Label Advertisement

The IGP and LDP on the ATM LSRs cannot run directly over the ATM interface and establish a neighborship. A control VC is needed for the IGP and LDP to run on between two adjacent ATM LSRs. When the IGP adjacency is built, the IGP can exchange IP prefixes which are put in the routing table. After LDP forms a session across the control VC, it can exchange label bindings. This in turn enables the ATM LSRs to populate the LIB with bindings. As you recall, a binding is a prefix and an associated label. Each IGP prefix in the routing table must be assigned a label. Each label value is mapped to a VPI/VCI value, and a virtual circuit is built for each label. Such a virtual circuit is called a label switched controlled virtual circuit (LVC) or tag switching controlled virtual circuit (TVC). To create these LVCs, you must configure the ATM interfaces on the ATM switches and routers to be Label Switching Controlled-ATM (LC-ATM) interfaces. Each such LC-ATM interface must have the control virtual circuit. On routers and ATM switches that are running Cisco IOS, this is by default the virtual circuit 0/32. The encapsulation for it must be LLC/SNAP. Figure 5-5 shows a typical MPLS network with ATM LSRs in the core.

Figure 5-5 ATM LSR Network

Loopback 0 10.200.253.5/32

Figure 5-5 ATM LSR Network

Loopback 0 10.200.253.5/32

Note the three ATM LSRs: washington-atm, denver-atm, and brussels-atm. Each ATM LSR has one router attached to it via an ATM interface. All six devices are in one OSPF area. The ATM interfaces between the ATM switches are enabled for MPLS with the mpls ip command, and they are IP unnumbered to the loopback 0 interface. LDP is running on the ATM interfaces, and LDP sessions are formed between the ATM LSRs over the control VC. On the three interfaces of the routers toward the ATM LSRs are subinterfaces with MPLS enabled and IP unnumbered to their loopback interface 0. Example 5-1 shows the configuration on the ATM LSR denver-atm and the edge LSR denver.

Example 5-1 Configuration of Denver LSRs hostname denver-atm !

mpls label protocol ldp tag-switching tdp router-id Loopback0 force !

interface Loopback0 ip address 10.200.253.1 255.255.255.255

interface ATM2/0/0 ip unnumbered Loopback0

tag-switching ip !

interface ATM2/0/1 ip unnumbered Loopback0 tag-switching ip

interface ATM2/0/2 ip unnumbered Loopback0 tag-switching ip

router ospf 1 log-adjacency-changes network 10.200.0.0 0.0.255.255 area 0

hostname denver

interface Loopback0 ip address 10.200.253.5 255.255.255.255

interface ATM1/0/0 no ip address

interface ATM1/0/0.10 mpls ip unnumbered Loopback0 mpls ip

router ospf 1 log-adjacency-changes network 10.200.0.0 0.0.255.255 area 0

NOTE The ATM subinterface with the mpls keyword at the end indicates that this is an LC-ATM subinterface on a router LSR.

Look at Example 5-2 to see how to verify that LDP runs over the ATM interfaces and how it uses the control VC 0/32.

Example 5-2 show mpls interfaces

denver-atm#show mpls interfaces

Interface IP Tunnel

Operational

ATM2/0/0 Yes (ldp) No

Yes

(ATM

labels)

ATM2/0/1 Yes (ldp) No

Yes

(ATM

labels)

ATM2/0/2 Yes (ldp) No

Yes

(ATM

labels)

denver-atm#show mpls interfaces detail

Interface ATM2/0/0:

IP labeling enabled (ldp)

LSP Tunnel labeling not enabled

MPLS operational

MTU = 4470

ATM tagging: Label VPI = 1

Label VCI range = 33 - 16383

Control VC = 0/32

Interface ATM2/0/1:

IP labeling enabled (ldp)

LSP Tunnel labeling not enabled

MPLS operational

MTU = 4470

ATM tagging: Label VPI = 1

Label VCI range = 33 - 16383

Control VC = 0/32

Interface ATM2/0/2:

IP labeling enabled (ldp)

LSP Tunnel labeling not enabled

MPLS operational

MTU = 4470

ATM tagging: Label VPI = 1

Label VCI range = 33 - 16383

Control VC = 0/32

OSPF is running on the ATM LSRs and the edge LSRs. OSPF neighborships are formed on the ATM links, and a routing table is built with the prefixes. Because all LSRs have only the loopback

0 configured with an IP address, the routing table shows one prefix per LSR in the MPLS network. Example 5-3 shows how to verify the OSPF neighborships and IP routing table.

