Subject: [Isis-wg] Ipv4/Ipv6 routers in 1 area

From: Koen Vermeulen <[email-masked]>
Date: Mon, 18 Mar 2002 12:02:36 +0100

Hello,

I have a question related to the support of IPv4/IPv6

mixed routing domains.

Every router will indicate inside its LSP what IP

version it is capable off using the 'protocols supported'

TLV: an IPv4-only router will indicate it supports

IPv4, and IPv6-only router will indicate it supports

IPv6 and an IPv4/IPv6 router indicates it supports both.

This 'protocols supported' TLV can be used inside the

SPF for a certain IP version (I assume that an IPv4/IPv6

router will need to run 2 SPFs, one for IPv4 and one for

IPv6) to determine whether ot not to take the related LSP

into account.

This mechanism will work fine as long as an IPv4/IPv6

capable router can forward both IPv4 and IPv6 traffic

on all its interfaces. Because of the 'node level' nature

of the 'protocols supported' TLV, it is not possible

to perform a correct calculation with IPv4/IPv6 routers

which have a mixture of IPv4-only and IPv6-only interfaces.

This can be shown easily with an example:

+----+

| R1 | IPv4/IPv6

+----+

|

| IPv6 capable link (1)

|

|

+----+

| R2 | IPv4/IPv6

+----+

|

| IPv4-only capable link (2)

|

|

+----+

| R3 | IPv4/IPv6

+----+

|

| IPv6 capable link (3)

|

|

+----+

| R4 | IPv4/IPv6

+----+

|

x

R1, R2, R3 and R4 are IPv4/IPv6 capable routers. Links (1) and

(3) can forward IPv6 packets, link (2) can only forward IPv4

traffic. x is an IPv6 network address.

Suppose R1 is calculating a route to IPv6 network x. Because

of the fact that R2 and R3 are both generating an LSP which

indicates they are IPv6 capable, the resulting route to x will

be via R2 and R3. Although, the link (2) between R2 and R3 is

not capable of forwarding IPv6 traffic.

I went back to RFC 1195 1.4 'Support of Mixed Routing Domains',

because here similar problems occur. From what I understand

from this text, is that a dual router here can both forward

CLNS and IP on all its interfaces, although it is not specifically

stated.

In case we take this rule also for IPv4/IPv6 capable routers

(all interfaces are IPv4/IPv6 capable), there is of course no

problem. On the other hand, during the transition of IPv4

to IPv6 I can image a lot of routers which will have a

mixture of IPv4-only and IPv6-only interfaces.

As a solution for this problem we were considering of announcing

in every IS-reachablity TLV via a sub-TLV the capability of

that specific interface (again via 'protocols supported')?

This can be picked up in the SPF to decide whether or not

take that link into account for a certain IP version.

All suggestions/remarks/corrections are welcome.

Thanks,

Koen

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---

From: "Philip Christian"<[email-masked]>
Date: Mon, 18 Mar 2002 13:41:10 -0000

According to the topological restrictions of RFC 1195, if IPv4 is to be
routed in an area, then all of the ISs in the area must be capable of
forwarding IPv4.
By the same logic, if IPv6 is to be routed in an area, then all of the ISs
in the area must be capable of forwarding IPv6.
 
Therefore in your example, all of the nodes would have to be IPv4 and IPv6
capable, as both network-layer protocols appear in the area.
 
This topological restriction applies due to the following clause in section
3.10 of RFC 1195
 
"The Dijkstra computation does not take into consideration whether a router
is IP-only, OSI-only, or dual. The topological restrictions specified in
section 1.4 ensure that IP packets will only be sent via IP-capable
routers, and OSI packets will only be sent via OSI-capable routers."
 
In other words, seperate SPF calculations for IPv4 and IPv6 are not
necessary, and in fact contrary to the above clause.
 
Not everyone may agree with this, but I believe that that is what it says.
 
What many vendors do do, is to gate adjacency formation on whether a
neighbour supports at least one network-layer protocol in common with you.
This prevents a useless adjacency from forming (between an IPv4-only and an
IPv6 only router).
This behaviour would appear to not be contrary to RFC 1195, and is fairly
normal practise.
 
Philip

-----Original Message-----
From: Koen Vermeulen [mailto:[email-masked]]
Sent: 18 March 2002 11:03
To: Isis-wg
Subject: [Isis-wg] Ipv4/Ipv6 routers in 1 area

Hello, 

                +----+ 
                | R1 | IPv4/IPv6 
                +----+ 
                   | 
                   | IPv6 capable link (1) 
                   | 
                   | 
                +----+ 
                | R2 | IPv4/IPv6 
                +----+ 
                   | 

Thanks, 
Koen _______________________________________________ Isis-wg mailing list -
[email-masked]
http://spider.juniper.net/mailman/listinfo/isis-wg

---

From: Jeff Learman <[email-masked]>
Date: Mon, 18 Mar 2002 09:42:12 -0500

Philip is correct; however, note that routers that run separate SPF
calculations for each protocol type will behave identically to those
that do not in any network that meets the topology restrictions.
Their behavior differs only in networks that violate the restrictions.

Failing to form adjacencies based on protocol is not mentioned anywhere in
1195, and again is outside the scope based on the topology restrictions.
However, on a LAN where some routers form adjacencies per 10589
and others reject adjacencies based on protocols supported, the DR
election process is not guaranteed to succeed, so DR priority should
be carefully managed on such LANs.

Regards,
Jeff

At 08:41 AM 3/18/2002, Philip Christian wrote:
>According to the topological restrictions of RFC 1195, if IPv4 is to be routed in an area, then all of the ISs in the area must be capable of forwarding IPv4.
>By the same logic, if IPv6 is to be routed in an area, then all of the ISs in the area must be capable of forwarding IPv6.
> 
>Therefore in your example, all of the nodes would have to be IPv4 and IPv6 capable, as both network-layer protocols appear in the area.
> 
>This topological restriction applies due to the following clause in section 3.10 of RFC 1195
> 
>In other words, seperate SPF calculations for IPv4 and IPv6 are not necessary, and in fact contrary to the above clause.
> 
>What many vendors do do, is to gate adjacency formation on whether a neighbour supports at least one network-layer protocol in common with you.
>This prevents a useless adjacency from forming (between an IPv4-only and an IPv6 only router).
>This behaviour would appear to not be contrary to RFC 1195, and is fairly normal practise.
> 
>Philip
>>Koen _______________________________________________ Isis-wg mailing list - [email-masked] http://spider.juniper.net/mailman/listinfo/isis-wg