lab03-ISIS-CLOS

Task Underlay. IS-IS

Set up IS-IS in the underlay network to establish IP connectivity between all network devices.

Network diagram

stand-plan

The lab is built using the following role-based device mapping host- linux, leaf - frr, spine - arista

Underlay IP Address Allocation

• Underlay addressing follows the format 10.x.y.z, where:

⁠◦ x = Data Center ID

⁠◦ y = Spine switch ID

⁠◦ z = Sequential host address per leaf connection

• Host addressing uses the 172.16.x.z/24 format, where:

⁠◦ x = Leaf switch ID

⁠◦ z = Sequential host address

⁠◦ Each leaf switch uses .1 as its own IP in the corresponding subnet

• Loopback addressing uses 192.168.a.b/32, where:

⁠◦ a = 1 for spine switches

⁠◦ a = 2 for leaf switches

⁠◦ b = Spine or leaf ID (assigned sequentially)

• IPv6 addressing uses fd00::[IPv4]

Interconnect ipv4 ipv6

Device A	Interface A	IPv4 A	IPv6 A	Device B	Interface B	IPv4 B	IPv6 B
Spine-1	Eth1	10.1.1.0/31	fd00::10:1:1:0/127	Leaf-1	Eth8	10.1.1.1/31	fd00::10:1:1:1/127
Spine-1	Eth2	10.1.1.2/31	fd00::10:1:1:2/127	Leaf-2	Eth8	10.1.1.3/31	fd00::10:1:1:3/127
Spine-1	Eth3	10.1.1.4/31	fd00::10:1:1:4/127	Leaf-3	Eth8	10.1.1.5/31	fd00::10:1:1:5/127
Spine-2	Eth2	10.1.2.0/31	fd00::10:2:1:0/127	Leaf-1	Eth9	10.1.2.1/31	fd00::10:2:1:1/127
Spine-2	Eth2	10.1.2.2/31	fd00::10:2:1:2/127	Leaf-2	Eth9	10.1.2.3/31	fd00::10:2:1:3/127
Spine-2	Eth3	10.1.2.4/31	fd00::10:2:1:4/127	Leaf-3	Eth9	10.1.2.5/31	fd00::10:2:1:5/127
Host-1	Eth1	172.16.1.2/24	fd00::172:16:1:2/116	Leaf-1	Eth1	172.16.1.1/24	fd00::172:16:1:1/116
Host-2	Eth1	172.16.2.2/24	fd00::172:16:2:2/116	Leaf-2	Eth1	172.16.2.1/24	fd00::172:16:2:1/116
Host-3	Eth1	172.16.3.2/24	fd00::172:16:3:2/116	Leaf-3	Eth1	172.16.3.1/24	fd00::172:16:3:1/116
Host-4	Eth1	172.16.4.2/24	fd00::172:16:4:2/116	Leaf-3	Eth2	172.16.4.1/24	fd00::172:16:4:1/116

loopback

Device	Loopback ipv4	loopback ipv6
Spine-1	192.168.1.1	fd00::192:168:1:1
Spine-2	192.168.1.2	fd00::192:168:1:2
Leaf-1	192.168.2.1	fd00::192:168:2:1
Leaf-2	192.168.2.2	fd00::192:168:2:2
Leaf-3	192.168.2.3	fd00::192:168:2:3

isis.net is configured manually. System id derived from loopback IP address, area id 1.

Table net

Device	NET
Spine-1	49.0001.1921.6801.0001.00
Spine-2	49.0001.1921.6801.0002.00
Leaf-1	49.0001.1921.6802.0001.00
Leaf-2	49.0001.1921.6802.0002.00
Leaf-3	49.0001.1921.6802.0003.00

Since task doesn't contain instructions for host IP addressing, and both leaf and spine nodes are implemented as routers, host IPs are assigned from separate subnets prior to overlay configuration. This ensures basic connectivity while keeping underlay and overlay roles clearly separated.

Once the overlay is deployed, we’ll place all hosts into a single subnet to emulate Layer 2 connectivity.

