Category: Data link layer

Link Aggregation – LACP Protocol

EtherChannel enables bundling multiple physical links connecting same devices into a single logical link. I will try to show you how it is configured and how it works.

The issue with one uplink

I made an example with 8 clients connected to two Cisco 3850 switches. For start, those two switches are connected together with 1G copper on Interface Ge1/23. The clients are also connected to 1G ports. In this case, when all of those four clients on the left side start simultaneously sending traffic at full speed to different computers on the right side, they will congest the uplink between switches and some traffic will be dropped.

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IPv6 Generated with EUI-64 Has a Strange Bit Inside

UPDATE on 22 Mar 2018:
Although the article is correctly explaining the IPv6 address generated in the example at the bottom, it seems that the process of generating the IPv6 address with EUI-64 has been changed from what is described in RFC2373 to something like RFC4291 and then to something like RFC7217. Thanks to @FernandoGont for pointing to this issue. A new article is here which describes newly suggested technique on how SLAAC implementation should work in new network device/NIC implementations.

What is universal/local bit in IPv6 EUI-64 address?

One of my readers contacted me with an interesting question in comments of “IPv6 – SLAAC EUI-64 Address Format” article. The question was:

“How come that the ipv6 address after the prefix is 21C:C4FF:FECF:4ED0 if the mac address is 00-1C-C4-CF-4E-D0?”

Of course, we all know from the previous article that EUI-64 process is taking the interface MAC address (if that is an Ethernet interface) and it creates 64 bits Interface ID with it by shimming additional FFFE (16bits in hex) in between the MAC address bits.

The reader was confused with an additional change that I did not cover in that article which is called universal/local bit of the IPv6 address Interface ID part.

Let’s go.

IPv6 address is 128 bit. First ‘n’ bits (first 64 bits of EUI-64 created IPv6 address) are called “subnet prefix” and the other half of bits are called “Interface ID”.

If we use EUI-64 process to generate a unique IPv6 address of the interface, then we are generating Interface ID from MAC address (or some other kind of L2 address if this is not about Ethernet).

|                     n bits                     |   128-n bits   |
|                  subnet prefix                 |  interface ID  |

On RFC 2373 page 18 chapter: “APPENDIX A : Creating EUI-64 based Interface Identifiers”, you can find that 7th bit on Interface ID part of an IPv6 address (the last 64 bits) is called “universal/local bit”.

If this bit is set to “0” it indicates local scope IPv6 address and if it is “1” then the generated IPv6 address has global scope (it is globally unique).

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MACSec – Media Access Control Security

Media Access Control Security or MACSec is the Layer 2 hop to hop network traffic protection.

Just like IPsec protects network layer, and SSL protects application data, MACSec protects traffic at data link layer (Layer 2).

MACSec is standardized IEEE 802.1AE hop-by-hop encryption that enables confidentiality and integrity of data at layer 2.

It encrypts entire Ethernet packet except Source and Destination MAC addresses on any device-to-device, switch-to-switch, router-to-switch, host-to-switch directly connected wired L2 connection.


If we compare MACSec with, for example, IPsec, MACsec provides same security but on layer 2 for each hop separately. On each hop, packets are received and decrypted enabling other features like QoS and load balancing to function. Same data is again encrypted before being sent out to next device on the way towards the destination. MACsec works on host-to-switch connection and on switch-to-switch connection enabling line-rate up to 10 GigabitEthernet fast encryption.

It uses, especially for that occasion developed and optimised Galois/Counter Mode Advanced Encryption Standard 128 (GCM-AES-128) which makes line-rate encryption/decryption possible.

There is some difference in MACSec on the connection between host and switch, and MACSec on the connection from switch to switch

MACsec uses MACsec Key Agreement (MKA) which is standardized protocol to provide the required session keys and manages the required encryption keys on a MACSec connection between hosts (PC, server, etc.) and switch.

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Proxy ARP

I found different kinds of explanations about what Proxy ARP is, just few of them were understandable at first. After merging all of them this explanation came out of my networking workshop:

  • Assuming that a router has an ARP entry of some IP address, if he receives an ARP request from somebody for that IP, the router will respond with its own MAC address.
  • Proxy ARP is fairly simple technique for nodes to get MAC address of a destination host that is on a different subnet but on the same router. Same router different interface.

And this one to:

  • If we have in the network one edge router that is our way out from the local LAN network. That router has Proxy ARP enabled by default. When it receives an ARP request on his interface for a client that is not actually from that local network it will try to be helpful and it will search his routing table if that network is locally connected on some other local interface. If he finds it, it will respond with his own MAC address to tell the source that he is the way to go towards that host.

If we look at the image below, I prepared a more detailed example for those who are still a bit confused about it.

It’s a technique that enables our R7 router on the image below to proxy ARP request from C1 computer which tries to find MAC address of computer C3.

You need to note that C1 has address from /16 range and that is why it thinks that is on the same subnet as . If that was not the case and C1 had the address, it would send the ARP asking what is MAC address of default gateway. It will go to default gateway because he will know that he is not directly connected to all of network We are then talking about standard routing by getting the packets from one subnet to another using routing table examination.

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L2TP – Layer 2 Tunneling Protocol

L2TP Attributes Summary

L2TP tunnelProjected L2TP standard was made available in the year 1999 by means of RFC 2661. It was originated primarily from two different tunneling protocols, named as: Point-to-Point communication protocol and PPTP (Point to Point Tunneling protocol). In other words, L2TP (Layer 2 Tunnel Protocol) is an up-and-coming IETF (Internet Engineering Task Force) standard that came in front with the traits of two on-hand tunneling protocols, named as: Cisco’s L2F  (Layer 2 Forwarding) and Microsoft’s PPTP (Point-to-Point Tunneling Protocol). L2TP protocol is actually an expanded form of the PPP (a significant constituent for VPNs).

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