Running IS-IS over Unnumbered Ethernet Interfaces

Last time we figured out that we cannot run OSPF over unnumbered interfaces that are not point-to-point links because OSPF makes assumptions about interface IP addresses. IS-IS makes no such assumptions; IPv4 and IPv6 prefixes are just a bunch of TLVs exchanged between routers over a dedicated layer-3 protocol with ridiculously long network addresses.

Could we thus build a totally unnumbered IP network with IS-IS even when the network contains multi-access segments? It depends:

  • It works like a charm on Arista EOS and Cisco IOS XE.
  • Cisco NXOS has a few quirks.
  • Junos vSRX works well with unnumbered IPv4 P2P links. IPv6 works like a charm (no surprise there due to link-local addresses).

We’ll use the following lab topology to run our tests. All devices run the same network operating system. All physical interfaces are unnumbered – the only IP addresses in the lab are assigned to loopback interfaces. P2P links have cost 10, the LAN link has cost 5.

Lab topology

Lab topology

Once I fixed the IS-IS network type (point-to-point network type should be used on P2P links but not on multi-access links), the adjacencies came up almost immediately (Gi3 is the multi-access interface):

r1#show isis neighbors

Tag Gandalf:
System Id       Type Interface     IP Address      State Holdtime Circuit Id
r2              L2   Gi2         UP    23       02
r2              L2   Gi3         UP    21       r3.01
r3              L2   Gi3         UP    8        r3.01

The IS-IS database has a pseudonode LSP (r3.01-00) confirming that one link in the topology works as a multi-access link:

r1#show isis database

Tag Gandalf:
IS-IS Level-2 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd      ATT/P/OL
r1.00-00            * 0x00000006   0x1A38                1009/*         0/0/0
r2.00-00              0x00000006   0x8499                1006/1200      0/0/0
r3.00-00              0x00000006   0x4DF8                1006/1197      0/0/0
r3.01-00              0x00000002   0xDCDC                1007/1199      0/0/0

All three routers are connected to the pseudonode LSP:

r1#show isis database r3.01-00 detail

Tag Gandalf:

IS-IS Level-2 LSP r3.01-00
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd      ATT/P/OL
r3.01-00              0x00000002   0xDCDC                 987/1199      0/0/0
  Metric: 0          IS-Extended r3.00
  Metric: 0          IS-Extended r1.00
  Metric: 0          IS-Extended r2.00

Mission accomplished. We got our network up and running over unnumbered interfaces.

Not so fast. Does IP routing work? Let’s inspect the IP routing table:

r1#show ip route | begin Gateway
Gateway of last resort is to network

S* [254/0] via is subnetted, 3 subnets
C is directly connected, Loopback0
i L2 [115/15] via, 00:17:05, GigabitEthernet3
i L2 [115/15] via, 00:16:39, GigabitEthernet3

Looks good. Loopback interfaces of other routers are reachable over GigabitEthernet3… but the next hop is the loopback IP address itself, so we need some ARP glue (more details). Do we have it?

r1#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet                -   5254.00ff.e287  ARPA   GigabitEthernet2
Internet                -   5254.00d3.58b7  ARPA   GigabitEthernet3
Internet               20   5254.00b1.edb0  ARPA   GigabitEthernet2
Internet               18   5254.00d9.55ec  ARPA   GigabitEthernet3
Internet               18   5254.007d.f1e5  ARPA   GigabitEthernet3

All the expected ARP glue entries are there. The final test: pinging remote loopback.

r1#ping r3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

OK, now we know it works 👍

Long story short: use IS-IS 😁

Addendum: NXOS Quirks

Cisco NXOS is different. Not only does it need an incredible amount of time to boot, it doesn’t like multi-access unnumbered IPv4 links… but you can still persuade it to use them (sort of).

Before someone starts yelling at me that I’m writing about irrelevant details: I know that. Most data center links are point-to-point links. It’s still interesting to see how different implementations behave though.

Here’s the full story:

  • To use ip unnumbered on NXOS you have to configure medium p2p
  • medium p2p seems to trigger point-to-point IS-IS links. With three NXOS switches connected to the same segment (R1, R2 and R3 are connected to Ethernet1/2) you get all the expected IS-IS adjacencies:
r1# show isis adjacency
IS-IS process: Gandalf VRF: default
IS-IS adjacency database:
Legend: '!': No AF level connectivity in given topology
System ID       SNPA            Level  State  Hold Time  Interface
r2              N/A             2      UP     00:00:31   Ethernet1/1
r2              N/A             2      UP     00:00:23   Ethernet1/2
r3              N/A             2      UP     00:00:23   Ethernet1/2

However, the IS-IS topology database does not contain the pseudonode LSP:

r1# show isis database
IS-IS Process: Gandalf LSP database VRF: default
IS-IS Level-1 Link State Database
  LSPID                 Seq Number   Checksum  Lifetime   A/P/O/T

IS-IS Level-2 Link State Database
  LSPID                 Seq Number   Checksum  Lifetime   A/P/O/T
  r1.00-00            * 0x00000009   0x1E07    967        0/0/0/3
  r2.00-00              0x00000009   0x72FD    1181       0/0/0/3
  r3.00-00              0x00000007   0xC0F9    1153       0/0/0/3

Also, the LSP generated by R1 contains P2P links to R2 and R3 instead of a link to the pseudonode LSP:

r1# show isis database detail r1.00-00
IS-IS Process: Gandalf LSP database VRF: default
IS-IS Level-1 Link State Database
  LSPID                 Seq Number   Checksum  Lifetime   A/P/O/T

IS-IS Level-2 Link State Database
  LSPID                 Seq Number   Checksum  Lifetime   A/P/O/T
  r1.00-00            * 0x00000009   0x1E07    921        0/0/0/3
    Instance      :  0x00000009
    Area Address  :  49.0001
    NLPID         :  0xCC 0x8E
    Router ID     :
    IP Address    :
    MT TopoId     : TopoId:2 Att: 0 Ol: 0
                    TopoId:0 Att: 0 Ol: 0
    Hostname      :  r1                 Length : 2
    TopoId: 2
    MtExtend IS   :  r3.00              Metric : 5
                     r2.00              Metric : 5
                     r2.00              Metric : 10
    Extended IS   :  r2.00              Metric : 5
    Extended IS   :  r3.00              Metric : 5
    Extended IS   :  r2.00              Metric : 10
    Extended IP   :  Metric : 1           (U)
    MT-IPv6 Prefx :  TopoId : 2
                    2001:db8:1:1::/64  Metric : 1           (U/I)
    Digest Offset :  0

While a network of Nexus switches works as expected, I wouldn’t expect a multi-vendor segment to work as everyone else insists on a different view of the world.

Latest blog posts in Unnumbered IPv4 Interfaces series


  1. this sure makes a ring topology deployment easy

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