Black Box®LB1350A/LB1351A
14. ADVANCED Features: Port Priority Schemes
The Black Box® LB1351A (only)can determine priority through three different
means.
The first method is a simple per port priority, the second is via the
802.1p frame tag and the third is by viewing the DSCP ( TOS ) field in the Ipv4 header. Two priority levels are deployed ( High and Low )
Per Port Priority
General priority can be specified on a per port basis. In this type of priority all traffic from the specified input port is considered high priority .
This can be useful in IP phone applications mixed with other data types . The IP phone traffic would be high priority .
802.1p Port Priority
This method works well when used with ports that have mixed data and media flows. The 802.1p priority tag ( 3 bits ) are used to determine frame priority. The inbound port examines the priority field in the tag and determines the high or low priority.
VLAN tagging and priority. When frames are sent across the network, there needs to be a way of indicating to which VLAN the frame belongs. This info is added to the Ethernet frame in the form of a tag header.
Ethernet Frame Tag Header consists of a Tag protocol identifier ( TPID ) and Tag control information ( TCI )
Ethernet frame :
DA | SA | TAG | PT |
| DATA | CRC |
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6B | 6B | 4B |
| 2B |
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4B |
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Tag Header
TPIDTCI
TPID is the tag protocol identifier which indicated that a tag header is following and the TCI contains the User priority, canonical format indicator ( CFI ) and the VLAN ID
TPID | User priority | CFI |
| VLAN ID |
16 b | 3b |
| 1b | 12b |
User priority is a 3 bit field which allows priority information to be encoded in the frame. 8 levels of priority are allowed, where zero is the lowest and 7 is the highest priority.(this field is used by 802.1p )
CFI : indicates presence/absence of RIF ( routing info field ) in the 802.3/Ethernet frames.
The VLAN ID is used uniquely to identify the VLAN to which the frame belongs. There can be a maximum of 4096 VLANs.
DSCP Port Priority
This is another per frame way to determine outbound priority.
The DSCP ( Differentiated Services Code Point – RFC 2474 ) method uses the
TOS
field in the IP header to determine high or low priority on a per code point basis. Each fully decoded code point can have either a high or low priority.
The most significant 6 bits of the TOS field are fully decoded into 64 possibilities,( up to 64 distinct behaviors ) and the resulted singular code is compared against the corresponding bit in the DSCP register. ( in Black Box® LB1351A unit )
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