Launching a lab in the Netlab

Netlab Usage Notes:

As in previous lab we need to enable bfd in frr.yml

Lab Notes:

Encountered issues while configuring BFD for IS-IS. After successfully enabling it manually, I concluded that Netlab’s documentation is inaccurate regarding the lack of BFD support for IS-IS on FRR instances. To enable BFD, I modified the module responsible for FRR IS-IS configuration: /usr/local/lib/python3.10/dist-packages/netsim/ansible/templates/isis/frr.macro.j2

Netlab is supposed to be about zero manual configuration, even if manual edits are faster and simpler. However, I ran into a known FRR limitation - Note: There will be just one BFD session per interface. If both IPv4 and IPv6 are configured, only an IPv6-based session is created.

Note that there will be just one BFD session per interface. In case both IPv4 and IPv6 support are configured then just a IPv6 based session is created. Checked the Arista EOS documentation and found that: BFD is not supported for IPv6 IS-IS. At this point, one could give up — but I suspected the lack of support was due to incomplete module implementation, not platform limitations. Manual testing confirmed that BFD for IS-IS over IPv6 works on EOS. To integrate it cleanly, I extended the EOS IS-IS module:

/usr/local/lib/python3.10/dist-packages/netsim/ansible/templates/isis/eos.macro.j2

Final caveat: EOS requires explicit protocol declaration for BFD. Since FRR runs in dual-stack mode, EOS must be explicitly told to use IPv6 for BFD sessions.

File Topology For quick reference, here’s the inline listing:

topology.yml

---
provider: clab
module: [ isis]

nodes:
s1:
device: eos
id: 1
isis:
  instance: netlab
  net: 49.0001.1921.6800.1001.00
loopback:
  ipv4: 192.168.1.1/32
  ipv6: fd00::192:168:1:1/128
s2:
device: eos
id: 2
isis:
  instance: netlab
  net: 49.0001.1921.6800.1002.00
loopback:
  ipv4: 192.168.1.2/32
  ipv6: fd00::192:168:1:2/128
l1:
device: frr
id: 3
isis:
  instance: netlab
  net: 49.0001.1921.6800.2001.00
loopback:
  ipv4: 192.168.2.1/32
  ipv6: fd00::192:168:2:1/128
l2:
device: frr
id: 4
isis:
  instance: netlab
  net: 49.0001.1921.6800.2002.00
loopback:
  ipv4: 192.168.2.2/32
  ipv6: fd00::192:168:2:2/128
l3:
device: frr
id: 5
isis:
  instance: netlab
  net: 49.0001.1921.6800.2003.00
loopback:
  ipv4: 192.168.2.3/32
  ipv6: fd00::192:168:2:3/128
h1:
device: linux
h2:
device: linux
h3:
device: linux
h4:
device: linux

