing the destination IP with
their own network addresses (interface IP/mask).
- can send an ARP to determine the layer 2 address of the destination
- add a layer 2 header
- send the packet on the wire
Disabling ip routing on a frame relay router, changes it to be a frame relay
bridge. The outgoing interface can still be determined from comparison with
local IP/Masks, but neither static ip maps nor inverse arp seem to operate
in this mode.
With ip routing disabled, the frame bridge (/ex-router) tries to perform ARP
by bridging packets out its frame relay intefaces, using the mac address of
one of its ethernet interfaces as the source. The ARPs are broadcasts, just
like other ethernet arps, and unfortunately frame relay does not natively
support broadcasts. Because static ip mappings are ignored, the frame
bridge (/ex-router) cannot determine which DLCI to send the ARP broadcasts
out. Hence the packets are dropped.
You can see this with the following back to back frame relay configuration:-
R5#sh run | inc ip routing
no ip routing
R5#
interface Serial0/0/1
description ==== Back to Back FR ====
ip address 192.168.45.5 255.255.255.0
encapsulation frame-relay
no ip route-cache
no keepalive
frame-relay map ip 192.168.45.4 45 broadcast ;ignored with ip routing
disabled
no frame-relay inverse-arp
frame-relay intf-type dce
end
R5#ping 192.168.45.4
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 192.168.45.4, timeout is 2 seconds:
*Mar 4 19:43:30.475: IP ARP: creating incomplete entry for IP address:
192.168.45.4 interface Serial0/0/0.1
*Mar 4 19:43:30.475: IP ARP: sent req src 132.1.35.5 0,
dst 192.168.45.4 0 Serial0/0/0.1
*Mar 4 19:43:30.479: Serial0/0/0.1: broadcast search
*Mar 4 19:43:30.479: Serial0/0/0.1(o): dlci 513(0x8011), pkt type 0x6558,
datagramsize 38
*Mar 4 19:43:30.479: bridged dest ffff.ffff.ffff, src 0014.a8cd.0648,
type 806
*Mar 4 19:43:30.479: IP ARP: sent req src 132.1.35.5 0014.a8cd.0648,
dst 192.168.45.4 0000.0000.0000 Serial0/0/0.1
*Mar 4 19:43:30.479: Serial0/0/0.1: broadcast search
*Mar 4 19:43:30.479: Serial0/0/0.1(o): dlci 513(0x8011), pkt type 0x6558,
datagramsize 46
*Mar 4 19:43:30.479: bridged dest ffff.ffff.ffff, src 0014.a8cd.0648,
type 806
*Mar 4 19:43:30.479: IP ARP: sent req src 192.168.45.5 0,
dst 192.168.45.4 0 Serial0/0/1
*Mar 4 19:43:30.479: Serial0/0/1: broadcast search
*Mar 4 19:43:30.479: Serial0/0/1:encaps failed on broadcast for link
38(BRIDGE)
*Mar 4 19:43:30.479: IP ARP: sent req src 192.168.45.5 0014.a8cd.0648,
dst 192.168.45.4 0000.0000.0000 Serial0/0/1
*Mar 4 19:43:30.479: Serial0/0/1: broadcast search
*Mar 4 19:43:30.479: Serial0/0/1:encaps failed on broadcast for link
38(BRIDGE)
*Mar 4 19:43:30.479: broadcast dequeue
....snip...
You can successfully send the frames out if you specify a static map for the
bridge group, so that the router knows which dlci to use for bridged
packets.
frame-relay map bridge 100 broadcast
But unless you configure the other device it will drop the packets.
If you disable ip routing and configure a static bridge mapping at both
devices, you can then ping between them.
Hope this helps,
Paul.
On Wed, Mar 4, 2009 at 7:25 PM, <joe_astorino@comcast.net> wrote:
> What nadeem said. Try "sh ip route" after you disable ip routing.
> ----- Original Message -----
> From: "khurram noor" <smartcapricon82@gmail.com>
> To: "Cisco certification" <ccielab@groupstudy.com>
> Sent: Wednesday, March 4, 2009 10:12:15 AM GMT -05:00 US/Canada Eastern
> Subject: IP Routing
>
> Hi fellows,
> *
> "Routers connected to each other using Frame relay,** either point to point
> or multipoint style, cannot **ping each other when ip routing is disabled
> on
> either end."*
>
> What is the theoretical concept behind this? I had a terrible time figuring
> it out during practicing one of IE's lab.
>
> --
> Khurram Noor
> CCIP, CCNA
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
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