Network Types (broadcast / point-to-point)

Network Types (broadcast / point-to-point)

When an Ethernet interface is configured for IS-IS, point-to-point may or may not be written under router isis. That one word decides, all together, whether there is a DIS election, a pseudonode, and periodic database synchronization.

The short answer: an Ethernet interface configured as point-to-point is a mode of operation that does not exist in ISO/IEC 10589:2002. The standard knows exactly two network types, and it was RFC 5309 that later set out the idea that a LAN with only two routers may be treated as point-to-point.

The Standard Knows Only Two Network Types

Clauses 3.6.8 and 3.6.9 of ISO/IEC 10589:2002 define these two.

The opening of clause 8 says the same thing outright: the only two types of circuit the Subnetwork Independent Functions recognise are broadcast and general topology.

There is no category anywhere for “point-to-point Ethernet”. By definition Ethernet is unambiguously a broadcast subnetwork, and following the standard puts it in clause 8.4 — the world of DIS election and pseudonodes.

RFC 5309: Treating a Two-Router LAN as Point-to-Point

So what is the point-to-point everyone uses in practice? It comes from RFC 5309, “Point-to-Point Operation over LAN in Link State Routing Protocols” (2008, Informational).

Its argument is simple: if a LAN segment holds only two routers, there is no reason to treat it as broadcast. It gives three reasons why a point-to-point circuit is more straightforward:

• No designated router is involved
• No pseudonode appears in the link state database
• For IS-IS specifically, there is also no periodic database synchronization

Crucially, RFC 5309 defines no new TLV and no new code point. It is not a protocol extension but a configuration and operational convention that runs the existing point-to-point procedures over LAN media.

That is also why this mode borrows 09:00:2B:00:00:05 as its destination MAC, an ISO 9542 (ES-IS) address. The standard never envisaged this mode, so no IS-IS multicast address of its own (01:80:C2:00:00:14 / :15) was ever assigned to it (the multicast addresses IS-IS uses are covered in IS-IS Packet Types and Header Format).

What Changes

The same two routers on the same cable behave differently depending on this one setting.

What this article checks is that they all arrive together, or all disappear together. For the individual mechanisms in depth: the IIH formats are covered in IS-IS Packet Types and Header Format, the two-way confirmation in Adjacency Formation and States, and the DIS and pseudonode in The DIS and the Pseudonode.

Verification on Real Equipment

The test environment

Only two routers were placed on an Ethernet segment (10.1.0.0/24) — precisely the case RFC 5309 addresses. Both are in area 49.0001 and both are level-1-2; the only thing that changes is whether point-to-point is present on the LAN side.

The test STEPs

The Default Is Broadcast (STEP 0)

  Circuit Type:             level-1-2
  Media Type:               LAN
  Level-1                   
    Adjacency Count:        1
    LAN ID:                 R2.01
    Priority (Local/DIS):   64/64
    Next LAN IIH in:        3 s
  Level-2                   
    Adjacency Count:        1
    LAN ID:                 R2.01
    Priority (Local/DIS):   64/64
    Next LAN IIH in:        3 s

Media Type: LAN. With nothing configured at all, Ethernet is treated as broadcast. LAN ID and Priority (Local/DIS) then appear per level, and a DIS (R2 here) has been elected. Next LAN IIH in is also tracked per level.

IS-IS 1 (Level-1) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL
R1.00-00            * 0x00000005   0x1506        1105 /*            0/0/0
R2.00-00              0x00000005   0x955a        1107 /1200         0/0/0
R2.01-00              0x00000002   0xac86        1107 /1200         0/0/0

 Total Level-1 LSP count: 3     Local Level-1 LSP count: 1

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL
R1.00-00            * 0x00000007   0x7603        1109 /*            0/0/0
R2.00-00              0x00000007   0xfb56        1111 /1200         0/0/0
R2.01-00              0x00000002   0xac86        1107 /1200         0/0/0

 Total Level-2 LSP count: 3     Local Level-2 LSP count: 1

There are two routers but three LSPs. The third, R2.01-00, is the pseudonode LSP the DIS creates.

Changing Only One Side (STEP 1)

Only R2 was set to point-to-point — a configuration slip that happens easily in practice.

router isis 1
 interface GigabitEthernet0/0/0/0
  point-to-point

RP/0/RP0/CPU0:R1#show isis neighbors
Wed Sep  9 08:07:39.035 UTC

IS-IS 1 neighbors:
System Id      Interface        SNPA           State Holdtime Type IETF-NSF

The adjacency is gone, and R2’s table is empty too.

An MTU mismatch is another cause of a failed IS-IS adjacency, and there the larger-MTU side stays in Init (covered in Adjacency Formation and States). With a network type mismatch neither side can see the other, and each router explains why.

  Circuit Type:             level-1-2
  Media Type:               LAN
  Circuit Number:           1
  Last IIH Received:        08:05:25 (00:02:39 ago), 1497 octets
  Last PDU Rejected:        08:07:56 (8.72 sec ago), 1497 octets, P2P IIH, P2P IIH on LAN Interface
  Last IIH Sent:            08:08:04 (0.28 sec ago), 1497 octets

  Media Type:               P2P
  Circuit Number:           0
  Extended Circuit Number:  0
  Last IIH Received:        08:05:23 (00:02:59 ago), 1497 octets
  Last PDU Rejected:        08:08:21 (2.19 sec ago), 1497 octets, L1 LAN IIH, LAN IIH on P2P Interface
  Last IIH Sent:            08:08:23 (0.01 sec ago), 1497 octets

The two reasons mirror each other.