Example 5-3 OSPF Neighbors and IP Routing Table denver-atm#show ip ospf neighbor

Neighbor ID

Pri

State

Dead Time

Address

Interface

10.200.253.2

1

FULL/ -

00:00:31

10.200.253

2

ATM2/0/0

10.200.253.3

1

FULL/ -

00:00:37

10.200.253

3

ATM2/0/1

10.200.253.5

1

FULL/ -

00:00:34

10.200.253

5

ATM2/0/2

denver-atm#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/32 is subnetted, 6 subnets

O

10.200

253

6

[110/3] via

10

0 0 2

253

3,

00

14:

15,

ATM2/0/1

O

10.200

253

5

[110/2] via

10

.200

253

5,

00

14:

15,

ATM2/0/2

O

10.200

253

4

[110/3] via

10

.200

253

2,

00

14:

15,

ATM2/0/0

O

10.200

253

3

[110/2] via

10

.200

253

3,

00

14:

15,

ATM2/0/1

O

10.200

253

2

[110/2] via

10

.200

253

2,

00

14:

15,

ATM2/0/0

C

10.200

253

1

is directly

connected,

Loopback0

The control VC 0/32 is set up between all devices. OSPF and LDP are running on this control VC on all ATM interfaces between the devices. Example 5-4 shows how to verify the existence of the control VC on the interfaces.

Example 5-4 Verifying the Control VC

denver-atm#show atm vc interface ATM 2/0/0

Interface

VPI

VCI

Type

X-Interface

X-VPI

X-VCI

Encap

Status

ATM2/0/0

0

5

PVC

ATM0

0

48

QSAAL

UP

ATM2/0/0

0

16

PVC

ATM0

0

35

ILMI

UP

ATM2/0/0

0

18

PVC

ATM0

0

203

PNNI

UP

ATM2/0/0

0

32

PVC

ATM0

0

230

SNAP

UP

ATM2/0/0

1

33

TVC(I)

ATM0

0

231

MUX

UP

ATM2/0/0

1

35

TVC(O)

ATM2/0/2

1

39

UP

ATM2/0/0

1

36

TVC(O)

ATM2/0/2

1

41

UP

ATM2/0/0

1

67

TVC(I)

ATM2/0/2

1

34

UP

Example 5-4 Verifying the Control VC (Continued)

denver-atm#show atm vc interface ATM 2/0/1

Interface

VPI

VCI

Type

X-Interface

X-VPI

X-VCI

Encap

Status

ATM2/0/1

0

5

PVC

ATM0

0

49

QSAAL

UP

ATM2/0/1

0

16

PVC

ATM0

0

36

ILMI

UP

ATM2/0/1

0

18

PVC

ATM0

0

204

PNNI

UP

ATM2/0/1

0

32

PVC

ATM0

0

221

SNAP

UP

ATM2/0/1

1

33

TVC(I)

ATM0

0

223

MUX

UP

ATM2/0/1

1

35

TVC(O)

ATM2/0/2

1

40

UP

ATM2/0/1

1

36

TVC(O)

ATM2/0/2

1

42

UP

ATM2/0/1

1

75

TVC(I)

ATM2/0/2

1

34

UP

To change the LDP control VC from 0/32 to another VPI/VCI pair, you can use the following interface command:

mpls atm control-vc vpi vci

Example 5-5 shows an example of how to change the control VC for LDP. Obviously, this must be configured on both LDP peers.

