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Understanding Transparent Tag Operations and IEEE 802.1p Inheritance

 

When swap-by-poppush is configured on IQ2 interfaces, 10-Gigabit Ethernet LAN/WAN PIC, IQ2-E interfaces, and MX Series interfaces, during a swap operation, the inner VLAN IEEE 802.1p bits are copied to the IEEE bits of the tag being swapped. If swap-by-poppush is not configured on the interface, the VLAN IEEE 802.1p bits of the tag being swapped remains same.

When swap-by-poppush is configured but the incoming packet has no inner VLAN tag (transparent tag), the IEEE 802.1p bits are set to zero .

Table 1 describes the relationship between the VLAN map operation and the inheritance of IEEE 802.1p from the transparent tag. It assumes the presence of the transparent tag in the incoming packet. If the transparent tag is not present, the IEEE 802.1p value is set to 0.

Table 1: VLAN Map Operation and IEEE 802.1p Inheritance

Rewrite Operation

Untagged Logical Interface

Transparent tag IEEE 802.1p Inheritance

Single-tagged Logical Interface

Transparent tag IEEE 802.1p Inheritance

Change in number of tags

push-push

yes

OUTER, INNER

NA

no operation

+2

swap-push

NA

no operation

yes

OUTER, INNER

+1

push

yes

OUTER

yes

*none

+1

swap

NA

NA

yes

OUTER

0

Note: *In a push operation on a single-tagged logical interface, none of the tags (inner, or outer) inherit the IEEE 802.1p bits from the transparent tag.

The following section shows four different examples of the inheritance of the transparent IEEE 802.1p values into the outer and inner VLAN tags.

Figure 1 shows an incoming packet with a transparent tag. A swap-push operation swaps the outer VLAN tag and pushes another VLAN tag. The IEEE 802.1p values are inherited from the transparent tag.

Figure 1: swap-push (transparent tag)
swap-push (transparent tag)

Figure 2 shows an incoming packet with no transparent tag. A swap-push operation swaps the outer VLAN tag and pushes another VLAN tag. The IEEE 802.1p value is set to zero, as there is no transparent tag.

Figure 2: swap-push (no transparent tag)
swap-push (no transparent tag)

Figure 3 shows an incoming packet with a transparent tag. A push operation pushes another VLAN tag. The IEEE 802.1p value is inherited from the transparent tag.

Figure 3: push (transparent tag)
push (transparent tag)

Figure 4 shows an incoming packet with a transparent tag. A push-push operation pushes the outer and inner VLAN tags, respectively. The IEEE 802.1p values are inherited from the transparent tag.

Figure 4: push-push (transparent tag)
push-push (transparent tag)