Understanding Cut-Through Switching Mode

Hello
i am learning about differents operation modes of Cisco Switch and i have a question about the Cut-Through Switching Mode:
Cut-Through mode tell the switch to forward a frame after opening only the first 14 bits. how this method can support Vlan Taging? the switch need to know if he can send into the trunk port he want to forward and need to check if the vlan he need to reach is allowed on the same port. the switch can see this only after seeing almost all the frame header.
thank you for your help

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
I mean, as part of the cut-through processing, the VLAN tag needs to be analyzed. If the ingress port is configured to support VLAN tags, then the cut-through needs to wait to see that portion of the frame. Likewise, if the egress port is VLAN tagged, the frame will need to held until the frame's VLAN tag can be constructed.
So, yes, cut-through could work with VLAN tags, it will just add a bit of latency. It's up to the device vendor to determine how, and under what conditions, cut-though will work.
Some cut-through swtiches support "fragment-fee", which means that look at more of the header. Some (at least years ago), would use fragment-free dynamically (based on loading). So, again, it's really up to the device vendor. (BTW, "basic" cut-though assumes the whole frame will be received, but that's not always a valid assumption.)

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Store & Forward vs cut-through switching

Hey Guys,
Looking for expert opinion on what layer (AGG/Tor) what type of packet forwarding should be used specially in DC.
is there no buffering at all when we use cut-through ? I understand on TOR Store and Forward only make sense if high buffer cheap switches are being used.
On cisco.com i found for nexus 3k -• Store-and-forward mode activates automatically for a port when the switch identifies that the port is
oversubscribed and the ingress rate is greater than the switching capacity of the egress port.For example,
when the port ingress rate is 10 gigabit and the switching capacity of the egress port is 1 gigabit
However on other hand for 5 K- 10 G to 1 G is Cut-through.
Forwarding Mode Behavior (Cut-Through or Store and Forward)
SOURCE INTERFACE
DESTINATION INTERFACE
SWITCHING MODE
10 GigabitEthernet
10 GigabitEthernet
Cut-Through
10 GigabitEthernet
1 GigabitEthernet
Cut-Through
1 GigabitEthernet
1 GigabitEthernet
Store-and-Forward
1 GigabitEthernet
10 GigabitEthernet
Store-and-Forward
This makes me little bit confused appreciate if someone can clarify.
Thanks
Ajay

Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
Posting
Sorry, I'm confused about what you're confused about. If you're asking about 3K Nexus forwarding operation, and why it is what it is, that's something only Cisco can answer.
If you're asking about store-and-forward vs. cut-through, the latter is designed to decrease store-and-foward latency.
Historically, when 10 Mbps switches first came out, they added latency (remember, its per hop) not seen with 10 Mbps hubs. When 100 Mbps came out, store-and-forward latency decreased so the need for cut-through fell out of favor. (BTW, keep in mind there are issues starting forwarding a frame before you know you have a good copy.)
With 1g, 10g, 40g and 100g, store-and-forward latency is decreased even more but now we have applications that required ultra low latency. We also now perhaps have applications using jumbo Ethernet. So, there's been a bit of a revival of cut-through.
PS:
BTW, cut-through cannot work with ingress having a slower bit rate than egress.

Cut through, fragment free config

Hi
Can someone please tell me how to configure cut through, fragment free and store & forward?
Can we configure this for each port seperately or do we have to do it to the whole switch?
How can we check the current switch type (cut through, fragment free or store and forward)?
What is the default mode?
Thanks in advance!

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Do all Sun Servers come with switch-mode power supply?

I was posted this question by my customer and i was dumb founded. Can anyone help me on this? Thank you!

I'm not absolutely certain I understand the term "switch-mode".
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ASA - cut through proxy authentication for RDP?

