IS-IS questions

IS-IS questions
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IS-IS questions pk 07-15-2007
Posted by pk on July 15, 2007, 3:11 pm
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I'm studying for the bsci, and I'm doing some IS-IS tests.
My topology is very simple: there are three routers (ra, rb, and rc),
connected via serial lines, each router has a LAN attached.

ra --conn1-- rb --conn2-- rc
| | |
LANa LANb LANc

LANa: 10.0.0.0/24
LANb: 10.0.1.0/24
LANc: 10.0.2.0/24
conn1: 172.16.0.0/24 (ra is .2, rb is .1)
conn2: 172.17.0.0/24 (rb is .1, rc is .2)

IS-IS is enabled on all routers on all interfaces, with the default
settings (ie, each router is a l1-l2). All routers are in the same area.

Now, I do a "sh isis database detail" on router ra. The l1 database
looks as expected: each router advertises one LSP, containing IP
reachability info for the connected IP networks.
I expected the l2 database to contain the same info. But instead, the l2
database shows more entries for each LSP:

ra#show isis database detail
[cut]
IS-IS Level-2 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
ra.00-00 * 0x00000004 0x4234 442 0/0/0
Area Address: 49.0001
NLPID: 0xCC
Hostname: ra
IP Address: 172.16.0.2
Metric: 10 IS rb.00
Metric: 10 IP 10.0.0.0 255.255.255.0
Metric: 20 IP 10.0.1.0 255.255.255.0
Metric: 30 IP 10.0.2.0 255.255.255.0
Metric: 10 IP 172.16.0.0 255.255.255.0
Metric: 20 IP 172.17.0.0 255.255.255.0
[cut]

why does ra advertise reachability for networks that are not connected
(eg, 10.0.2.0 or 172.17.0.0)?

And, how is the address shown in the line "IP Address:" chosen for
routers that have more than one IP address? For example, for rb LSP it's
172.17.0.1. Why is that one chosen, instead of 172.16.0.1 or 10.0.1.x?
Is this the same "highest IP address" rule used for eg ospf router id?

Thanks for any help.

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Posted by pk on July 16, 2007, 7:28 am
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pk wrote:

> And, how is the address shown in the line "IP Address:" chosen for
> routers that have more than one IP address? For example, for rb LSP it's
> 172.17.0.1. Why is that one chosen, instead of 172.16.0.1 or 10.0.1.x?
> Is this the same "highest IP address" rule used for eg ospf router id?

And a last question, why are pseudonode LSP not generated for the attached
LANs?

Thanks


Posted by Ern on July 17, 2007, 7:04 am
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pk napisa=B3(a):
> I'm studying for the bsci, and I'm doing some IS-IS tests.
> My topology is very simple: there are three routers (ra, rb, and rc),=20
> connected via serial lines, each router has a LAN attached.
>=20
> ra --conn1-- rb --conn2-- rc
> | | |
> LANa LANb LANc
>=20
> LANa: 10.0.0.0/24
> LANb: 10.0.1.0/24
> LANc: 10.0.2.0/24
> conn1: 172.16.0.0/24 (ra is .2, rb is .1)
> conn2: 172.17.0.0/24 (rb is .1, rc is .2)
>=20
> IS-IS is enabled on all routers on all interfaces, with the default=20
> settings (ie, each router is a l1-l2). All routers are in the same area=
=2E
>=20
> Now, I do a "sh isis database detail" on router ra. The l1 database=20
> looks as expected: each router advertises one LSP, containing IP=20
> reachability info for the connected IP networks.
> I expected the l2 database to contain the same info. But instead, the l=
2=20
> database shows more entries for each LSP:
>=20
> ra#show isis database detail
> [cut]
> IS-IS Level-2 Link State Database:
> LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/O=
L
> ra.00-00 * 0x00000004 0x4234 442 0/0/0
> Area Address: 49.0001
> NLPID: 0xCC
> Hostname: ra
> IP Address: 172.16.0.2
> Metric: 10 IS rb.00
> Metric: 10 IP 10.0.0.0 255.255.255.0
> Metric: 20 IP 10.0.1.0 255.255.255.0
> Metric: 30 IP 10.0.2.0 255.255.255.0
> Metric: 10 IP 172.16.0.0 255.255.255.0
> Metric: 20 IP 172.17.0.0 255.255.255.0
> [cut]
>=20

L1/2 router acts like an ABR in OSPF - it takes all internal routes (L1) =

and represents them in its own level 2 LSP (at least until you use=20
summarization)


> why does ra advertise reachability for networks that are not connected =

> (eg, 10.0.2.0 or 172.17.0.0)?

Level 1/2 represents its area to the core (level 2), not only its=20
directly connected networks, but all networks from level 1 - look above ;=
)

>=20
> And, how is the address shown in the line "IP Address:" chosen for=20
> routers that have more than one IP address? For example, for rb LSP it'=
s=20
> 172.17.0.1. Why is that one chosen, instead of 172.16.0.1 or 10.0.1.x?=
=20
> Is this the same "highest IP address" rule used for eg ospf router id?
>=20

it looks like it does similarly to OSPF (I haven't found it in=20
documentation)

> And a last question, why are pseudonode LSP not generated for the=20
> attached LANs?

