VCMerge

As mentioned, the upstream LSR requests a label for a prefix from its downstream LSR and so on, until it reaches the egress LSR. However, without VC-merge, the label requests are propagated from the ingress LSR to the egress LSR, even if an intermediate LSR has already received an outgoing label from its downstream LSR for that prefix. Look at Figure 5-10, where the ATM LSR brussels-atm has already received a label for the prefix 10.200.253.6/32 from its downstream LSR brussels. This first label was outgoing label 1/34 from brussels-atm to brussels. For the traffic from washington-atm to brussels, a second label will be requested from brussels by brussels-atm. This is the label 1/33.

Figure 5-10 Two Upstream LSRs

Figure 5-10 Two Upstream LSRs

Why does the same destination 10.200.253.6/32 on brussels-atm have a second outgoing label? One VC is from denver-atm to brussels, and the second VC is from washington-atm to brussels. What if the LSR brussels-atm does not request a second label for the second upstream LSR washington-atm but uses the label that it already received from LSR brussels? There would be a problem. Look at Figure 5-11 to see the interleaving of cells problem.

Figure 5-11 Interleaving of Cells

Figure 5-11 Interleaving of Cells

LSR brussels-atm has two incoming labels—one for each upstream LSR and only one outgoing label toward LSR brussels. Therefore, cells from both the LSR denver-atm and the LSR washington-atm are interleaved onto the same LVC; this means they have the same VPI/VCI value toward the LSR brussels. The egress LSR brussels—which needs to reassemble the ATM cells into frames—does not know which cells belong to which of the two streams. That is not a good idea. It might work, however, if the cells forming one frame are not interleaved with cells from another frame from a different upstream LSR. You can do this if the merging LSR (here LSR brussels-atm) buffers the cells until it detects that it has received the last cell from the frame. This detection can be accomplished by looking at the end-of-frame bit in the cell header. The merging LSR can then send the cells without interleaving the cells with cells from another upstream LSR. The cells do need to be buffered, which requires extra memory on the ATM LSR. The procedure of buffering the cells and only using one outgoing label per prefix for all upstream ATM LSRs is called VC-Merge. Different incoming LVCs are merged into one outgoing LVC. Look at Figure 5-12 to see VC-Merge.

134 Chapter 5: MPLS and ATM Architecture Figure 5-12 VC-Merge

134 Chapter 5: MPLS and ATM Architecture Figure 5-12 VC-Merge

The obvious advantage of VC-Merge is that the number of needed VCs is reduced. If the router brussels-atm had five upstream LSRs for a set of 50 prefixes, there would already be (5 - 1) * 50 = 200 LVCs less in this simple example.

Following is the global Cisco IOS command to enable VC-Merge:

mpls ldp atm vc-merge

VC-Merge is on by default on Cisco ATM Switches.

Look at Example 5-19 to see the LVCs before disabling VC-Merge on the LSR brussels-atm. Two LVCs are incoming, but only one LVC is outgoing for the prefix 10.200.253.6/32.

Example 5-19 LVCs Before Disabling VC-Merge brussels-atm#show mpls atm-ldp bindings 10.200.253.6 32

Destination: 10.200.253.6/32

Transit ATM2/0/0 1/36 Active -> ATM2/0/2 1/33 Active Transit ATM2/0/1 1/36 Active -> ATM2/0/2 1/33 Active

Examine Example 5-20. Notice the VCs after disabling VC-Merge on the LSR brussels-atm. You can see two outgoing LVCs for the prefix 10.200.253.6/32.

Example 5-20 LVCs After Disabling VC-Merge brussels-atm#conf t

Enter configuration commands, one per line. End with CNTL/Z. brussels-atm(config)#no mpls ldp atm vc-merge brussels-atm(config)#~Z brussels-atm#

brussels-atm#show mpls atm-ldp bindings 10.200.253.6 32

Destination: 10.200.253.6/32

Transit ATM2/0/1 1/34 Active -> ATM2/0/2 1/33 Active Transit ATM2/0/0 1/33 Active -> ATM2/0/2 1/34 Active

Some ATM switches in the network might not run MPLS. The solution to this problem is to run Virtual Path (VP) tunnels across the non-MPLS-aware ATM switches. Figure 5-13 shows a network with ATM switches that are not MPLS-aware. A VP tunnel is created across those ATM switches to carry the LVCs, which are created as needed on the VP tunnel. The VCIs are mapped to the MPLS labels and carry the "labeled" ATM cells.

Figure 5-13 MPLS over VP Tunnels

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