EAP-Message

RADIUS encapsulates EAP packets in the EAP-Message attribute, as shown in Figure 302. The Type field takes 79, and the Value field can be up to 253 bytes. If an EAP packet is longer than 253 bytes, RADIUS encapsulates it in multiple EAP-Message attributes.

Figure 302 EAP-Message attribute format

0

7

15

N

 

 

 

 

 

Type

Length

String

 

 

 

 

EAP packets

Message-Authenticator

RADIUS includes the Message-Authenticator attribute in all packets that have an EAP-Message attribute to check their integrity. The packet receiver drops the packet if the calculated packet integrity checksum is different from the Message-Authenticator attribute value. The Message-Authenticator prevents EAP authentication packets from being tampered with during EAP authentication.

Figure 303 Message-Authenticator attribute format

Initiating 802.1X authentication

Both the 802.1X client and the access device can initiate 802.1X authentication.

802.1X client as the initiator

The client sends an EAPOL-Start packet to the access device to initiate 802.1X authentication. The destination MAC address of the packet is the IEEE 802.1X specified multicast address 01-80-C2-00-00-03 or the broadcast MAC address. If any intermediate device between the client and the authentication server does not support the multicast address, you must use an 802.1X client (for example, the HP iNode 802.1X client) that can send broadcast EAPOL-Start packets.

Access device as the initiator

The access device initiates authentication, if a client cannot send EAPOL-Start packets. One example is the 802.1X client available with Windows XP.

The access device supports the following modes:

•Multicast trigger mode—The access device multicasts Identity EAP-Request packets periodically (every 30 seconds by default) to initiate 802.1X authentication.

•Unicast trigger mode—Upon receiving a frame with the source MAC address not in the MAC address table, the access device sends an Identity EAP-Request packet out of the receiving port to the unknown MAC address. It retransmits the packet if no response has been received within a certain time interval.

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