Deborah,
As far as Option B goes,
you need to have MP-eBGP session between the ASBR's which are directly connected.
They will redistribute the VPN-IPv4 labels of it's AS to the ASBR.
You can have multiple ASBR-ASBR Ebgp session incase there are many VPN's or if you want
redundancy(L-Pref).
Option C
The redistribution of the VPN-IPv4 routes can happen via Multi-Hop MP-BGP session between
the PE's or RR's.
The ASBR will not hold any VPN-IPv4 labels or redistribute them,but has to have an
/32(loopback) labeled route for PE in its AS and will redistribute this to the other AS.
This is the Igp Label(PE Loopback) which will enable a packet to be forwarded from the Ingress PE to the Egress PE(LSP from Ingress PE to Egress PE).
Incase the /32 label is also redistributed to the P routers also...
it will require a label stack of 2 labels.
If they are unkown to P routers the label stack required will be 3.
The bottom label will be the VPN-IPv4 label,
middle label will be the Egress PE label redistributed by the ASBR of its AS
and the Top label will be assigned by the Ingress PE to reach its ASBR.
Coming back to the your question the major difference between option A and B are.
OptionB-ASBR redistributes VPN-IPv4 labels.
OptionC-ASBR doesn't hold any VPN-IPv4 labels.
As far as I can understand,you can have multiple ASBR's in all the 3 Options.
Regards,
Aditya
