Tag Archives: CCIE

Demystifying the IS-IS database

I’ve gone over the OSPFv2 and OSPFv3 databases in depth before. Now is the time for IS-IS. As always, I’ll start from a basic two router set up and add devices to the topology.

Basic LSPs

In OSPF we use the term LSA, Link-State Advertisement. In IS-IS we use the term LSP – Link-State PDUs. Further expanded into Link-State Protocol Data Units. Not to be confused with Label Switched Paths.

This is the topology we’ll start with:
IS IS 1 Demystifying the IS IS database
Like OSPF, IS-IS will treat ethernet links as broadcast by default. In OSPF a DR and BDR will be elected. In IS-IS a single DIS (Designated Intermediate System) is elected with no backup DIS. This DIS election is also pre-emtptive, unlike OSPF. The DIS will originate an LSP representing the DIS. This means I should have three LSPs in the database currently:

RP/0/0/CPU0:XR1#show isis database
Tue Aug 12 17:34:21.594 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00           * 0x00000003   0x8577        736             0/0/0
XR1.01-00             0x00000002   0x1fba        931             0/0/0
XR2.00-00             0x00000005   0x856b        806             0/0/0

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

XR2 has a single LSP with XR1 has two. The XR1.01 LSP is the DIS LSP. Dig deeper into the LSPs to see their current content:

RP/0/0/CPU0:XR1#show isis database XR1.00-00 detail
Tue Wed 12 17:38:23.307 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00           * 0x00000003   0x8577        494             0/0/0
  Area Address: 49.0001
  NLPID:        0xcc
  Hostname:     XR1
  IP Address:   1.1.1.1
  Metric: 10         IS XR1.01
  Metric: 10         IP 1.1.1.1/32
  Metric: 10         IP 10.0.12.0/24

XR1 has originated an LSP stating what area it’s in and hostname. Notice the NLPID value. This means Network Layer Protocol IDentifier. The value of 0xcc translates to IPv4. Further down the LSP contains the IS of XR1 itself, plus two IP ranges. All these with metrics to those IS and IPs. I’ll get onto the ATT/P/OL bits later so ignore those for now.

It’s important to note that an LSP is made up of several TLVs. On the wire multiple TLVs can be grouped together in a single frame. If large enough, IS-IS will fragment these frames.

As XR1 is the DIS, there is a separate DIS LSP, let’s take a look at that:

RP/0/0/CPU0:XR1#show isis database XR1.01-00 detail
Tue Aug 12 17:43:00.448 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.01-00             0x00000003   0x1dbb        1161            0/0/0
  Metric: 0          IS XR1.00
  Metric: 0          IS XR2.00

The DIS LSP advertises all the IS’ that are on the segment in which the DIS sits.

If I change the segment to point-to-point, this removes the need of a DIS and as such there will be no DIS LSP.

router isis 1
!
 interface GigabitEthernet0/0/0/1
  point-to-point
RP/0/0/CPU0:XR1#show isis database
Tue Aug 12 18:46:50.566 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00           * 0x0000000b   0x7480        674             0/0/0
XR2.00-00             0x0000000d   0x5297        543             0/0/0

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

Externals

I’m going to add another loopback interface on XR1 and redistribute that loopback into IS-IS. This will make the route external

interface Loopback100
 ipv4 address 100.100.100.100 255.255.255.255
!
prefix-set LOOPBACK100
  100.100.100.100/32
end-set
!
route-policy RP-100
  if destination in LOOPBACK100 then
    done
  else
    drop
  endif
end-policy
!
router isis 1
 address-family ipv4 unicast
  redistribute connected level-2 route-policy RP-100

As I mentioned above, IS-IS has separate TLVs that make up the LSP. Therefore there is still only a single LSP from XR1:

RP/0/0/CPU0:XR2#sh isis database
Tue Aug 12 19:03:31.569 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00             0x0000000d   0x6be5        1043            0/0/0
XR2.00-00           * 0x00000010   0x9c8f        1094            0/0/0

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

The external route can be seen in the detailed output under that LSP:

RP/0/0/CPU0:XR2#sh isis database XR1.00-00 detail
Tue Aug 12 19:03:58.637 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00             0x0000000d   0x6be5        1016            0/0/0
  Area Address: 49.0001
  NLPID:        0xcc
  Hostname:     XR1
  IP Address:   1.1.1.1
  Metric: 10         IS XR2.00
  Metric: 10         IP 1.1.1.1/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 0          IP-External 100.100.100.100/32

Inter-Area

XR3 has now been added to the topology. I’ve had to move XR2 into the same area as XR3 otherwise they will not be able to form a L1 adjacency:
IS IS 2 Demystifying the IS IS database

the R2-R3 link has not been changed to point-to-point, and as such I would expect to see three LSPs in XR3s database:

RP/0/0/CPU0:XR3#show isis database
Tue Aug 12 09:44:40.660 UTC

IS-IS 1 (Level-1) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x00000008   0xd230        1107            1/0/0
XR3.00-00           * 0x00000008   0xf1be        1105            0/0/0
XR3.07-00             0x00000003   0xfcd3        1105            0/0/0