links:
#spine1-leaf1,2,3
- interfaces:
    - node: s1
      ifname: eth1
      isis:
        network_type: point-to-point
        bfd:
          ipv6: True
      ipv4: 10.1.1.0
      ipv6: fd00::10:1:1:0
    - node: l1
      ifname: eth8
      isis:
        network_type: point-to-point
        bfd: True
      ipv4: 10.1.1.1
      ipv6: fd00::10:1:1:1
  prefix:
    ipv4: 10.1.1.0/31
    ipv6: fd00::10:1:1:0/127
- interfaces:
    - node: s1
      ifname: eth2
      ipv4: 10.1.1.2
      ipv6: fd00::10:1:1:2
      isis:
        network_type: point-to-point
        bfd:
          ipv6: True
    - node: l2
      ifname: eth8
      ipv4: 10.1.1.3
      ipv6: fd00::10:1:1:3
      isis:
        network_type: point-to-point
        bfd: True
  prefix:
    ipv4: 10.1.1.2/31
    ipv6: fd00::10:1:1:2/127
- interfaces:
    - node: s1
      ifname: eth3
      ipv4: 10.1.1.4
      ipv6: fd00::10:1:1:4
      isis:
        bfd:
          ipv6: True
    - node: l3
      ifname: eth8
      ipv4: 10.1.1.5
      ipv6: fd00::10:1:1:5
      isis:
        bfd: True
  prefix:
    ipv4: 10.1.1.4/31
    ipv6: fd00::10:1:1:4/127
#spine2-leaf1,2,3
- interfaces:
    - node: s2
      ifname: eth1
      ipv4: 10.1.2.0
      ipv6: fd00::10:1:2:0
      isis:
        network_type: point-to-point
        bfd:
          ipv6: true
    - node: l1
      ifname: eth9
      ipv4: 10.1.2.1
      ipv6: fd00::10:1:2:1
      isis:
        bfd: true
  prefix:
    ipv4: 10.1.2.0/31
    ipv6: fd00::10:1:2:0/127
- interfaces:
    - node: s2
      ifname: eth2
      ipv4: 10.1.2.2
      ipv6: fd00::10:1:2:2
      isis:
        bfd:
          ipv6: true
    - node: l2
      ifname: eth9
      ipv4: 10.1.2.3
      ipv6: fd00::10:1:2:3
      isis:
        bfd: true
  prefix:
    ipv4: 10.1.2.2/31
    ipv6: fd00::10:1:2:2/127
- interfaces:
    - node: s2
      ifname: eth3
      ipv4: 10.1.2.4
      ipv6: fd00::10:1:2:4
      isis:
        bfd:
          ipv6: true
    - node: l3
      ifname: eth9
      ipv4: 10.1.2.5
      ipv6: fd00::10:1:2:5
      isis:
        bfd: true
  prefix:
    ipv4: 10.1.2.4/31
    ipv6: fd00::10:1:2:4/127
#host1
- interfaces:
    - node: h1
      ifname: eth1
      ipv4: 172.16.1.2
      ipv6: fd00::172:16:1:2
    - node: l1
      ifname: eth1
      isis: false
      ipv4: 172.16.1.1
      ipv6: fd00::172:16:1:1
  prefix:
    ipv4: 172.16.1.0/24
    ipv6: fd00::172:16:1:0/116
#host2
- interfaces:
    - node: h2
      ifname: eth1
      ipv4: 172.16.2.2
      ipv6: fd00::172:16:2:2
    - node: l2
      ifname: eth1
      isis: false
      ipv4: 172.16.2.1
      ipv6: fd00::172:16:2:1
  prefix:
    ipv4: 172.16.2.0/24
    ipv6: fd00::172:16:1:0/116
#host3
- interfaces:
    - node: h3
      ifname: eth1
      ipv4: 172.16.3.2
      ipv6: fd00::172:16:3:2
    - node: l3
      ifname: eth1
      isis: false
      ipv4: 172.16.3.1
      ipv6: fd00::172:16:3:1
  prefix:
    ipv4: 172.16.3.0/24
    ipv6: fd00::172:16:3:0/116
#host4
- interfaces:
    - node: h4
      ifname: eth1
      ipv4: 172.16.4.2
      ipv6: fd00::172:16:4:2
    - node: l3
      ifname: eth2
      isis: false
      ipv4: 172.16.4.1
      ipv6: fd00::172:16:4:1
  prefix:
    ipv4: 172.16.4.0/24
    ipv6: fd00::172:16:4:0/116

Validating

spine-1

s1#show ip ro

VRF: default

Gateway of last resort is not set

C        10.1.1.0/31
         directly connected, Ethernet1
C        10.1.1.2/31
         directly connected, Ethernet2
C        10.1.1.4/31
         directly connected, Ethernet3
I L2     10.1.2.0/31 [115/20]
         via 10.1.1.1, Ethernet1
I L2     10.1.2.2/31 [115/20]
         via 10.1.1.3, Ethernet2
I L2     10.1.2.4/31 [115/20]
         via 10.1.1.5, Ethernet3
C        192.168.1.1/32
         directly connected, Loopback0
I L2     192.168.1.2/32 [115/30]
         via 10.1.1.1, Ethernet1
         via 10.1.1.3, Ethernet2
         via 10.1.1.5, Ethernet3
I L2     192.168.2.1/32 [115/20]
         via 10.1.1.1, Ethernet1
I L2     192.168.2.2/32 [115/20]
         via 10.1.1.3, Ethernet2
I L2     192.168.2.3/32 [115/20]
         via 10.1.1.5, Ethernet3


