Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Troubleshooting IP Layer 3 Connections
The following is an explanation of the information in the Next Hop Column:
• Attached—This is a directly connected interface subnet. For example, 10.85.40.0/24 is the IP subnet
assigned to interface Fast Ethernet1/0/15 with a 24-bit mask.
• Received—These entries are ARP entries for the directly connected interfaces. You will see three
entries here for each directly connected interface. For example, prefix 10.85.40.254/32 is the IP
address for interface Fast Ethernet 1/0/15. Prefix 10.85.40.0/32 using IP conventions means that this
specific interface and prefix 10.85.40.255/32 is the broadcast address.
• xxxx.yyyy.zzzz.aaaa—These IP addresses belong to either the end station connected to the interface
(ARP entries) or the next-hop router for a specific subnet. For example, prefix 10.85.40.5/32 is an
end station. The prefix entry and next-hop entry are the same. Prefix entry 10.85.45.0/24 is a route
learned via next-hop 10.85.66.5.
Step 2 From the Catalyst 8540-1, use the show ip cef command with the destination network IP address to
display the CEF FIB table entry for the network connecting Host B to Catalyst 8540-2 with IP address
10.85.45.0.
C8540CSR-1# show ip cef 10.85.45.0
10.85.45.0/24, version 22, cached adjacency 10.85.66.5
via 10.85.66.5, GigabitEthernet0/0/0, 0 dependencies
next hop 10.85.66.5, GigabitEthernet0/0/0
valid cached adjacency
The display confirms that the next hop IP address 10.85.66.5 is valid and has a valid cached adjacency.
Step 3 From the Catalyst 8540-1, use the show adjacency command to display the MAC address rewrite
information for the connection from Catalyst 8540-1 to Catalyst 8540-2 with IP address 10.85.66.5.
C8540CSR-1# show adjacency GigabitEthernet 0/0/0 detail
Protocol Interface Address
IP GigabitEthernet0/0/0 10.85.66.5(9)
0 packets, 0 bytes
009021DDDDDD009021CCCCCC0800
ARP 03:59:57
The display confirms the MAC address rewrite information as:
• 009021DDDDDD—the Catalyst 8540-2 destination MAC address
• 009021CCCCCC—the Catalyst 8540-1 source MAC address
• 0800—protocol field, IP ARPA (IP Ethernet type [hex 0800])
Note The MAC addresses of the source and destination interfaces is displayed using the show interface
command.
If the next hop interface does not display the correct MAC address rewrite information, use the show arp
command to confirm the MAC addresses.