All Entries in the "Cisco-switching" Category
Cisco: How to use CEF for load-balancing
According to Cisco’s website, Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions.
The term Load balancing describes a functionality in a router that distributes packets across multiple links based on layer 3 routing information.
Now, putting this two terms together we obtain Load balancing with CEF. Cisco IOS software basically supports two modes of load balancing: On per-destination or per-packet basis.
In per-destination mode all packets for a given destination are forwarded along the same path. Usage of only one path will lead to a unequal usage of the lines, as the packets to the same destination will use only one line, leaving the other unused. This can be a problem in a small environment where let’s say that a location is having 2 x E1 connection. In per-destination mode, all the packets to one destination will use one E1 connection, and the other line will remain unused. In addition to this, if this one destination is a server on which most on the site’s users will connect to, the per-destination mode will lead to an exhaust of the bandwidth available on one E1 line leaving the other one empty. On the other hand, on more developed location, where there are more destination available, the per-destination method will not have too much impact about the usage of the lines, as in this case the traffic will be split for the multiple destinations over multiple paths.
The per-packet mode guarantees equal load across all links because the forwarding process determines the outgoing interface for each packet by looking up the route table and picking the least used interface. If you read until now, than most probably you will say that this method is the best and then why the per-destination mode is the default one. Some issues with the per-packet mode would be that this method will almost always result in out-of-order packets, as advised by Ivan Pepelnjak, on his blog. What I can tell you is that this out-of-order packets has not a big impact on low-speed environment where TCP stack can deal with this problem. On the other hand on high-speed environment where video or voice traffic is expected, you can have big problems and it’s better to avoid the usage of per-packet load-balancing. Also, Cisco is advising that this ensures equal utilization of the links, but is a processor intensive task and impacts the overall forwarding performance. How much will impact on the CPU / resources usage? I cannot tell exactly because this depend on the task and traffic that the device has to handle.
OK, so if this per-packet mode is not so great, than why we should use it? The answer is that in some particular topologies or environments you cannot use other method of load-balancing and you are in desperate need of such mechanism. Below you will see an example of what kind of topology can force us to use per-packet load-balancing.
Before you configure load-balancing, you have to be sure that IP CEF is enabled on your router. In case that it is disabled, please enable it:
configure terminal
ip cef
If you want to fine tune the IP CEF load-balancing algorithm you can do this with the command:
configure terminal
ip cef load-balancing algorithm “name parameter”
where for “name” you have 3 choices:
original – Sets the load sharing algorithm to the original based on a source and destination hash.
tunnel – Sets the load sharing algorithm for use in tunnel environments or in environments where there are only a few IP source and destination address pairs.
universal – Sets the load sharing algorithm to the universal algorithm that uses a source and destination, and ID hash.
Skipping the tunnel option which you should use only if you are sure that you need it, the other choices would be universal and original. Original algorithm use IP addresses to generate the 4-bit hash. On the other hand universal algorithm add a router specific information to the hash leading to a more complex development of the hash value. Since universal option is the default one, exception the case where you know what you are doing, you should not change this value.
To enable IP CEF load-balancing on per-destination base, you don’t have to modify anything, as it’s enabled by default. For per-packet mode, you have to use the following commands:
configure terminal
interface FastEthernet x/y
ip load-balancing per-packet
One scenario where I had to use per-packet load-balancing is the one below.
Let’s assume that we have client (c-ubuntu-1, 10.10.20.100) which is sending traffic to one server (s-ubuntu-1,10.10.10.100). The routing protocol is already configured in such way that CE1 and CE2 routers are announcing, through OSPF, 2 default routes (equal cost) to SC device. The rest of the routing part is assure by BGP towards PE routers and another IGP protocols, but this has no importance for the topic discussed here.
SC1#sh ip route | i 110/1
O*E2 0.0.0.0/0 [110/1] via 172.29.190.237, 02:35:46, Vlan23
[110/1] via 172.29.190.229, 02:35:46, Vlan13
On the SC devices I used L3 interface vlan 23 to connect to CE2 and vlan 13 to connect to CE1. With the CEF enabled and per-destination load-balancing mechanism, only one path (either vlan 23 or vlan 13) was used, leading to only one WAN serial connections to be used. Since this was client to server traffic, and quite a lot, from time to time one WAN connection was exhausted while the other one remains unused. You can see this, in the excerpt below:
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
After I enabled the per-packet load-balancing, the situation has changed since both lines to CE routers where used, leading to a equal utilization of the WAN lines:
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan13 (next hop 172.29.190.229)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan13 (next hop 172.29.190.229)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan13 (next hop 172.29.190.229)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan13 (next hop 172.29.190.229)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan13 (next hop 172.29.190.229)
SC#sh ip cef exact-route 10.10.20.100 10.10.10.100
10.10.20.100 -> 10.10.10.100 : Vlan23 (next hop 172.29.190.237)
I had to choose this method to force the equal usage of the WAN links and I rely on TCP stack to solve the out-of-order packets as there is not so high traffic over this interfaces.
Popularity: 13% [?]
Cisco: Layer 2 traffic filtering
Layer 2 traffic filtering can be very useful when you want to drop packets closer to
the source because you can do this on L2 next-hop which is the switch where the
devices are connected. Based on mac-address, Layer 2 filtering can be apply using
one of the two most common method: Port Security and MAC Access Groups.
Port Security is the more secure method of the two. To use it, map a switch port to the
specific MAC address of the connected device. It gives you more possibility than just
drop the packets from a specific source, depending on what you want to achieve on
the interface where it is applied.
MAC Access Groups are generally used for small networks of 20 devices or less. Add
a permit statement for all of your devices interface MAC addresses and apply the access
list to switch interface. This will limit inbound traffic to that interface to only those
MAC addresses on your list. Is not recommended when you have many MAC addresses,
because MAC access-list are the same like IP address access-list, so they consume a
lot of resources of the machine where it is applied.
For this tutorial we will use a Cisco 3750 in which it is connected a router ( R4 ). To test
Layer 2 traffic filtering, we have a point-to-point Layer 3 connection in between
( 10.0.0.0 /30 ), with physical interface used on the R4 and a Vlan 4 interface on the
switch. The port on the switch were R4 is connected is an access port in vlan 4.
Please see the tutorial below:
Popularity: 4% [?]
Interface macro command on a Cisco switch
From the beginning let me tell you that I don’t see very useful this command, as I prefer to use “interface range…” syntax, but since I saw it as a requirement in one of the task for CCIE RS lab exam, and maybe somebody will find it usable in real environment, I said I should put it here in a tutorial.
As many of you already know, you can control a range of interfaces by typing the command “interface range Fa0/1 – 6″ (for example), but there is another way to do this by using the interface macro style. For those how are beginners, this interface range or macro syntax spare you from typing 6 commands under 6 interfaces (stick to the example above), but issue only one command under interface range or macro.
Please see the tutorial below:
Popularity: 5% [?]
Limit traffic on a Cisco switch L2 port with minimal configuration
Let’s say that somebody (or some task in a test) ask you to limit the inbound traffic on a switch Layer 2 port by using minimal configuration possible. I must say that in the first steps I failed this task miserable, but actually is very simple to do it.
I will use a plain layer 2 Cisco 2950 switch for this task. I observed that I could not implement this on a Cisco 3500XL. I don’t know if the IOS image was wrong, but I didn’t investigate too much in that area as I cannot stand 3500XL switches and they are actually pretty old piece of hardware.
No topology is needed for this as I will only show how to do it and not testing it with real traffic. I will do testing later when I’ll have some more time, or you can do it on your own.
See the tutorial below:
Popularity: 4% [?]
