Troubleshooting
Unusual Network Activity
that happens to include the switch’s IP address. For an example of
this problem, refer to the section titled “General ACL Operating
Notes” in the “Access Control Lists (ACLs)” chapter of the latest
Access Security Guide for your switch.
Routing Through a Gateway on the Switch Fails
Configuring a “deny” ACE that includes a gateway address can block traffic
attempting to use the gateway as a next-hop.
Remote Gateway Case. For example, configuring ACL “101” (below) and
applying it outbound on VLAN 1 in figure C-4 includes the router gateway
(10.0.8.1) needed by devices on other networks. This can prevent the switch
from sending ARP and other routing messages to the gateway router to
support traffic from authorized remote networks.
In figure C-4, this ACE
denies access to the
10 Net’s 10.0.8.1 router
gateway needed by
the 20 Net.
(Subnet mask is
255.255.255.0.)
Figure C-3. Example of ACE Blocking an Entire Subnet
30 Net
IP: 30.29.16.1
(Deflt. Gateway)
Router X
10 Net
IP: 10.0.8.1
8212zl
10 Net -- VLAN 1
IP: 10.08.15
(Deflt. G’Way = 10.0.8.1)
Switch 1
20 Net -- VLAN 2
IP: 20.0.8.21
(Deflt. G’way = 20.0.8.1)
20 Net VLAN 2
IP: 20.0.8.1
(Deflt. G’way
f o r 2 0 . 0 . 8 . 1 )
30.29.16.91
Switch 2
10 Net -- VLAN 1
IP: 10.0.8.16
(Deflt. G’way = 10.0.8.1)
Switch 1 cannot
access the 30 Net on
Router X because ACL
101 on the Switch
8212zl denies routed,
outbound IP traffic to
the 10 Net.
Figure C-4. Example of Inadvertently Blocking a Gateway
C-12
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