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

If you look at XR2′s L1 LSP in detail you now see the ATT bit set. Also note it’s advertising only it’s directly connected interfaces:

RP/0/0/CPU0:XR3#show isis database XR2.00-00 detail
Tue Aug 12 19:45:51.025 UTC

IS-IS 1 (Level-1) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x00000008   0xd230        1037            1/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  Hostname:     XR2
  IP Address:   2.2.2.2
  Metric: 10         IS XR3.07
  Metric: 10         IP 2.2.2.2/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 10         IP 10.0.23.0/24

XR2 has set the ATT bit which is the attached bit. An L1/L2 router will set this bit in the LSP inside the L1 area it’s connected to. This is to inform the L1 routers that it is attached to the L2 domain. No actual default route is advertised, but L1 routers can create their own defaults pointing towards the attached routers:

RP/0/0/CPU0:XR3#sh ip route 0.0.0.0
Tue Aug 12 19:47:07.839 UTC

Routing entry for 0.0.0.0/0
  Known via "isis 1", distance 115, metric 10, candidate default path, type level-1
  Installed Aug 12 19:43:09.476 for 00:03:58
  Routing Descriptor Blocks
    10.0.23.2, from 2.2.2.2, via GigabitEthernet0/0/0/0.23
      Route metric is 10
  No advertising protos.

Notice from XR1′s persepctive, that any routes coming from an L1 area is simple flooded from the L1/L2 router as normal routes:

RP/0/0/CPU0:XR1#show isis database XR2.00-00 detail
Tue Aug 12 19:50:08.676 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x0000001b   0x5b3d        778             0/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  Hostname:     XR2
  IP Address:   2.2.2.2
  Metric: 10         IS XR1.00
  Metric: 10         IP 2.2.2.2/32
  Metric: 20         IP 3.3.3.3/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 10         IP 10.0.23.0/24
  Metric: 10         IP 200.200.200.200/32

IS-IS gives you the ability to leak L2 prefixes into the L1 domain. This is handy when you have two L1/L2 border routers and want to engineer destiations to go on particular paths. From XR2 I’ll leak XR1′s loopback into the L1 domain. The database now shows:

RP/0/0/CPU0:XR3#show isis database XR2.00-00 detail
Tue Aug 12 21:53:13.981 UTC

IS-IS 1 (Level-1) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x0000002f   0x4e13        1193            1/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  Hostname:     XR2
  IP Address:   2.2.2.2
  Router Cap:   2.2.2.2, D:0, S:0
  Metric: 10         IS XR3.07
  Metric: 20         IP-Interarea 1.1.1.1/32
  Metric: 10         IP 2.2.2.2/32
  Metric: 10         IP 10.0.23.0/24

1.1.1.1/32 shows up in LSP as an IP-Interarea route. Again a TLV is used for this.

IPv6

When running both IPv4 and IPv6 at the same time, IS-IS can be run in single-topology or multi-topolgy mode. In single topology, all your IS-IS links need to have both v4 and v6 addresses as the SPF tree is run indenpently of prefix information. If the SPF tree is calculated to use a link without a v6 address, IPv6 traffic will be blackholed over that link.

For now I’ve added an IPv6 loopback and interface address. I’ve got IS-IS running in multi topology mode. I should still only see two LSPs from XR1′s perspective:

RP/0/0/CPU0:XR1#show isis database
Tue Aug 12 23:47:02.152 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00           * 0x0000001e   0x9683        1115            0/0/0
XR2.00-00             0x0000002b   0x62fa        1117            0/0/0

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

IPv6 information is carried inside another TLV. Note also that there is a new NLPID value of 0x8e in the LSP. As you would guess this value represents IPv6:

RP/0/0/CPU0:XR1#show isis database detail XR2.00-00
Tue Aug 12 23:47:50.899 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x0000002b   0x62fa        1068            0/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  NLPID:        0x8e
  MT:           Standard (IPv4 Unicast)
  MT:           IPv6 Unicast                                     0/0/0
  Hostname:     XR2
  IP Address:   2.2.2.2
  IPv6 Address: 2001:db8:2:2::2
  Metric: 10         IS XR1.00
  Metric: 10         IP 2.2.2.2/32
  Metric: 20         IP 3.3.3.3/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 10         IP 10.0.23.0/24
  Metric: 10         IP 200.200.200.200/32
  Metric: 10         MT (IPv6 Unicast) IS-Extended XR1.00
  Metric: 10         MT (IPv6 Unicast) IPv6 2001:db8:2:2::2/128
  Metric: 10         MT (IPv6 Unicast) IPv6 2001:db8:12::/64

When running multi-topology mode, you’ll see MT: plus the address families configured for multi-topology. If I change this to single topology:

RP/0/0/CPU0:XR1#show isis database XR2.00-00 detail
Tue Aug 12 23:11:20.989 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x00000023   0xd22a        1196            0/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  NLPID:        0x8e
  Hostname:     XR2
  IP Address:   2.2.2.2
  IPv6 Address: 2001:db8:2:2::2
  Metric: 10         IS XR1.00
  Metric: 10         IP 2.2.2.2/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 10         IP 10.0.23.0/24
  Metric: 10         IP 200.200.200.200/32
  Metric: 10         IPv6 2001:db8:2:2::2/128
  Metric: 10         IPv6 2001:db8:12::/64

MT no longer shows up, and all TLVs are added as-is to the LSP.