s1#show ipv6 ro

VRF: default

C        fd00::10:1:1:0/127 [0/0]
         via Ethernet1, directly connected
C        fd00::10:1:1:2/127 [0/0]
         via Ethernet2, directly connected
C        fd00::10:1:1:4/127 [0/0]
         via Ethernet3, directly connected
I L2     fd00::10:1:2:0/127 [115/20]
         via fe80::a8c1:abff:fef4:6f56, Ethernet1
I L2     fd00::10:1:2:2/127 [115/20]
         via fe80::a8c1:abff:fe01:dde1, Ethernet2
I L2     fd00::10:1:2:4/127 [115/20]
         via fe80::a8c1:abff:fe7b:3d0c, Ethernet3
C        fd00::192:168:1:1/128 [0/0]
         via Loopback0, directly connected
I L2     fd00::192:168:1:2/128 [115/30]
         via fe80::a8c1:abff:fef4:6f56, Ethernet1
         via fe80::a8c1:abff:fe01:dde1, Ethernet2
         via fe80::a8c1:abff:fe7b:3d0c, Ethernet3
I L2     fd00::192:168:2:1/128 [115/20]
         via fe80::a8c1:abff:fef4:6f56, Ethernet1
I L2     fd00::192:168:2:2/128 [115/20]
         via fe80::a8c1:abff:fe01:dde1, Ethernet2
I L2     fd00::192:168:2:3/128 [115/20]
         via fe80::a8c1:abff:fe7b:3d0c, Ethernet3

s1#show bfd peers
VRF name: default
------------------
DstAddr                                MyDisc         YourDisc       Interface/Transport         Type               LastUp       LastDown            LastDiag    State
------------------------------- ---------------- ---------------- ------------------------- ------------ -------------------- -------------- ------------------- -----
fe80::a8c1:abff:fe60:94a3          2730906329       2680068865           Ethernet2(2177)       normal       08/27/25 15:43             NA       No Diagnostic       Up
fe80::a8c1:abff:fe88:9959          2530925411       1836350440           Ethernet3(2183)       normal       08/27/25 15:16             NA       No Diagnostic       Up
fe80::a8c1:abff:fe92:99c2          1861245626       1908179683           Ethernet1(2173)       normal       08/27/25 15:16             NA       No Diagnostic       Up

s1#show isis neighbors

Instance  VRF      System Id        Type Interface          SNPA              State Hold time   Circuit Id
netlab    default  l1               L2   Ethernet1          P2P               UP    29          00
netlab    default  l2               L2   Ethernet2          P2P               UP    30          00
netlab    default  l3               L2   Ethernet3          P2P               UP    28          00

s1#show isis network topology

IS-IS Instance: netlab VRF: default
IS-IS IPv4 paths to level-2 routers
  System Id        Metric   IA Metric Next-Hop         Interface                SNPA
  s2               20       0         l1               Ethernet1                P2P
                                      l2               Ethernet2                P2P
                                      l3               Ethernet3                P2P
  l1               10       0         l1               Ethernet1                P2P
  l2               10       0         l2               Ethernet2                P2P
  l3               10       0         l3               Ethernet3                P2P
IS-IS IPv6 paths to level-2 routers
  System Id        Metric   IA Metric Next-Hop         Interface                SNPA
  s2               20       0         l1               Ethernet1                P2P
                                      l2               Ethernet2                P2P
                                      l3               Ethernet3                P2P
  l1               10       0         l1               Ethernet1                P2P
  l2               10       0         l2               Ethernet2                P2P
  l3               10       0         l3               Ethernet3                P2P

