Figure 1 BGP EPE diagram
PE 1 Loop1
AS 100
BGP EPE
ASBR 1
18001
PeerAdj SID
PeerAdj SID
18002
28001
ASBR 3
PeerNode SID
PeerNode
SID
28002
PeerNode
SID
28003
ASBR 4
PE 2
ASBR 2
PeerNode
SID
28004
PeerSet SID
29001 ASBR 5
Loop1
AS 200
AS 300
PE 3
PE 4
BGP EPE
As shown in Figure 1 , BGP EPE allocates SID labels as follows:
•
ASBR 1 and ASBR 3 are directly connected through two physical links. The two ASBRs establish an EBGP peer relationship with each other through loopback interfaces. BGP EPE assigns Peer-Node-SID 28001 to the peer node and assigns Peer-Adj-SIDs 18001 and 18002 to the two physical links. Traffic between the two ASBRs will be load-shared on the two physical links when it is forwarded based on the Peer-Node-SID.
•
An EBGP peer relationship is established over a physical link between ASBR 1 and ASBR 5,
ASBR 2 and ASBR 4, and ASBR 2 and ASBR 5. BGP EPE only assigns labels
(Peer-Node-SIDs 28002, 28003, and 28004) to the peer nodes.
•
ASBR 4 and ASBR 5 both establish an EBGP peer relationship with ASBR 2. BGP EPE adds
ASBR 4 and ASBR 5 to a BGP EPE peer set and assigns the set Peer-Set-SID 29001. Traffic will be sent to the devices in the peer set for load sharing when it is forwarded based on the
Peer-Set-SID.
SIDs that BGP EPE assigns are locally significant. These SIDs are not advertised to other devices and are not affected by route types exchanged between BGP peers.
SID allocation through BGP EPE is typically used for MPLS VPN inter-AS option B.
Label forwarding entry installation based on SIDs
Label forwarding entry installation based on IGP prefix SIDs
Label forwarding entries based on prefix SIDs can be static or dynamic.
•
Static label forwarding entries —The device creates local label forwarding entries based on manually configured incoming labels, outgoing labels, and next hops.
•
Dynamic label forwarding entries —The device uses the IGP to flood in the SR domain the local SRGB and the index of the prefix SID for the local loopback interface. Based on the flooded information, the other devices in the domain calculates their local label forwarding entries by using the following rules:
Incoming label = Local SRGB base value + Index
Outgoing label = SRGB base value of the next hop + Index
4
Figure 2 Creating label forwarding entries based on IGP prefix SIDs
In Label
16201
Nexthop
Device B
Out Label
16201
In Label
16201
Nexthop
Device C
Out Label
16201
In Label
16201
.1
/32
.1
.1
0
201
Device A
SRGB Base
16000
Range
1000
Device B
SRGB Base
16000
Range
1000
Device C
SRGB Base
16000
Range
1000
Figure 2
shows how dynamic label forwarding entries are created. After you assign index value 201 to loopback address 1.1.1.1/32 on Device C, Device C uses an IGP packet to advertise the index value and its local SRGB. Then, the devices calculate incoming and outgoing labels according to the previously mentioned label calculation rules.
•
Devices C calculates its incoming label, which is 16201.
•
Device B calculates its incoming label and outgoing label and creates a label forwarding entry.
The incoming label is 16201. The outgoing label is 16201. The next hop is Device C.
•
Device A calculates its incoming label and outgoing label and creates a label forwarding entry.
The incoming label is 16201. The outgoing label is 16201. The next hop is Device B.
Label forwarding entry installation based on BGP prefix SIDs
The device redistributes routes to BGP and assigns prefix SIDs to redistributed routes based on routing policies. Then, the device uses BGP to flood in the SR domain the prefix SIDs and the indexes of the prefix SIDs. Based on the flooded information, the other devices in the domain calculates their local label forwarding entries by using the following rules:
•
Incoming label = Local SRGB base value + Index
•
Outgoing label = Received SID
Figure 3 Creating label forwarding entries based on BGP prefix SIDs
In Label
16201
Nexthop
Device C
Out Label
16201 In Label
16201
In Label
16201
Nexthop
Device B
Out Label
16201
In Label
16201
Nexthop
Device D
Out Label
16201
AS 100 AS 200
IBGP EBGP IBGP
.1
/32
.1
.1
0
201
SRGB Base
16000
Device A
Range
8000
SRGB Base
16000
Device B
Range
8000
Device C
SRGB Base
16000
Range
8000
Device D
SRGB Base
16000
Range
8000
Figure 3
shows how label forwarding entries are created. After you assign index value 201 to loopback address 1.1.1.1/32 on Device D, Device D redistributes the route of the address and assigns it an SID (16201 in this example). Then, the device uses a BGP packet to advertise the SID and index value. The devices in the SR domain calculate incoming and outgoing labels according to the previously mentioned label calculation rules and create label forwarding entries.
•
Devices D calculates its incoming label, which is 16201.
•
Device C calculates its incoming label and outgoing label and creates a label forwarding entry.
The incoming label is 16201. The outgoing label is 16201. The next hop is Device D.
5