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[*]OUTSIDE 1.1.1.1 to INSIDE 10.10.70.100
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[code]
ASA Version 8.2(5)
hostname ASA5505
names
name 10.10.0.0 LANTraffic
name 10.10.30.0 SALES
name 10.10.40.0 FoodServices
name 10.10.99.0 Management
name 10.10.20.0 Office
name 10.10.80.0 Printshop
name 10.10.60.0 Regional
name 10.10.70.0 Servers
name 10.10.50.0 ShoreTel
name 10.10.100.0 Surveillance
name 10.10.90.0 Wireless
interface Ethernet0/0
description TO INTERNET
switchport access vlan 11
interface Ethernet0/1
description TO INSIDE 3560X
switchport access vlan 10
interface Ethernet0/2
shutdown
interface Ethernet0/3
shutdown
interface Ethernet0/4
shutdown
interface Ethernet0/5
shutdown
interface Ethernet0/6
shutdown
interface Ethernet0/7
shutdown
interface Vlan1
no nameif
security-level 50
no ip address
interface Vlan10
description Cisco 3560x
nameif INSIDE
security-level 100
ip address 10.10.1.1 255.255.255.252
interface Vlan11
description Internet Interface
nameif OUTSIDE
security-level 0
ip address 1.1.1.1 255.255.255.224
ftp mode passive
clock timezone PST -8
clock summer-time PDT recurring
dns domain-lookup OUTSIDE
dns server-group DefaultDNS
name-server 8.8.8.8
name-server 4.2.2.2
domain-name test.local
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access-list RDP-INBOUND extended permit tcp any host 1.1.1.2 eq 3389
pager lines 24
logging enable
logging timestamp
logging trap warnings
logging device-id hostname
logging host INSIDE 10.10.70.100
mtu INSIDE 1500
mtu OUTSIDE 1500
ip verify reverse-path interface OUTSIDE
icmp unreachable rate-limit 1 burst-size 1
asdm image disk0:/asdm-645.bin
no asdm history enable
arp timeout 14400
global (OUTSIDE) 1 interface
nat (INSIDE) 1 LANTraffic 255.255.0.0
static (INSIDE,OUTSIDE) tcp interface 3389 10.10.70.100 3389 netmask 255.255.255.255
static (INSIDE,OUTSIDE) tcp 1.1.1.2 3389 10.10.50.200 3389 netmask 255.255.255.255
access-group RDP-INBOUND in interface OUTSIDE
route OUTSIDE 0.0.0.0 0.0.0.0 1.1.1.1 1
route INSIDE LANTraffic 255.255.0.0 10.10.1.2 1
timeout xlate 3:00:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 mgcp 0:05:00 mgcp-pat 0:05:00
timeout sip 0:30:00 sip_media 0:02:00 sip-invite 0:03:00 sip-disconnect 0:02:00
timeout sip-provisional-media 0:02:00 uauth 0:05:00 absolute
timeout tcp-proxy-reassembly 0:01:00
timeout floating-conn 0:00:00
dynamic-access-policy-record DfltAccessPolicy
aaa authentication ssh console LOCAL
aaa authentication http console LOCAL
http server enable
http Management 255.255.255.0 INSIDE
no snmp-server location
no snmp-server contact
snmp-server enable traps snmp authentication linkup linkdown coldstart
crypto ipsec security-association lifetime seconds 28800
crypto ipsec security-association lifetime kilobytes 4608000
telnet timeout 5
ssh 10.10.70.100 255.255.255.255 INSIDE
ssh Management 255.255.255.0 INSIDE
ssh 0.0.0.0 0.0.0.0 OUTSIDE
ssh timeout 5
ssh version 2
console timeout 0
threat-detection basic-threat
threat-detection scanning-threat shun
threat-detection statistics access-list
threat-detection statistics tcp-intercept rate-interval 30 burst-rate 400 average-rate 200
webvpn
username scott password CNjeKgq88PLZXETE encrypted privilege 15
class-map inspection_default
match default-inspection-traffic
policy-map type inspect dns preset_dns_map
parameters
message-length maximum client auto
message-length maximum 512
policy-map global_policy
class inspection_default
inspect dns preset_dns_map
inspect ftp
inspect h323 h225
inspect h323 ras
inspect ip-options
inspect netbios
inspect rsh
inspect rtsp
inspect skinny
inspect esmtp
inspect sqlnet
inspect sunrpc
inspect tftp
inspect sip
inspect xdmcp
service-policy global_policy global
prompt hostname context
no call-home reporting anonymous
call-home
profile CiscoTAC-1
no active
destination address http https://tools.cisco.com/its/service/oddce/services/DDCEService
destination address email [email protected]
destination transport-method http
subscribe-to-alert-group diagnostic
subscribe-to-alert-group environment
subscribe-to-alert-group inventory periodic monthly
subscribe-to-alert-group configuration periodic monthly
subscribe-to-alert-group telemetry periodic daily
Cryptochecksum:1e9d278ce656f22829809f4c46b04a07
: end
[/code]

You're running ASA 8.2(5). In 8.4(2) Cisco added support for what they call Identity Firewall rules. That is, you can make access-lists entries specific to users (or object groups containing users).
There's an overview document on this posted here. It's a bit dated but I believe the only change is that Cisco is now preferring use of the more current Context Directory Agent (CDA) - a free VM they provide - vs. the deprecated AD agent (software service that runs on your DC).

Switching mode

Can switching mode be specified? Ex. Store-and-forward, cut-through, fragment-free, adaptive cut-through? How?
1.10

Hi
Thanks for your reply,
how would a 1924 switch running cut-through be be able to determine,when receiving a frame on a trunk port be to to read the 802.1q vlan id, since a cut through enabled switch only reads the dest. mac address and then it will forward the frame, I just cant see how it would be able to read the VLAN-ID
cheers
per

Why is the Camera app so slow when switching modes?