spine-2

s2#show ip ro

Gateway of last resort is not set

I L2     10.1.1.0/31 [115/20]
         via 10.1.2.1, Ethernet1
I L2     10.1.1.2/31 [115/20]
         via 10.1.2.3, Ethernet2
I L2     10.1.1.4/31 [115/20]
         via 10.1.2.5, Ethernet3
C        10.1.2.0/31
         directly connected, Ethernet1
C        10.1.2.2/31
         directly connected, Ethernet2
C        10.1.2.4/31
         directly connected, Ethernet3
I L2     192.168.1.1/32 [115/30]
         via 10.1.2.1, Ethernet1
         via 10.1.2.3, Ethernet2
         via 10.1.2.5, Ethernet3
C        192.168.1.2/32
         directly connected, Loopback0
I L2     192.168.2.1/32 [115/20]
         via 10.1.2.1, Ethernet1
I L2     192.168.2.2/32 [115/20]
         via 10.1.2.3, Ethernet2
I L2     192.168.2.3/32 [115/20]
         via 10.1.2.5, Ethernet3

s2#show ipv6 ro

VRF: default

I L2     fd00::10:1:1:0/127 [115/20]
         via fe80::a8c1:abff:fe8b:1dd7, Ethernet1
I L2     fd00::10:1:1:2/127 [115/20]
         via fe80::a8c1:abff:fe10:3ceb, Ethernet2
I L2     fd00::10:1:1:4/127 [115/20]
         via fe80::a8c1:abff:fee5:14ed, Ethernet3
C        fd00::10:1:2:0/127 [0/0]
         via Ethernet1, directly connected
C        fd00::10:1:2:2/127 [0/0]
         via Ethernet2, directly connected
C        fd00::10:1:2:4/127 [0/0]
         via Ethernet3, directly connected
I L2     fd00::192:168:1:1/128 [115/30]
         via fe80::a8c1:abff:fe8b:1dd7, Ethernet1
         via fe80::a8c1:abff:fe10:3ceb, Ethernet2
         via fe80::a8c1:abff:fee5:14ed, Ethernet3
C        fd00::192:168:1:2/128 [0/0]
         via Loopback0, directly connected
I L2     fd00::192:168:2:1/128 [115/20]
         via fe80::a8c1:abff:fe8b:1dd7, Ethernet1
I L2     fd00::192:168:2:2/128 [115/20]
         via fe80::a8c1:abff:fe10:3ceb, Ethernet2
I L2     fd00::192:168:2:3/128 [115/20]
         via fe80::a8c1:abff:fee5:14ed, Ethernet3

s2#show bfd peers
VRF name: default
-----------------
DstAddr                                MyDisc         YourDisc       Interface/Transport         Type               LastUp       LastDown            LastDiag    State
------------------------------- ---------------- ---------------- ------------------------- ------------ -------------------- -------------- ------------------- -----
fe80::a8c1:abff:fe10:3ceb          2399755991       2604022487           Ethernet2(2171)       normal       08/27/25 15:16             NA       No Diagnostic       Up
fe80::a8c1:abff:fe8b:1dd7           854137237       4290215108           Ethernet1(2169)       normal       08/27/25 15:16             NA       No Diagnostic       Up
fe80::a8c1:abff:fee5:14ed          1230089738         45823714           Ethernet3(2179)       normal       08/27/25 15:16             NA       No Diagnostic       Up

s2#show isis neighbors

Instance  VRF      System Id        Type Interface          SNPA              State Hold time   Circuit Id
netlab    default  l1               L2   Ethernet1          P2P               UP    28          00
netlab    default  l2               L2   Ethernet2          P2P               UP    28          00
netlab    default  l3               L2   Ethernet3          P2P               UP    29          00


s2#show isis network topology

IS-IS Instance: netlab VRF: default
IS-IS IPv4 paths to level-2 routers
  System Id        Metric   IA Metric Next-Hop         Interface                SNPA
  s1               20       0         l1               Ethernet1                P2P
                                      l2               Ethernet2                P2P
                                      l3               Ethernet3                P2P
  l1               10       0         l1               Ethernet1                P2P
  l2               10       0         l2               Ethernet2                P2P
  l3               10       0         l3               Ethernet3                P2P
IS-IS IPv6 paths to level-2 routers
  System Id        Metric   IA Metric Next-Hop         Interface                SNPA
  s1               20       0         l1               Ethernet1                P2P
                                      l2               Ethernet2                P2P
                                      l3               Ethernet3                P2P
  l1               10       0         l1               Ethernet1                P2P
  l2               10       0         l2               Ethernet2                P2P
  l3               10       0         l3               Ethernet3                P2P

