You can configure this command on the ATM LSRs to stop them from sending Label Requests for IP prefixes that do not need an LSP to be set up. Example 5-18 shows the edge ATM LSR denver with this command and 1 stopping it from sending Label Requests for anything other than IP addresses from the range 10.200.253.0/24.
In general, you want to block LVCs from being set up for IP prefixes that are not important—IP prefixes that do not carry customer or through traffic. In the example of MPLS VPN, the important prefixes in the MPLS network are the PE loopback IP addresses because they are the BGP next-hop IP addresses. These IP addresses carry the VPN customer traffic across the MPLS cloud. Refer to Chapter 7, “MPLS VPN,” to understand why the BGP next-hop IP addresses are important prefixes in MPLS VPN networks.
Aggregate Labels
Aggregate labels can be the result of aggregation or summarization of IP prefixes in the network.
You can aggregate multiple IPv4 prefixes into one prefix with a smaller mask capturing all the component prefixes with longer masks. You can do this in an MPLS network, but it might not be a good idea. When the labeled packets arrive at the aggregation router, it removes the label and performs an IP lookup, and then it labels them again and forwards them. Do not aggregate on ATM LSRs for that reason. The aggregation label entails that the labeled packets must become unlabeled on the ATM LSR. For this to happen, the aggregating ATM LSR first reassembles the cells into a frame. When the aggregating ATM LSR forwards the packet by doing an IP lookup, it chops the frame into cells again. This is an expensive operation and has a serious performance impact. That is why you should always avoid aggregation on ATM LSRs.
Example 5-18 Blocking Label Requests on Edge ATM LSR
!
hostname denver
!
mpls label protocol ldp
mpls ldp router-id Loopback0 force mpls ldp request-labels for 1
!
!
access-list 1 permit 10.200.253.0 0.0.0.255 access-list 1 deny any
!
VC-Merge
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
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.
ATM 2/0/1
ATM 2/0/1
ATM 0/0/2 ATM 0/0/0 ATM 2/0/0 ATM 2/0/2 ATM 3/0.10
Loopback 0 10.200.253.2/32
Loopback 0 10.200.253.1/32 ATM 2/0/2
MPLS Network
Loopback 0 10.200.253.6/32 Loopback 0
10.200.253.3/32
10.200.253.6/32
washington-ATM denver-ATM
brussels-ATM brussels
1/34 1/33
1/33 1/33
1/33
1/34 1/34 1/33 1/34 1/34
VC-Merge 133
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.
ATM 2/0/1
ATM 2/0/1
ATM 0/0/2 ATM 0/0/0 ATM 2/0/0 ATM 2/0/2 ATM 3/0.10
Loopback 0 10.200.253.2/32
Loopback 0 10.200.253.1/32 ATM 2/0/2
MPLS Network
Interleaving
Loopback 0 10.200.253.6/32 Loopback 0
10.200.253.3/32
10.200.253.6/32
washington-ATM denver-ATM
brussels-ATM brussels
1/34 1/33
1/33 1/33
1/33
1/33 1/33 1/33 1/33 1/34
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:
mmm
mppppllsllsss llllddddpp pp aataatttmmmm vvvvcc-cc---mmemmeeerrrrggggeeee
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#sssshhhhoooowwww mmmmpplpplllssss aaaattmttmmm--l--lllddddpppp bbibbiiinndnndddiiiinnnnggggss ss 11011000....2222000000.00...225225553333....6666 333322 22 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
ATM 2/0/1
ATM 2/0/1
ATM 0/0/2 ATM 0/0/0 ATM 2/0/0 ATM 2/0/2 ATM 3/0.10
Loopback 0 10.200.253.2/32
Loopback 0 10.200.253.1/32 ATM 2/0/2
MPLS Network
No Interleaving
VC-Merge
Loopback 0 10.200.253.6/32 Loopback 0
10.200.253.3/32
10.200.253.6/32
washington-ATM denver-ATM
brussels-ATM brussels
1/34 1/33
1/33 1/33
1/33
1/33 1/33 1/33 1/33 1/34