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core switch catalyst 4507R default gateway...


hai all...... newbie here,


i got issue, when i change my ip firewall subnet besides 0/16 that core switch can't grab that ip..

my ip using here in network class A...


sorry for bad english



7 Replies 7

Hello Diley,


I hope you are fine!


From what I understand on your description when you change the ip of the interface of the firewall the switch cannot connect to it, could you pleas send the configuration changes, the mac of both interfaces (ASA and Switch) and a show arp of the asa.

hii Kornelia Gutierrez....


sorry for late feedback... got long training course....

yup, if i change my ip interface firewall other from  /16 that coreswitch cant grab...


default route my core

my firewall ip 

i need to change my ip firewall to


show arp asa? please enlighten me

Hi Diiey,


Sorry for the late response, I have been busy, on the ASA I would need you to run the following command:


show arp | in 10.38.50


Also could you please let me know what is the ip for the switch? I understand the ASA's ip is, who is



hai, Kornelia Gutierrez

thanks for your support....


hope this its what u need


SGBNTCCSW-J2.2-1#sh arp | in 10.38.50
Internet - 000f.342a.46bf ARPA Vlan50
Internet 0 001e.4f27.41f7 ARPA Vlan50

SGBNTCCSW-J2.2-1#sh ip route
Gateway of last resort is to network is subnetted, 1 subnets
C is directly connected, Vlan51 is variably subnetted, 20 subnets, 5 masks
C is directly connected, Vlan52
C is directly connected, Vlan53
C is directly connected, Vlan10
C is directly connected, Vlan54
C is directly connected, Vlan50
C is directly connected, Vlan110
C is directly connected, Vlan101
C is directly connected, Vlan124
C is directly connected, Vlan125
C is directly connected, Vlan120
C is directly connected, Vlan121
C is directly connected, Vlan122
C is directly connected, Vlan123
C is directly connected, Vlan116
C is directly connected, Vlan117
C is directly connected, Vlan118
C is directly connected, Vlan119
C is directly connected, Vlan112
C is directly connected, Vlan113
C is directly connected, Vlan114
S* [1/0] via


interface Vlan50
description Transit
ip address


SGBNTCCSW-J2.2-1#sh interfaces vlan 50
Vlan50 is up, line protocol is up
Hardware is Ethernet SVI, address is 000f.342a.46bf (bia 000f.342a.46bf)
Description: Transit
Internet address is
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 2/255
Encapsulation ARPA, loopback not set
ARP type: ARPA, ARP Timeout 04:00:00


core switch IP
interface Vlan10
description Management IP
ip address


Hi Diiey,


I think I know what the problem is, is about subnetting, please check the following:


When the ASA is configured with ip address  with mask /16, it is within this network range:




With a range of hosts from to, within this range ip (ASA's ip) and (Core SW ip) resides on the same network space, for that reason they are able to communicate.


However when the ASA is configured with ip with mask /30, ip and ip are no longer in the same network addressing space, since ASA ip address belongs to the following network range:



IP hosts range: to (from .1 to .2, son is no longer in the picture)


For that reason they are not able to communicate, if you want to change the ASA subnet mask and still able to communicate to the core switch, then you should configure both ASA and SW with an ip address and subnet mask that belongs to the same network addressing space. 


Hope this helps! 

hai... Kornelia Gutierrez

 thanks for your support....

very nice explanation, that mean i need change my ASA's ip to core switch range...


it should be  or (ASA's ip) ?

Hi Diley,


Your ASA FW interface ip is connected to your core sw vl 50 with ip


On this scenario you have different broadcast address for the same subnet, so I suggest you to change your ASA FW ip address to /29, same with your core sw were is connected to or you can create two host between the ASA with Core SW so will be and


Hope this help!


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