Verify ping reachability to all device loopbacks from leaf-3

leaf-3 pings

l3# ping l1
PING l1 (192.168.2.1): 56 data bytes
64 bytes from 192.168.2.1: seq=0 ttl=63 time=1.610 ms
64 bytes from 192.168.2.1: seq=1 ttl=63 time=0.901 ms
64 bytes from 192.168.2.1: seq=2 ttl=63 time=0.808 ms
^C
--- l1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.808/1.106/1.610 ms
l3# ping l2
PING l2 (192.168.2.2): 56 data bytes
64 bytes from 192.168.2.2: seq=0 ttl=63 time=1.253 ms
64 bytes from 192.168.2.2: seq=1 ttl=63 time=0.875 ms
64 bytes from 192.168.2.2: seq=2 ttl=63 time=0.850 ms
^C
--- l2 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.850/0.992/1.253 ms
l3# ping s1
PING s1 (192.168.1.1): 56 data bytes
64 bytes from 192.168.1.1: seq=0 ttl=64 time=0.074 ms
64 bytes from 192.168.1.1: seq=1 ttl=64 time=0.098 ms
64 bytes from 192.168.1.1: seq=2 ttl=64 time=0.095 ms
64 bytes from 192.168.1.1: seq=3 ttl=64 time=0.092 ms
64 bytes from 192.168.1.1: seq=4 ttl=64 time=0.115 ms
^C
--- s1 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 0.074/0.094/0.115 ms
l3# ping s2
PING s2 (192.168.1.2): 56 data bytes
64 bytes from 192.168.1.2: seq=0 ttl=64 time=0.103 ms
64 bytes from 192.168.1.2: seq=1 ttl=64 time=0.094 ms
64 bytes from 192.168.1.2: seq=2 ttl=64 time=0.173 ms
64 bytes from 192.168.1.2: seq=3 ttl=64 time=0.106 ms
64 bytes from 192.168.1.2: seq=4 ttl=64 time=0.111 ms
^C
--- s2 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 0.094/0.117/0.173 ms

l3# ping ipv6 l1
PING l1 (fd00::192:168:2:1): 56 data bytes
64 bytes from fd00::192:168:2:1: seq=0 ttl=63 time=1.274 ms
64 bytes from fd00::192:168:2:1: seq=1 ttl=63 time=1.009 ms
64 bytes from fd00::192:168:2:1: seq=2 ttl=63 time=0.636 ms
^C
--- l1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.636/0.973/1.274 ms
l3# ping ipv6 l2
PING l2 (fd00::192:168:2:2): 56 data bytes
64 bytes from fd00::192:168:2:2: seq=0 ttl=63 time=1.395 ms
64 bytes from fd00::192:168:2:2: seq=1 ttl=63 time=1.001 ms
64 bytes from fd00::192:168:2:2: seq=2 ttl=63 time=1.386 ms
^C
--- l2 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 1.001/1.260/1.395 ms
l3# ping ipv6 s1
PING s1 (fd00::192:168:1:1): 56 data bytes
64 bytes from fd00::192:168:1:1: seq=0 ttl=64 time=0.140 ms
64 bytes from fd00::192:168:1:1: seq=1 ttl=64 time=0.115 ms
64 bytes from fd00::192:168:1:1: seq=2 ttl=64 time=0.096 ms
^C
--- s1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.096/0.117/0.140 ms
l3# ping ipv6 s2
PING s2 (fd00::192:168:1:2): 56 data bytes
64 bytes from fd00::192:168:1:2: seq=0 ttl=64 time=0.105 ms
64 bytes from fd00::192:168:1:2: seq=1 ttl=64 time=0.098 ms
64 bytes from fd00::192:168:1:2: seq=2 ttl=64 time=0.202 ms
64 bytes from fd00::192:168:1:2: seq=3 ttl=64 time=0.090 ms
^C
--- s2 ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 0.090/0.123/0.202 ms

Device configuration files:

Spine-1 Spine-2 Leaf-1 Leaf-2 Leaf-3