System information

Troubleshooting XNS
Book Title
14-306
Backdoor bridge
between segments
Step 1 Use the show xns traffic exec command to determine whether the bad hop count
field is incrementing. The XNS network updates by default occur every 30
seconds:
C4000#show xns traffic
Rec: 3968 total, 0 format errors, 0 checksum errors,
0 bad hop count,
3968 local destination, 0 multicast
[...]
Step 2
If this counter is increasing, use a network analyzer to look for packet loops on
suspect segments. Look for routing updates. If a backdoor bridge exists, you will
probably see hop counts that increment up to 15, at which point the route
disappears. The route reappears unpredictably.
Step 3 Use a network analyzer to examine the traffic on each segment. Look for known
remote network numbers that appear on the local network. That is, look for
packets from a remote network whose source address is not the source address of
the router.
The backdoor is located on the segment on which a packet from a remote network
appears whose source address is not the source address of a local router. To
prevent XNS routing updates from being learned from the interface connected to
the same segment as the backdoor bridge, you can use the xns
input-network-filter command.
Example:
In the following example, access list 476 controls which networks are added to the
routing table when RIP packets are received on Ethernet interface 1. Network 16
is the only network whose information will be added to the routing table. Routing
updates for all other networks are implicitly denied and are not added to the
routing table:
access-list 476 permit 16
interface ethernet 1
xns input-network-filter 476
1 RIP = Routing Information Protocol
Possible Problem Solution