Traffic Engineering

To enable TE, wide-metrics need to be enabled. Up until this point I’ve been using narrow metrics. Once enabled You can see the TE information in the LSP by doing a verbose output:

RP/0/0/CPU0:XR1#show isis database verbose XR2.00-00
Tue Aug 12 23:42:09.932 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x00000026   0x2dd8        910             0/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  NLPID:        0x8e
  Hostname:     XR2
  IP Address:   2.2.2.2
  IPv6 Address: 2001:db8:2:2::2
  Router ID:    2.2.2.2
  Metric: 10         IS-Extended XR1.00
    Affinity: 0x00000000
    Interface IP Address: 10.0.12.2
    Neighbor IP Address: 10.0.12.1
    Physical BW: 1000000 kbits/sec
    Reservable Global pool BW: 0 kbits/sec
    Global Pool BW Unreserved:
      [0]: 0        kbits/sec          [1]: 0        kbits/sec
      [2]: 0        kbits/sec          [3]: 0        kbits/sec
      [4]: 0        kbits/sec          [5]: 0        kbits/sec
      [6]: 0        kbits/sec          [7]: 0        kbits/sec
    Admin. Weight: 167772160
    Ext Admin Group: Length: 32
      0x00000000   0x00000000
      0x00000000   0x00000000
      0x00000000   0x00000000
      0x00000000   0x00000000
  Metric: 10         IP-Extended 2.2.2.2/32
  Metric: 10         IP-Extended 10.0.12.0/24
  Metric: 10         IP-Extended 10.0.23.0/24
  Metric: 10         IP-Extended 200.200.200.200/32
  Metric: 10         IPv6 2001:db8:2:2::2/128
  Metric: 10         IPv6 2001:db8:12::/64

Notice there there is no new NLPID value for TE. TE extensions are enabled under address-family ipv4 and hence it uses the 0xcc id. If/when RSVP-TE can use IPv6 natively, I could expect to see only the IPv6 ID.

Overload

IS-IS has the ability to set the overload bit in an LSP. This could be originated by the router itself if it was overwhelmed, but it can also be hard set when doing planned works for example. If the overload bit is set, other routers will route around the router.

router isis 1
 set-overload-bit

Note that OL bit set in the LSP:

RP/0/0/CPU0:XR1#show isis database
Tue Aug 12 23:32:58.107 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR1.00-00           * 0x0000001f   0x9484        947             0/0/0
XR2.00-00             0x0000002e   0x97a4        1151            0/0/1

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

I no longer have access to R3 now as R2 is the only router connecting these two devices:

RP/0/0/CPU0:XR1#ping 3.3.3.3
Tue Aug 12 23:08:44.083 UTC
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
UUUUU
Success rate is 0 percent (0/5)

I am still able to ping XR2 itself though:

RP/0/0/CPU0:XR1#ping 2.2.2.2
Tue Aug 12 23:09:32.870 UTC
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

We’ve now seen the purpose of both the ATT and OL bits, so what is the P bit for? that bit is for the Partition Repair Bit which no vendor has implemented. i.e. it should always show 0.

Segment Routing

IS-IS is easily extended using new TLVs. If I enable segment routing under my IS-IS process, I see it added as a new TLV in the LSP:

RP/0/0/CPU0:XR1#show isis database verbose XR2.00-00
Tue Aug 12 23:50:35.855 UTC

IS-IS 1 (Level-2) Link State Database
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
XR2.00-00             0x00000036   0x252b        954             0/0/0
  Area Address: 49.0023
  NLPID:        0xcc
  NLPID:        0x8e
  MT:           Standard (IPv4 Unicast)
  MT:           IPv6 Unicast                                     0/0/0
  Hostname:     XR2
  IP Address:   2.2.2.2
  IPv6 Address: 2001:db8:2:2::2
  Router Cap:   2.2.2.2, D:0, S:0
    Segment Routing: I:1 V:0, SRGB Base: 900000 Range: 65535
  Metric: 10         IS XR1.00
  Metric: 10         IP 2.2.2.2/32
  Metric: 20         IP 3.3.3.3/32
  Metric: 10         IP 10.0.12.0/24
  Metric: 10         IP 10.0.23.0/24
  Metric: 10         IP 200.200.200.200/32
  Metric: 10         MT (IPv6 Unicast) IS-Extended XR1.00
  Metric: 10         MT (IPv6 Unicast) IPv6 2001:db8:2:2::2/128
  Metric: 10         MT (IPv6 Unicast) IPv6 2001:db8:12::/64