Example 5-5 Changing the Control VC denver-atm#conf t

Enter configuration commands, one per line. End with CNTL/Z. denver-atm(config)#int atm 2/0/2 denver-atm(config-if)#mpls atm control-vc 0 1000 denver-atm(config-if)#"Z denver-atm#

denver-atm#show mpls interfaces ATM 2/0/2 detail

Interface ATM2/0/2:

IP labeling enabled (ldp) LSP Tunnel labeling not enabled MPLS operational MTU = 4470

ATM tagging: Label VPI = 1

Label VCI range = 33 - 16383 Control VC = 0/1000

On the ATM LSR, you can change the VPI/VCI range that MPLS uses for the LVCs per ATM interface. The default VPI used for MPLS is 1. The Cisco IOS interface command to change the VPI/VCI range is as follows:

Example 5-6 shows that the VPI range is changed to 2 and the VCI range is changed to 33-2000. Example 5-6 Changing the VPI/VCI Range for LVCs interface ATM2/0/0 ip unnumbered Loopback0 mpls label protocol ldp tag-switching atm vpi 2 vci-range 33-2000 tag-switching ip

brussels-atm#show mpls interfaces ATM 2/0/0 detail

Interface ATM2/0/0:

IP labeling enabled (ldp) LSP Tunnel labeling not enabled MPLS not operational MTU = 4470

ATM tagging: Label VPI = 2

Remember that each prefix that is present in the routing table creates a virtual circuit through the network. Therefore, in the interest of scalability, it is better to limit the number of prefixes in the routing table. One way of doing this, which is highly advisable, is to have the ATM interfaces as IP unnumbered interfaces. You need a loopback interface anyway as LDP router ID and IGP router ID, and the IP unnumbered interfaces can point to the loopback interface. When you do not use IP unnumbered interfaces, you allocate a label and a virtual circuit to each IP prefix that is configured on a link. These unimportant prefixes do not forward traffic through the ATM network, so unused LVCs are set up.

Downstream-on-Demand Label Advertisement

To avoid unnecessary label advertisement for prefixes in the routing table, the ATM LSR does not operate in Unsolicited Downstream (UD) label advertisement mode. Rather, it operates in Downstream-on-Demand (DoD) label advertisement mode. This means that an ATM LSR only advertises a label (binding) when it is requested to. The upstream LSR requests the downstream LSR for a label for a particular prefix. The upstream ATM LSR knows who the downstream LSR is by looking up the next hop for the prefix in the routing table. ATM LSRs (and routers with LC-ATM interfaces) use the Ordered LSP Control mode (as discussed in Chapter 2, "MPLS Architecture") by default, whereas routers (non-LC-ATM interfaces) use the Independent LSP Control mode.

With Ordered LSP Control mode, the downstream ATM LSR only replies with a label if it has received a label for the prefix from its downstream LSR. The egress LSR at the end of the LSP is the first to allocate a (local) label to the prefix, and it sends this label to the upstream LSR. In turn, this LSR allocates a (local) label and sends it to the upstream LSR. Figure 5-6 shows an example of ATM LSRs and DoD Label Advertisement mode with Ordered LSP Control mode.

Figure 5-6 DoD Label Advertisement with Ordered LSP Control Mode

Loopback 0 10.200.253.5/32

Figure 5-6 DoD Label Advertisement with Ordered LSP Control Mode

Loopback 0 10.200.253.5/32

Washington washington-ATM

brussels-ATM

brussels

Label Mapping for 10.200.253.6/32

Label Mapping for 10.200.253.6/32

Label Mapping for 10.200.253.6/32

Washington washington-ATM

brussels-ATM

brussels

Label Mapping for 10.200.253.6/32

Label Mapping for 10.200.253.6/32

Label Mapping for 10.200.253.6/32

When a new prefix is learned throughout the network via the router brussels, the routers denver and washington send an LDP Label Request message to their downstream ATM LSR neighbor, requesting a label for the prefix. They in turn send an LDP Label Request message to the downstream LSRs. This continues until the Label Request reaches the edge ATM LSR brussels. This router returns an LDP Label Mapping message to its upstream neighbor. The upstream neighbor sends an LDP Label Mapping message upstream and so on, until the edge ATM LSR is reached. At that point, every ATM LSR has the label binding for the destination. Because the ATM LSRs run in DoD mode, they only request a label from the next-hop LSR as indicated by the routing table. Thus, the label retention mode is conservative for LSRs that are running DoD label advertisement mode.

