SRv6 uSID w/ SR-TE Explicit Paths

Load top1.vpnv4v6.srv6.usid.configured.cfg

#IOS-XR
configure
load top1.vpnv4v6.srv6.usid.configured.cfg
commit replace
y

#IOS-XE
config replace flash:top1.vpnv4v6.srv6.usid.configured.cfg

L3VPN for IPv4 and IPv6 is currently functional using SRv6 uSID in the core.

Configure an SR-TE policy on XR5 that uses an explicit SID list with uSID. The path should go via XR1, XR2, XR4 and then XR7. Color routes OSPF routes redistributed into BGP on XR7 to use this policy.

Answer

#XR5
segment-routing
 traffic-eng
  srv6
   locator CCIE binding-sid dynamic behavior ub6-insert-reduced
  !
  segment-lists
   srv6
    sid-format usid-f3216
   !
   segment-list LIST1
    srv6
     index 1 sid fd00:0:1::
     index 2 sid fd00:0:2::
     index 3 sid fd00:0:4::
    !
   !
  !
  policy POL1
   srv6
   !
   color 10 end-point ipv6 2001:db8:200::7
   candidate-paths
    preference 100
     explicit segment-list LIST1


#XR7
extcommunity-set opaque COLOR_10
  10
end-set
!
route-policy SET_COLOR
  set extcommunity color COLOR_10
end-policy
!
router bgp 100
 vrf CUSTOMER
  address-family ipv4 unicast
   redistribute ospf CUSTOMER route-policy SET_COLOR
  !
  address-family ipv6 unicast
   redistribute ospfv3 CUSTOMER route-policy SET_COLOR

Explanation

While SRv6-TE ODN policies are more scalable, it is still good to know how to configure SRv6-TE policies that use explicit paths. Using an explicit path with SRv6-TE requires several lines of configuration.

First we specify the SRv6 locator that is used to allocate a BSID. This is done when using an SRv6-TE policy (ODN or explicit paths).

segment-routing
 traffic-eng
  srv6
   locator CCIE binding-sid dynamic behavior ub6-insert-reduced

Next, we configure an SRv6 SID list. We must specify that the SID list uses the f3216 format. (This is our only option). Remember not to include the endpoint itself (XR7) in the SID list. This is because the BGP SID already will be steered towards the endpoint. (The SID comes from the egress PE’s locator block already).

segment-routing
 traffic-eng
  segment-lists
   srv6
    sid-format usid-f3216
   !
   segment-list LIST1
    srv6
     index 1 sid fd00:0:1::
     index 2 sid fd00:0:2::
     index 3 sid fd00:0:4::

Finally we apply the SID list to the policy as a candidate path. Make sure to enable SRv6 under the policy.

segment-routing
 traffic-eng
  policy POL1
   srv6
   !
   color 10 end-point ipv6 2001:db8:200::7
   candidate-paths
    preference 100
     explicit segment-list LIST1

The policy should now be up. A BSID was allocated from the specified locator:

After XR7 colors the routes, traffic towards CE3 should take this SRv6-TE policy.

Taking a pcap of a ping from CE1 to CE3 shows the proper uSID container as the IPv6 destination address:

CE1 receives ping replies: