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Unequal-Cost Multipath with BGP DMZ Link Bandwidth

In the previous blog post in this series, I described why it’s (almost) impossible to implement unequal-cost multipathing for anycast services (multiple servers advertising the same IP address or range) with OSPF. Now let’s see how easy it is to solve the same challenge with BGP DMZ Link Bandwidth attribute.

I didn’t want to listen to the fan noise generated by my measly Intel NUC when simulating a full leaf-and-spine fabric, so I decided to implement a slightly smaller network:

X1 and X2 are Cisco IOS devices advertising the same prefix – 10.42.42.0/24. E1 and E2 have the same function as the ToR switches in a leaf-and-spine fabric with a slight twist: every link between AS 65000 and AS 65001 has a different bandwidth. PE1 is an equivalent of a spine switch in a leaf-and-spine fabric.

After we get BGP and OSPF up and running, E1 receives two copies of 10.42.42.0/24 prefix (one from X1 and another one from X2), but it only uses one of them for packet forwarding.

Initial BGP and IP routing table on E1
e1#sh ip bgp 10.42.42.0
BGP routing table entry for 10.42.42.0/24, version 2
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     1          2
  Refresh Epoch 1
  65100
    10.1.0.10 from 10.1.0.10 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0
  Refresh Epoch 2
  65100
    10.1.0.14 from 10.1.0.14 (10.0.0.5)
      Origin IGP, metric 0, localpref 100, valid, external
      rx pathid: 0, tx pathid: 0
e1#sh ip route 10.42.42.0
Routing entry for 10.42.42.0/24
  Known via "bgp 65000", distance 20, metric 0
  Tag 65100, type external
  Last update from 10.1.0.10 00:00:45 ago
  Routing Descriptor Blocks:
  * 10.1.0.10, from 10.1.0.10, 00:00:45 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 65100
      MPLS label: none

The caveat: default value of BGP maximum-paths on Cisco IOS is one. After changing that to eight (because why not), E1 uses both prefixes… but in 1:1 ratio.

IP routing table contains both BGP prefixes after the maximum-paths have been configured
e1#sh ip route 10.42.42.0
Routing entry for 10.42.42.0/24
  Known via "bgp 65000", distance 20, metric 0
  Tag 65100, type external
  Last update from 10.1.0.14 00:00:05 ago
  Routing Descriptor Blocks:
    10.1.0.14, from 10.1.0.14, 00:00:05 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 65100
      MPLS label: none
  * 10.1.0.10, from 10.1.0.10, 00:00:05 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 65100
      MPLS label: none

Time to enter the magical world of DMZ Link Bandwidth. We have to configure:

  • neighbor dmzlink-bw on all EBGP neighbors to attach DMZ Link Bandwidth extended BGP community to all BGP prefixes received from EBGP neighbors1.
  • bgp dmzlink-bw within an address family to use the link bandwidth extended community to calculate traffic shares.
BGP configuration changes on E1
router bgp 65000
 !
 address-family ipv4
  bgp dmzlink-bw
  neighbor 10.1.0.10 dmzlink-bw
  neighbor 10.1.0.14 dmzlink-bw
  maximum-paths 8

After configuring DMZ Link Bandwidth on E1 and E2, you’ll notice additional communities attached to BGP prefix 10.42.42.0/24… and a modified traffic share on E1 (E2 has only one link into AS 65001).

External BGP prefixes on E1 contain DMZ-Link BW community
e1#show ip bgp 10.42.42.0
BGP routing table entry for 10.42.42.0/24, version 5
Paths: (2 available, best #1, table default)
Multipath: eBGP iBGP
  Advertised to update-groups:
     1          2
  Refresh Epoch 2
  65100
    10.1.0.10 from 10.1.0.10 (10.0.0.4)
      Origin IGP, metric 0, localpref 100, valid, external, multipath, best
      DMZ-Link Bw 250 kbytes
      rx pathid: 0, tx pathid: 0x0
  Refresh Epoch 3
  65100
    10.1.0.14 from 10.1.0.14 (10.0.0.5)
      Origin IGP, metric 0, localpref 100, valid, external, multipath(oldest)
      DMZ-Link Bw 375 kbytes
      rx pathid: 0, tx pathid: 0
Traffic share ratio between E1-X1 and E2-X2 is 2:3. We have a winner ūüėČ
e1#show ip route 10.42.42.0
Routing entry for 10.42.42.0/24
  Known via "bgp 65000", distance 20, metric 0
  Tag 65100, type external
  Last update from 10.1.0.14 00:01:43 ago
  Routing Descriptor Blocks:
    10.1.0.14, from 10.1.0.14, 00:01:43 ago
      Route metric is 0, traffic share count is 3
      AS Hops 1
      Route tag 65100
      MPLS label: none
  * 10.1.0.10, from 10.1.0.10, 00:01:43 ago
      Route metric is 0, traffic share count is 2
      AS Hops 1
      Route tag 65100
      MPLS label: none

Now for a sprinkle of Pure Magic‚ĄĘ – let’s inspect the BGP table and IP routing table on PE1 without doing anything else but configuring maximum-paths and bgp dmzlink-bw on it:

BGP prefix 10.42.42.0/24 on PE1
pe1#show ip bgp 10.42.42.0
BGP routing table entry for 10.42.42.0/24, version 9
Paths: (2 available, best #1, table default)
Multipath: eBGP iBGP
  Advertised to update-groups:
     1
  Refresh Epoch 1
  65100, (Received from a RR-client)
    10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath, best
      DMZ-Link Bw 625 kbytes
      rx pathid: 0, tx pathid: 0x0
  Refresh Epoch 3
  65100, (Received from a RR-client)
    10.0.0.2 (metric 2) from 10.0.0.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, valid, internal, multipath(oldest)
      DMZ-Link Bw 125 kbytes
      rx pathid: 0, tx pathid: 0

The 10.42.42.0/24 prefix received from E2 contains the expected DMZ Link Bandwidth (1000 kbps), while the prefix received from E1 contains the sum of all DMZ Link Bandwidths in E1 BGP table, resulting in perfect UCMP traffic ratio. Problem solved.

pe1#show ip route 10.42.42.0
Routing entry for 10.42.42.0/24
  Known via "bgp 65000", distance 200, metric 0
  Tag 65100, type internal
  Last update from 10.0.0.2 00:11:18 ago
  Routing Descriptor Blocks:
    10.0.0.2, from 10.0.0.2, 00:11:18 ago
      Route metric is 0, traffic share count is 23
      AS Hops 1
      Route tag 65100
      MPLS label: none
  * 10.0.0.1, from 10.0.0.1, 00:11:18 ago
      Route metric is 0, traffic share count is 120
      AS Hops 1
      Route tag 65100
      MPLS label: none

Unfortunately, there’s always another twist in every saga. While the IP routing table entry on PE1 looks great, and the overview part of the CEF table entry looks equally impressive…

CEF table entry for 10.42.42.0/24 (slightly abridged to fit onto the page)
pe1#sh ip cef 10.42.42.0/24 internal
10.42.42.0/24, epoch 2, flags [rnolbl, rlbls], RIB[B], refcnt 6...
  sources: RIB
  feature space:
    IPRM: 0x00018000
    Broker: linked, distributed at 4th priority
  ifnums:
    GigabitEthernet2(8): 10.1.0.1
    GigabitEthernet3(9): 10.1.0.5
  path list 7F72505C4090, 3 locks, per-destination, flags 0x269 
    path 7F724DEDA0D8, share 23/23, type recursive, for IPv4
      recursive via 10.0.0.2[IPv4:Default], fib 7F72AE3E3B88, 1 terminal fib...
      path list 7F72505C4130, 3 locks, per-destination, flags 0x49 [shble, rif, hwcn]
          path 7F724DEDA4F8, share 1/1, type attached nexthop, for IPv4
            nexthop 10.1.0.5 GigabitEthernet3, IP adj out of GigabitEthernet3...
    path 7F724DEDA448, share 120/120, type recursive, for IPv4
      recursive via 10.0.0.1[IPv4:Default], fib 7F72AE3E3C88, 1 terminal fib...
      path list 7F72505C4270, 3 locks, per-destination, flags 0x49 [shble, rif, hwcn]
          path 7F724DEDA658, share 1/1, type attached nexthop, for IPv4
            nexthop 10.1.0.1 GigabitEthernet2, IP adj out of GigabitEthernet2...

… the hashing buckets don’t reflect the desired traffic sharing. The traffic is sent toward E1 and E2 in 8:8 (i.e. 1:1) ratio. Bummer.

CEF hashing buckets for 10.42.42.0/24 on Cisco IOS XE 16.06.01
  output chain:
    loadinfo 80007F72AF328F60, per-session, 2 choices, flags 0003, 5 locks
      flags [Per-session, for-rx-IPv4]
      16 hash buckets
        < 0 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        < 1 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        < 2 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        < 3 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        < 4 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        < 5 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        < 6 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        < 7 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        < 8 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        < 9 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        <10 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        <11 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        <12 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        <13 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010
        <14 > IP adj out of GigabitEthernet3, addr 10.1.0.5 7F7249F23E20
        <15 > IP adj out of GigabitEthernet2, addr 10.1.0.1 7F7249F24010

If you want to reproduce my results, you’ll find the lab topology and configuration files on GitHub.

Back to Leaf-and-Spine Fabrics

Can we use the same trick in a leaf-and-spine fabric? Absolutely. Figuring out how to apply these concepts to leaf-and-spine fabrics is left as an exercise for the reader.

The example used IBGP over OSPF. Can we use EBGP-only design in a leaf-and-spine fabric? According to the original DMZ Link Bandwidth draft, the answer is NO. The DMZ Link Bandwidth community is not supposed to be propagated beyond a single AS2. Of course that didn’t stop anyone – there’s another draft saying it’s OK to propagate DMZ Link Bandwidth over EBGP.

I used Cisco IOS XE in the examples. What about data center switches? Similar functionality is implemented in Arista EOS and FRR (and thus Cumulus Linux, SoNIC, and whoever else uses FRR).

The Arista EOS implementation (described in the Data Center Fabrics webinars in June 2016) is even better than what’s available in Cisco IOS XE. You can aggregate the bandwidth advertised from downstream BGP speakers (servers or leaf switches), and split the bandwidth when advertising the same prefix toward upstream BGP speakers (spine switches or ingress leaf switches).

I haven’t found anything similar in Nexus OS; corrections are (as always) most welcome.


  1. As is often the case when configuring BGP, the changes apply to new incoming updates. Do clear ip bgp soft in first. ↩︎

  2. Long-time readers might remember that I was always telling people to use IBGP+OSPF instead of EBGP as better IGP hype in small data center fabrics. Not that anyone ever listens. ↩︎

Blog posts in this series

3 comments:

  1. It seems X1 and X2 routers have wrong loopback IP addresses in your network diagram. Any idea why the hash buckets are evenly distributed accross the interfaces and not according to the share ratio on PE1? Is this a bug?

  2. @Anonymous: Fixed the diagram. Thank you. Also added lab setup instructions to the GitHub repository.

    As for the CEF table: It could be a bug, or it could be irrelevant. I have no idea whether CSR 1000v uses CEF table for packet forwarding. If it uses the underlying Linux routing table, it doesn't matter what's in the CEF table.

  3. X1 and X2 looks like are in AS65100 based on cli output. In the diagram X1,X2 are in AS65001.

    Replies
    1. Thank you, fixed. Hope this is the last error in that diagram 😕

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