When the routing adjacencies are up, the IP prefixes are propagated, the routing table is built on the LSRs, and LDP forms the LDP neighborships and advertises the label bindings for the prefixes, the ATM LSRs can build the LVCs between them.

NOTE LVCs and ATM Forum virtual circuits can exist on the same ATM switches and the same ATM interfaces.

Look at Example 5-7. If IP traffic needs to go from the denver edge LSR to the brussels edge LSR (destination 10.200.253.6), it takes the following LVCs (TVCs):

To see the ATM LDP bindings, use the show mpls atm-ldp bindings command. Example 5-7 shows you how to use that command to track the LVCs through the ATM LSRs.

Example 5-7 Checking the LVCs

denver#show mpls forwarding-table

Local

Outgoing

Prefix

Bytes tag

Outgoing

Next Hop

tag

tag or VC

or Tunnel Id

switched

interface

17

1/38

10.200.253.1/32

0

AT1/0/0.10

point2point

18

1/39

10.200.253.2/32

0

AT1/0/0.10

point2point

19

1/40

10.200.253.3/32

0

AT1/0/0.10

point2point

20

1/41

10.200.253.4/32

0

AT1/0/0.10

point2point

21

1/42

10.200.253.6/32

0

AT1/0/0.10

point2point

denver#show atm vc interface ATM 1/0/0.10

VCD / Peak Avg/Min Burst denver#show atm vc interface ATM 1/0/0.10

VCD / Peak Avg/Min Burst

Interface

Name

VPI VCI

Type

Encaps

Kbps

Kbps Cells

Sts

1/0/0.10

56

0 32

PVC

SNAP

149760

N/A

UP

1/0/0.10

62

1 34

TVC

MUX

149760

N/A

UP

1/0/0.10

63

1 38

TVC

MUX

149760

N/A

UP

1/0/0.10

66

1 39

TVC

MUX

149760

N/A

UP

1/0/0.10

64

1 40

TVC

MUX

149760

N/A

UP

1/0/0.10

67

1 41

TVC

MUX

149760

N/A

UP

1/0/0.10

65

1 42

TVC

MUX

149760

N/A

UP

denver#show mpls forwarding-table 10.200.253.6 detail

Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface

21 1/42 10.200.253.6/32 0 AT1/0/0.10 point2point

MAC/Encaps=4/8, MRU=4470, Tag Stack{1/42(vcd=65)} 00418847 00041000 No output feature configured

Example 5-7 Checking the LVCs (Continued)

denver#show mpls atm-ldp bindings 10.200.253.6 32

Destination: 10.200.253.6/32

Headend Router ATM1/0/0.10 (3 hops) 1/42 Active, VCD=65

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

Destination: 10.200.253.6/32

Transit ATM2/0/2 1/42 Active -> ATM2/0/1 1/36 Active denver-atm#show atm vc interface ATM 2/0/2 1 42

Interface: ATM2/0/2, Type: oc3suni VPI = 1 VCI = 42 Status: UP

Time-since-last-status-change: 00:18:52

Connection-type: TVC(I)

Cast-type: multipoint-to-point-input

Cross-connect-interface: ATM2/0/1, Type: oc3suni Cross-connect-VPI = 1 Cross-connect-VCI = 36 Cross-connect-UPC: pass 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 brussels-atm#show atm vc interface ATM 2/0/1 1 36

Interface: ATM2/0/1, Type: oc3suni VPI = 1 VCI = 36 Status: UP

Time-since-last-status-change: 00:20:00 Connection-type: TVC(I) Cast-type: multipoint-to-point-input Packet-discard-option: enabled Usage-Parameter-Control (UPC): pass Wrr weight: 2

Number of OAM-configured connections: 0 OAM-configuration: disabled OAM-states: Not-applicable

Cross-connect-interface: ATM2/0/2, Type: oc3suni Cross-connect-VPI = 1 Cross-connect-VCI = 33 Cross-connect-UPC: pass continues