That Last IIH Received is frozen at a time before the change on both routers confirms that neither has accepted one since.

The capture shows the two of them endlessly exchanging IIHs that never match.

$ tshark -r isis-nwtype-mismatch.pcap -n -Y "isis.type == 15 || isis.type == 16 || isis.type == 17" \
    -T fields -e eth.src -e eth.dst -e isis.type | sort | uniq -c
  26 52:54:00:1f:66:bf	01:80:c2:00:00:14	15
  26 52:54:00:1f:66:bf	01:80:c2:00:00:15	16
   5 52:54:00:4b:d0:88	01:80:c2:00:00:14	15
   5 52:54:00:4b:d0:88	01:80:c2:00:00:15	16
   23 52:54:00:4b:d0:88	09:00:2b:00:00:05	17

The five 15 and 16 frames from R2 predate the change. After that, R1 keeps sending 15 and 16 to 01:80:c2:00:00:14 / :15 while R2 sends 17 to 09:00:2b:00:00:05. Neither the PDU type nor the destination MAC lines up, so neither router’s IIH ever reaches the other.

This is where it differs from an MTU mismatch. If only the MTUs disagree, the smaller side’s IIHs still arrive, so one end lingers in Init. With a network type mismatch neither router accepts a single IIH from the other, and both go completely blind.

Setting Both Sides to Point-to-Point (STEP 2)

R1 was set to point-to-point as well.

RP/0/RP0/CPU0:R1#show isis neighbors
Wed Sep  9 08:11:03.517 UTC

IS-IS 1 neighbors:
System Id      Interface        SNPA           State Holdtime Type IETF-NSF
R2             Gi0/0/0/0        *PtoP*         Up    24       L1L2 Capable 

Total neighbor count: 1

The adjacency is back and SNPA reads *PtoP*. The interface display changes considerably too.

  Circuit Type:             level-1-2
  Media Type:               P2P
  Extended Circuit Number:  3
  Level-1                   
    Adjacency Count:        1
  Level-2                   
    Adjacency Count:        1

The LAN ID, Priority (Local/DIS) and Next LAN IIH in from STEP 0 have vanished entirely, replaced by Extended Circuit Number. With no DIS, the fields that exist for the DIS are simply not there.

IS-IS 1 (Level-1) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL
R1.00-00            * 0x00000009   0xcc4b        1100 /*            0/0/0
R2.00-00              0x00000008   0x3cd3        1099 /1199         0/0/0

 Total Level-1 LSP count: 2     Local Level-1 LSP count: 1

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime/Rcvd  ATT/P/OL
R1.00-00            * 0x0000000d   0x1e56        1100 /*            0/0/0
R2.00-00              0x0000000b   0xfa76        1101 /1200         0/0/0

 Total Level-2 LSP count: 2     Local Level-2 LSP count: 1

The pseudonode LSP R2.01-00 is gone and the count drops from three LSPs per level to two — matching the number of routers.

How much traffic is saved

Roughly 130 seconds of steady state were captured under each setting and set side by side.

On the same two routers over the same link, the protocol packet count fell to a fifth.

IIHs dropped from 121 to 30 because broadcast sends a separate IIH per level whereas point-to-point uses one IIH whose Circuit type covers both. CSNPs went to exactly zero, just as RFC 5309 says when it notes that for IS-IS there is also no periodic database synchronization.

show isis trace shows the DIS election stopping at the moment of the change.

Sep  9 08:09:22.593 isis/1/det ... isis_dis_run_election:739  DIS_ELECTION_RESULT_NO_DIS L1 Gi0/0/0/0
Sep  9 08:09:22.593 isis/1/det ... isis_dis_run_election:719  DIS_RUN_ELECTION L2 Gi0/0/0/0
Sep  9 08:09:22.593 isis/1/det ... isis_dis_run_election:739  DIS_ELECTION_RESULT_NO_DIS L2 Gi0/0/0/0

Which to Choose

If a LAN holds only two routers, point-to-point is the default choice. The DIS election, the pseudonode LSP and the periodic CSNPs do nothing useful between two routers; the measurements above show what they cost.

The absolute condition is configuring it at both ends. As STEP 1 showed, one side alone means no adjacency at all — and since the reason is only visible in Last PDU Rejected, a missed side tends to be found late.

Do not use point-to-point when three or more routers share the segment. RFC 5309 is explicitly about the two-router case.

STEP 3 returned both sides to broadcast, matching the initial state.

Verification Config and show Output

At every STEP the following three files were collected from both routers, separately per router. The verification config is the ..._run.txt file (the final state being the STEP 3 one).

In the final configuration neither router has point-to-point configured, leaving the default broadcast behaviour.

STEP 0: both sides broadcast (default)

STEP 1: only R2 set to point-to-point

STEP 2: both sides point-to-point

STEP 3: both sides back to broadcast (final state)

Download the broadcast steady-state capture, STEP 0 (isis-nwtype-broadcast.pcap)

Download the network type mismatch capture, STEP 1 (isis-nwtype-mismatch.pcap)

Download the point-to-point steady-state capture, STEP 2 (isis-nwtype-p2p.pcap)

References

Related Articles

• What Is IS-IS
• NSAP Addresses and the NET (System ID)
• Level 1 and Level 2 (the IS-IS hierarchy)
• IS-IS Packet Types and Header Format
• Adjacency Formation and States
• The DIS and the Pseudonode
• Network Types (broadcast / point-to-point)