Tip: OSPF unequal-cost (UCMP) multipath in IOS-XR

Nov 03, 21

One of the not-so-well-known features IOS-XR platforms supports that IOS-XE does not is that it allows all IGP’s as well as Static Routing to perform unequal-cost multipath (UCMP). By leveraging the weight distribution in CEF, if the IGP considers that the alternate path is loop-free, it allows the alternate path to be installed in the forwarding plane with a weight value that is derived from the metric assigned to the path allowing load-sharing proportionally based on the given cost. This is what IOS-XE is known as traffic share count.

Note: For further information on how IOS-XR computes weight distributions, refer to the document on leveraging IOS-XR weights I wrote while working in Cisco TAC back in the day.

Initial configurations

/XR1
hostname XR1
interface Loopback0
 ipv4 address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0/0/0.12
 ipv4 address 12.0.0.1 255.255.255.0
 encapsulation dot1q 12
!
interface GigabitEthernet0/0/0/0.21
 ipv4 address 21.0.0.1 255.255.255.0
 encapsulation dot1q 21
!
router ospf 12
 router-id 1.1.1.1
 address-family ipv4
 area 0
  interface Loopback0
  !
  interface GigabitEthernet0/0/0/0.12
   network point-to-point
  !
  interface GigabitEthernet0/0/0/0.21
   network point-to-point
  !
 !
!
end

/XR2
hostname XR2
interface Loopback0
 ipv4 address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet0/0/0/0.12
 ipv4 address 12.0.0.2 255.255.255.0
 encapsulation dot1q 12
!
interface GigabitEthernet0/0/0/0.21
 ipv4 address 21.0.0.2 255.255.255.0
 encapsulation dot1q 21
!
router ospf 12
 router-id 2.2.2.2
 address-family ipv4
 area 0
  interface Loopback0
  !
  interface GigabitEthernet0/0/0/0.12
   network point-to-point
  !
  interface GigabitEthernet0/0/0/0.21
   network point-to-point
  !
 !
!
end

Initially XR1 is seeing the prefix 2.2.2.2/32 coming from XR1 as equally shared across interface gi0/0/0/0.12 and gi0/0/0/0.21 respectively.

RP/0/0/CPU0:XR1#show route 2.2.2.2/32
<snip>
Routing entry for 2.2.2.2/32
  Known via "ospf 12", distance 110, metric 2, type intra area
  Routing Descriptor Blocks
	12.0.0.2, from 2.2.2.2, via GigabitEthernet0/0/0/0.12
  	Route metric is 2
	21.0.0.2, from 2.2.2.2, via GigabitEthernet0/0/0/0.21
  	Route metric is 2
  No advertising protos.

Same weight has been assigned to both links.

RP/0/0/CPU0:XR1#show cef ipv4 2.2.2.2/32 hardware egress detail
<snip>
2.2.2.2/32, version 15, internal 0x1000001 0x0 (ptr 0xa1423bf4) [1], 0x0 (0xa13ef464), 0x0 (0x0)
   via 12.0.0.2/32, GigabitEthernet0/0/0/0.12, 5 dependencies, weight 0, class 0 [flags 0x0]
	path-idx 0 NHID 0x0 [0xa0f571fc 0x0]
	next hop 12.0.0.2/32
	local adjacency
   via 21.0.0.2/32, GigabitEthernet0/0/0/0.21, 5 dependencies, weight 0, class 0 [flags 0x0]
	path-idx 1 NHID 0x0 [0xa0f571a8 0x0]
	next hop 21.0.0.2/32
	local adjacency
	Load distribution: 0 1 (refcount 2)
	Hash  OK  Interface             	Address
	0 	Y   GigabitEthernet0/0/0/0.12 12.0.0.2  	 
	1 	Y   GigabitEthernet0/0/0/0.21 21.0.0.2

Let’s change the cost on XR1 link gi0/0/0/0.21 towards XR2 on to 5 and set the variance multiplier to consider the alternate path.

router ospf 12
 ucmp variance 10000
!
 interface GigabitEthernet0/0/0/0.21
  cost 5
 !
end

Note: The variance does not have influence in the resulting weight assignments by CEF.

RP/0/0/CPU0:XR1#show route 2.2.2.2/32   	 
<snip>
Routing entry for 2.2.2.2/32
  Known via "ospf 12", distance 110, metric 2, type intra area
  Routing Descriptor Blocks
	12.0.0.2, from 2.2.2.2, via GigabitEthernet0/0/0/0.12
  	Route metric is 2, Wt is 4294967295 <<
	21.0.0.2, from 2.2.2.2, via GigabitEthernet0/0/0/0.21
  	Route metric is 2, Wt is 1431655764 <<
  No advertising protos.

Note that the weight assigned to each interface is 4294967295 for the primary path and 1431655764 for the secondary path, this means that the primary path would take 3x the load of the secondary, since its metric is 2 versus 6, is 3x better than the alternate path, this allows the router to load-sharing based on the metric values coming from the IGP.