Example 5-7 Checking the LVCs (Continued)

brussels#show mpls atm-ldp bindings 10.200.253.6 32

Destination: 10.200.253.6/32

Tailend Router ATM3/0.10 1/33 Active, VCD=597

Look at Example 5-8 to see the complete label information base (LIB) for the ATM LSR denver. You can see one entry per IP prefix in the routing table. An ATM switch can be in three positions regarding the LSPs (LVCs): tail end switch, transit, or head end switch. A head end switch means that the ATM LSR is the ingress LSR, and a tail end switch means that the ATM LSR is the egress LSR. A transit LSR is the ATM LSR between the ingress and egress LSR on the LSP. The Tailend Switch entries in Example 5-8 are the entries for the prefix 10.200.253.1/32 because this prefix is directly connected on the LSR denver-atm. Three entries exist because the ATM LSR denver-atm has three upstream ATM LSR neighbors for that prefix: denver, washington-atm, and brussels-atm. These LDP label bindings lead to the LVCs (TVCs) set up as in Example 5-4.

Example 5-8 ATM Label Bindings

denver-atm#show mpls

atm-ldp bindings

Destination: 10.200

253.1/32

Tailend Switch ATM2/0/1 1/33

Active

-> Terminating Active,

VCD=

223

Tailend Switch ATM2/0/0 1/33

Active

-> Terminating Active,

VCD=

231

Tailend Switch ATM2/0/2 1/38

Active

-> Terminating Active,

VCD=

240

Destination: 10.200

253.5/32

Transit ATM2/0/1

1/75 Active

-> ATM2/0/2

1/34

Active

Transit ATM2/0/0

1/67 Active

-> ATM2/0/2

1/34

Active

Destination: 10.200

253.2/32

Transit ATM2/0/2

1/39 Active

-> ATM2/0/0

1/35

Active

Destination: 10.200

253.3/32

Transit ATM2/0/2

1/40 Active

-> ATM2/0/1

1/35

Active

Destination: 10.200

253.4/32

Transit ATM2/0/2

1/41 Active

-> ATM2/0/0

1/36

Active

Destination: 10.200

253.6/32

Transit ATM2/0/2

1/42 Active

-> ATM2/0/1

1/36

Active

You can see that the LDP-learned labels are VPI/VCI values, and the VCs have been set up in the data plane as a result of the learned MPLS labels. The command show mpls ip binding gives another nice overview of the label bindings per prefix. Example 5-9 shows the output of this command.

Example 5-9 Show MPLS IP Binding

denver-atm#show mpls

ip binding

10.200.253.1/32

in label:

imp-null

in vc label:

1/33

lsr: 10

.200

253.2

1

ATM2/0/0

Active

egress

(vcd

231)

in vc label:

1/38

lsr: 10

.200

253.5

1

ATM2/0/2

Active

egress

(vcd

240)

in vc label:

1/33

lsr: 10

.200

253.3

2

ATM2/0/1

Active

egress

(vcd

246)

10.200.253.2/32

in label:

16

in vc label:

1/39

lsr: 10

.200

253.5

1

ATM2/0/2

Active

transit

out vc label:

1/35

lsr: 10

.200

253.2

1

ATM2/0/0

Active

transit

10.200.253.3/32

in label:

17

in vc label:

1/91

lsr: 10

.200

253.5

1

ATM2/0/2

Active

transit

out vc label:

1/33

lsr: 10

.200

253.3

2

ATM2/0/1

Active

transit

10.200.253.4/32

in label:

18

in vc label:

1/41

lsr: 10

.200

253.5

1

ATM2/0/2

Active

transit

out vc label:

1/36

lsr: 10

.200

253.2

1

ATM2/0/0

Active

transit

10.200.253.5/32

in label:

25

in vc label:

1/67

lsr: 10

.200

253.2

1

ATM2/0/0

Active

transit

in vc label:

1/34

lsr: 10

.200

253.3

2

ATM2/0/1

Active

transit

out vc label:

1/34

lsr: 10

.200

253.5

1

ATM2/0/2

Active

transit

10.200.253.6/32

in label:

19

in vc label:

1/92

lsr: 10

.200

253.5

1

ATM2/0/2

Active

transit

out vc label:

1/36

lsr: 10

.200

253.3

2

ATM2/0/1

Active

transit

LDP Control Mode for ATM

Chapter 2 mentioned two possible control modes for label advertisement: Ordered and Independent Control mode. The Independent Control mode means that an LSR immediately responds to a Label Request message from the upstream LSR. Ordered means that the LSR only responds to the Label Request message from the upstream LSR when it received a response to its Label Request message from its downstream LSR. In Figure 5-6, the Control mode was Ordered. Look at Figure 5-7 to see the two different Control modes side by side.

Figure 5-7 LDP Control Modes

Label Request I

Label Request I

Ordered Control Mode ill—

I Label Mapping | Label Mapping]

® ® Label Request Label Request

Independent Control Mode in—m—I

| Label Mapping | Label Mapping]

The default Control mode in Cisco IOS on ATM switches is Ordered Control. To change the Control mode, use the following global Cisco IOS command:

mpls ldp atm control-mode {ordered | independent}

The disadvantage of Ordered Control mode is the delay in receiving a label binding. The advantage is that no packets are lost, because the ingress ATM LSR only has an outgoing label (and hence an outgoing LVC) if all ATM LSRs along the path have one. Packets can be lost in Independent Control mode until all ATM LSRs have the label bindings for that LSP. An ATM LSR that does not have an outgoing LVC must perform an expensive operation—namely, it must reassemble the cells into a packet and then discard the packet.

Look at Example 5-10 to see Label Request and Label Mapping messages received and sent out on the denver-atm LSR running Independent Control mode for a new prefix learned downstream from denver-atm. (The downstream LSR is brussels-atm.)

Example 5-10 Independent Control Mode denver-atm# denver-atm#conf t

Enter configuration commands, one per line. End with CNTL/Z. denver-atm(config)#mpls ldp atm control-mode independent denver-atm(config)#~Z denver-atm#

denver-atm#deb mpls ldp messages sent

LDP sent PDUs, excluding periodic Keep Alives debugging is on denver-atm#deb mpls ldp messages received

LDP received messages, excluding periodic Keep Alives debugging is on denver-atm#show debugging

MPLS ldp:

LDP received messages, excluding periodic Keep Alives debugging is on LDP sent PDUs, excluding periodic Keep Alives debugging is on ldp: Rcvd label request msg from 10.200.253.5:1 (pp 0x6302D8C4)

ldp: Sent label mapping msg to 10.200.253.5:1 (pp 0x6302D8C4)

ldp: Sent label request msg to 10.200.253.3:2 (pp 0x6302E47C)

ldp: Rcvd label mapping msg from 10.200.253.3:2 (pp 0x6302E47C)

ldp: Rcvd label mapping msg from 10.200.253.3:2 (pp 0x6302E47C)

ldp: Sent label mapping msg to 10.200.253.5:1 (pp 0x6302D8C4)

You can see that the LSR denver-atm sends a Label Mapping message immediately in response to the Label request message from the LSR denver (10.200.253.5). In Ordered Control mode, the LSR denver-atm sends a Label Request message to the downstream LSR brussels-atm (10.200.253.3) and awaits the receipt of the Label Mapping message from it before sending a Label Mapping message back to the denver LSR. This is depicted in Example 5-11.

Example 5-11 Ordered Control Mode

denver-atm#

ldp: Rcvd label

request

msg

from

10.200.253

5

1 (pp 0X6302D8C4)

ldp: Sent label

request

msg

to 1<

.200.253.3

2

(pp 0X6302E47C)

ldp: Rcvd label

mapping

msg

from

10.200.253

3:

2 (pp 0X6302E47C)

ldp: Sent label

mapping

msg

to 1<

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Micro Expression Master

Micro Expression Master

If You Could Read Everyone Life A Book You Can Have Better Career, Great Relationships And Become Successful. This Book Is One Of The Most Valuable Resources In The World When It Comes To Reading the smallest and tiniest body Language and know what people are thinking about.

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