Troubleshooting guide

11-3
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Overview of Layer 3 Switching
Figure 11-1 Phase 1 — Layer 3 Packet Flow
Figure 11-2 shows and describes, in Steps 5 through 7, how the route processor sends the ARP and
propagates the updated routing tables to the interfaces.
Note In Figure 11-2, the ARP requests are described only for illustration purposes. In most cases, if you
are running a dynamic protocol, the switches will have already sent and received ARP packets, and
built the route tables.
ARP Request Packet:
Source MAC: 00:10:e3:aa...
Destination MAC:
Source IP:172.1.1.5
Destination IP: 172.1.3.10
Ingress Interface Route Table:
IP Add: 172.1.1.5
MAC Add: 00:10:e3:aa...
IF: Fa 3/0/0
IP Add:
MySubnet:
My MAC: 00:90:21:bb...
IF-Map: Fa 3/0/0
Broute VC-79
... (other routes)
Central CPU Route Table:
IP Add: 172.1.1.5
MAC Add: 00:10:e3:aa...
IF: Fa 3/0/0
IP Add:
MySubnet:
My MAC: 00:90:21:bb...
IF-Map: Fa 3/0/0
Broute VC-79
... (other routes)
Copy
Host A
IP Add: 172.1.1.5
MAC Add: 00:10:e3:aa...
Egress Interface:
Giga 9/1/0
IP: 172.1.2.2
MAC: 00:90:21:cc...
CPU
Ingress Interface:
Giga 1/0/0
IP: 172.1.2.5
MAC: 00:90:21:dd...
Egress Interface:
Fast 2/1/5
IP: 172.1.3.8
MAC: 00:90:21:ee...
Host B
IP Add: 172.1.3.10
MAC Add: 00:01:02:ff...
C8540CSR-1
C8540CSR-2
CPU
Ingress Interface:
Fast 3/0/0
IP: 172.1.1.8
MAC: 00:90:21:bb...
Step 1: Host A sends ARP
request to learn MAC address
of the ingress interface
Step 2:
.
Step 3: CPU updates central
route table with Host A
information and updates all
interface route tables
Step 4. CPU sends ARP
response back to Host A
with ingress interface
MAC address
49954
Ingress interface forwards
ARP request to CPU for processing