Hi there,
I have couple of questions about some Segment Routing and BGP-LU.
Above is my network topology. So here are the details and I will omit some details just to keep things simple.
* orange area is ISIS AGG1. Yellow is ISIS BB. Grey is AGG2
* no redistribution between ISIS instance. ABRs belong to both BB and AGGx
* black lines are BGP labeled-unicast sessions.
* XR8 is central route-reflector
* ABRs are route-reflectors too with next-hop-self. This is to propagate the loopbacks from AGGx to BB and vice versa.
* All routers belong to BGP AS#1234
* All routers are running segment-routing.
* Loopback addresses are 192.168.255.x/32 where X is the router number.
Here's my configuration for PE routers and it applies to all. I am using index 1 for xr1, 2 for xr2, and so on. Using default SRGB.
prefix-set LOOPBACK
0.0.0.0/0 eq 32
end-set
!
route-policy LOOP($SID)
if destination in LOOPBACK then
set label-index $SID
endif
end-policy
!
router bgp 1234
ibgp policy out enforce-modifications
address-family ipv4 unicast
redistribute connected route-policy LOOP(1)
!
router isis AGG1
interface Loopback0
address-family ipv4 unicast
tag 100
prefix-sid index 1 explicit-null
My question is about this command, "ibgp policy out enforce-modifications".
The lab is working fine and I can ping from ios1 to ios3. I have another VPNv4 peering using XR7 as the sRR so that's a different story.
So now, I am just wondering how this ibgp policy out command works. When I turned off ibgp policy out (I'll keep the command short) on XR3 and XR4, XR8 see these routes.
RP/0/0/CPU0:xr8#show bgp ipv4 labeled-unicast labels | b Origin
Mon Oct 19 04:30:16.846 UTC
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Rcvd Label Local Label
*>i192.168.37.0/24 192.168.255.7 3 16007
*>i192.168.57.0/24 192.168.255.7 3 16007
* i192.168.255.1/32 192.168.255.3 3 16001
*>i 192.168.255.4 3 16001
* i192.168.255.2/32 192.168.255.3 3 16002
*>i 192.168.255.4 3 16002
*>i192.168.255.7/32 192.168.255.7 3 16007
* i192.168.255.9/32 192.168.255.5 16009 16009
*>i 192.168.255.6 16009 16009
* i192.168.255.10/32 192.168.255.5 16010 16010
*>i 192.168.255.6 16010 16010
I don’t know why it says here Rcvd Label 3 (implicit-null) but the next-hop is correct which is 192.168.255.4 because XR4 is advertising the route to XR8 with next-hop-self. (I'm just getting it from XR4 for now because XR3 is getting the routes from RR too. Will deal with that later)
XR4’s BGP table is the same.
RP/0/0/CPU0:xr4#show bgp ipv4 labeled-unicast labels | b Origin
Sat Oct 10 12:57:48.667 UTC
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Rcvd Label Local Label
*>i192.168.255.1/32 192.168.255.1 3 16001
*>i192.168.255.2/32 192.168.255.2 3 16002
*>i192.168.255.7/32 192.168.255.7 3 16007
*>i192.168.255.9/32 192.168.255.6 16009 16009
*>i192.168.255.10/32 192.168.255.6 16010 16010
It says Rcvd Label is 3 (implicit-null)
Why is the local label correct for the corresponding loopbacks, i.e. 16001 for XR1 and 16002 for XR2. I enabled the ibgp policy out on XR5 and XR6 so the Rcvd Labels are correct for XR9 and XR10, 16009 and 16010 respectively.
For XR1, I am advertising the loopback route with SID1 (which I suspect should be SRGB+index value) but the output in both XR3 and XR4 says the received label is 3. I also do not know how it was able to assign a correct "local label" of 16001 and 16002.
If I turn on ibgp out policy command on XR3 and XR4, the table changes to this.
XR4 remains to have Rcvd Label 3 but XR8 is now correct with 16001 and 16002.
RP/0/0/CPU0:xr4#show bgp ipv4 labeled-unicast labels | b Origin
Sat Oct 10 13:06:39.601 UTC
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Rcvd Label Local Label
*>i192.168.255.1/32 192.168.255.1 3 16001
*>i192.168.255.2/32 192.168.255.2 3 16002
*>i192.168.255.7/32 192.168.255.7 3 16007
*>i192.168.255.9/32 192.168.255.6 16009 16009
*>i192.168.255.10/32 192.168.255.6 16010 16010
RP/0/0/CPU0:xr8#show bgp ipv4 labeled-unicast labels | b Origin
Sat Oct 10 13:07:30.479 UTC
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Rcvd Label Local Label
* i192.168.255.1/32 192.168.255.3 16001 16001
*>i 192.168.255.4 16001 16001
* i192.168.255.2/32 192.168.255.3 16002 16002
*>i 192.168.255.4 16002 16002
*>i192.168.255.7/32 192.168.255.7 3 16007
* i192.168.255.9/32 192.168.255.5 16009 16009
*>i 192.168.255.6 16009 16009
* i192.168.255.10/32 192.168.255.5 16010 16010
*>i 192.168.255.6 16010 16010
Can someone help me understand how the labels are being assigned here and advertised? If I do not enable ibgp policy out command, the end to end VPN doesn't work.