Because these LANs are stub networks - there are no other IS-IS capable=20
routers/neighbors. It would be not efficient to select DIS and create=20
pseudonode on stub networks :)

> Thanks for any help.

You should always (when possible) use summarization and set appropriate=20
IS level to every router.

I hope I have explained that :)

Best regards
------------------------
Ern
XV535 90' - fajna by=B3a
YZF600R Miau :)
gg - 1078599; ern ma=B3pa ern waw pl

Posted by pk on July 17, 2007, 8:47 am
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Ern wrote:

> L1/2 router acts like an ABR in OSPF - it takes all internal routes (L1)
> and represents them in its own level 2 LSP (at least until you use
> summarization)

Yes, but OSPF ABRs advertise networks reachable inside an area *to other
areas*, not back to the same area.

However, reading rfc 1195 did prove useful here:

------
The "IP Internal Reachability Information" field may be present in all LSPs
transmitted by IP-capable routers. If present, it identifies a list of zero
or more [IP address, subnet mask, metrics] reachable by the router which
originates the LSP.
[cut]
If included in level 1 LSPs, this field includes only entries directly
reachable by the router which originates the LSP, via one of its
interfaces.
->->->-> If included in level 2 LSPs, this field includes only entries
reachable by the router which originates the LSP, either via one of its
interfaces, or indirectly via level 1 routing.
------

I know that in practice, since internal routers prefer l1 routes, and
because one should always set the correct IS level for each router, this is
not an issue, but I was curious.

>> And, how is the address shown in the line "IP Address:" chosen for
>> routers that have more than one IP address? For example, for rb LSP it's
>> 172.17.0.1. Why is that one chosen, instead of 172.16.0.1 or 10.0.1.x?
>> Is this the same "highest IP address" rule used for eg ospf router id?
>>
>
> it looks like it does similarly to OSPF (I haven't found it in
> documentation)

I did some captures with wireshark, and it looks like the address in that
line is the one the rfc calls "interface address".
What confused me is that, for hello PDUs, the interface address is always
the address of the interface out of which the hello is sent (thus different
for different neighbors), but for LSPs it is always the same regardless of
the neighbor.
rfc 1195 is a bit vague here (at least imho):

------
The "IP Interface Address" is included in all IS-IS Hello packets and
LSPs transmitted by IP-only and dual routers. In the Hello packets,
this field occurs once only, and contains the IP address(es) of the
interface on which the Hello packet is transmitted (up to a maximum
of 63 IP addresses on each interface). If an IS-IS Hello is
transmitted over an interface which does not have an IP address
assigned, then this field may be omitted, or may be included with
zero entries.

->->->-> In Link State Packets, this field contains a list of
one or more IP addresses corresponding to one or more interfaces of
the router which originates the LSP. Each IP-capable router must
include this field in its LSPs. This field may occur multiple times
in an LSP, and may occur in an LSP with any LSP number.
------

And, below:

------
Where a router is both a level 1 and level 2 router, it must include the
same IP addresses in its level 1 and level 2 LSPs.
------

It seems that, for LSP PDUs, cisco implementation puts the highest loopback
or physical IP address in this field.


>> And a last question, why are pseudonode LSP not generated for the
>> attached LANs?
>
> Because these LANs are stub networks - there are no other IS-IS capable
> routers/neighbors. It would be not efficient to select DIS and create
> pseudonode on stub networks :)

Yes, you are right, OSPF does not generate network LSAs for stub networks
either. My fault.

>> Thanks for any help.
>
> You should always (when possible) use summarization and set appropriate
> IS level to every router.

Yes, of course. But being a student (vs. one that must manage a production
network), I think that, to better understand how things work, one should
take the liberty of experimenting a bit and playing around.

> I hope I have explained that :)
>
> Best regards

Thanks for your time!


Posted by Ern on July 17, 2007, 9:09 am
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pk napisa=B3(a):
> Ern wrote:
>=20
>> L1/2 router acts like an ABR in OSPF - it takes all internal routes (L=
1)
>> and represents them in its own level 2 LSP (at least until you use
>> summarization)
>=20
> Yes, but OSPF ABRs advertise networks reachable inside an area *to othe=
r
> areas*, not back to the same area.
>=20

Yes, but you think in OSPF terms ;) That's because in OSPF area borders=20
fall on routers (ABRs) and in IS-IS on links. So level1/2 IS takes all=20
its level 1 networks and presents them to backbone (level 2) - don't=20
think it resends them to the same area! :)

As you noticed - reading RFCs sometimes helps :)

Regards
--=20
------------------------
Ern
XV535 90' - fajna by=B3a
YZF600R Miau :)
gg - 1078599; ern ma=B3pa ern waw pl

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