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I get that an app can take a few seconds to load, but switching camera modes should be nearly instantaneous, shouldnt it? Do other people see the same delay, or is it just me? Are there any known tweaks to improve this delay?
On an iPhone 4 with latest iOS.
--Tim

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Outline Fonts do not cut through tint panel underneath

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ClarisWorks outline fonts did, and splendidly. Even <spit> AppleWorks did. Pages, however, does not.
An Outline font should show the character in white, with a black outline. The white character should stay white when set over the top of a grey tinted panel.
The Editor is not happy about this. He is now wasting precious time in trying to make it work as it did using previous s/w.
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To fill text with color, gradient, or image – with or without outline
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Cut-through authentication vs. virtual telnet/http

Hi,
I'm having difficulties understanding the meaning of the virtual telnet/http commands on the ASA.
I have configured an ASA and defined an access-list with all the traffic which is to be authenticated. These are protocols like RDP, which can't be intercepted by the ASA, but also HTTP and HTTPS which can indeed be intercepted (this is also referred as cut-trough authentication).
The setup principially works. Then a few consultants came and checked my config for errors. They also performed a portscan, where they found out that all protected services (which should only work after authenticating) were answered by the ASA (a tcp-session was started), so an attacker would know what potential services are behind the firewall.
The customer (and me) disliked this behaviour, and I thought this could be solved by using the virtual http feature. Define a seperate IP-Adress, to which you can connect via HTTPs and authenticate, after which you can reach all other services.
Can this be done with this feature? My testresults showed just the behaviour, that you can authenticate at the virtual http-address, but the cut-through authentication is still active, so that's not the solution.
To be honest, I even believe that the virtual telnet/http feature is completely useless! Why? Because to make it work, you have to
1) allow the ip an the inbound ACL
2) add the ip in the ACL where the authenticated traffic is defined
3) configure a NAT for this ip to be routed inside
I don't really see a practical reason for this command - Thanks for your thought...
Florian

Hi Florian / Jeff
I agree largely with what you are saying and have found similiar issues with it. if you are already authenticating to a web service the additional config of a virtual http service seems unnecessary.
But i think one instance where virtual telnet is useful is if you have services such as RDP etc. that you need to authenticate but you don't have a web server or telnet server to authenticate against.
Without virtual telnet i'm not sure how you could setup access to these services so you would need virtual telnet in this case.
Where i find the command particularly useless is that i want to authenticate people accessing for example terminal servers on a particular subnet. This subnet is also running web servers.
Now say i want to do this via http authentication. I'm trying to authentciate them because i don't know their IP addresses. So i enter an authentication command for http but now everyone who wants to use http has to authenticate and not just people who are going to be using terminal services.
Regards

ASA Cut Through (Authentication) Proxy for a Single ACL

I have a customer that wants to authenticate users at the ASA before being allowed access from the outside into a payroll server on the DMZ. I am aware of the cut through proxy feature, but doesn't that affect all traffic entering the DMZ? Is there a way to only authenticate users accessing one server?

Hi,
Seems to me the easiest way to do this is you are connecting to the destination server with either Browser or CLI based connection.
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username password privilege
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E2000 in Switch Mode

My home network is configured with an 8 port Netgear box as the DHCP server and an E2000 for wireless access (and some wired ports as well) in switch mode.
Since the E2000 is no longer a DHCP server it loses its IP address of 192.168.1.1 and appears to be assigned as 192.168.0.8. When I try to access the E2000 at this new address using the browser based utility, the expected login screen appears, but there doesn't appear to be a password that works. According to the instructions the WAP key is supposed to be the password, and this worked before when the E2000 was set up as a router.
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Solved!
Go to Solution.

Reset the E2000 to factory defaults. Unplug it from your modem. Wire a single computer to the E2000.
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5. Unplug the computer.
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Thanks.
Steve

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Global Switching Mode

I have two Sup 720 (WS-SUP720-BASE ) installed on out 6500 switch and I just found that my global switching mode is "Flow through", does that mean that all modules are running on bus mode?.. does that mean that my Sup 720 does not have switching fabric built-in? if this is true, I thought all sup 720 have switching fabrics. Please help.
Regards

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E72: shortcut for switching mode ?

I wanted to removed 'switch mode' short cut from the home screen, so that I can use some other useful shortcut there ?
Is there any shortcut for switching mode ?
Thanks

There are actually two sets of shortcuts for switching windows in Photoshop CC. One is the OS shortcut that Photoshop respects, which c.pfaffenbichler covered. The other is the older traditional Photoshop shortcut for switching windows, which on the Mac is Control-Tab or Shift-Control-Tab (that really does use the Control key on the Mac, not Command). Both ways work for me in Photoshop CC in Mavericks.
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Understanding Cut-Through Switching Mode

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