Solved: Re: ipv6 address and link-local address - Page 2

sarahr202 · ‎08-25-2010

Hi every body.

I have few questions.

1) one of the feature of ipv6 is auto-configuration. With auto-configuration, a ipv6-device can configure itself with IP address without relying on dhcp server.But what about other parameters for e.g dns? So with ipv6 the need for dhcp is not completely removed am i correct?

2) Let say a ipv6-host boots up and configures itself with link-local address,FE80:: mac address.

Next host receives a prefix 2001:: and host configures another ip address based on prefix received from router. Will router apply both addresses i.e link -local and address based on received prefix from router to a interface? If yes , which one will be used for communication?

Thanks

sarahr202 · ‎08-26-2010

Thanks Peter

Could you please give me an example how are these local-link addresses belonging to different devices on the same physical link,are shown in routing table of ipv6 router?

thanks and have a great day.

Peter Paluch · ‎08-26-2010

Hello Sarah,

This is an example of a backbone created by having three routers running IPv6 and RIPng connected to a common switch. The common backbone is 2001:1:1:123::/64. Each of these three routers also has a loopback with the IPv6 address 2001:1:1:fffX::/64 where X is the number of the router (1, 2 or 3).

The routing table on R1 shows:

R1#show ipv6 route
IPv6 Routing Table - 8 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
       U - Per-user Static route
       I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
       O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
       ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
C   2001:1:1:123::/64 [0/0]
     via ::, FastEthernet0/0
L   2001:1:1:123::1/128 [0/0]
     via ::, FastEthernet0/0
C   2001:1:1:FFF1::/64 [0/0]
     via ::, Loopback0
L   2001:1:1:FFF1::1/128 [0/0]
     via ::, Loopback0
R   2001:1:1:FFF2::/64 [120/2]
     via FE80::C201:65FF:FE02:0, FastEthernet0/0
R   2001:1:1:FFF3::/64 [120/2]
     via FE80::C202:65FF:FE02:0, FastEthernet0/0
L   FE80::/10 [0/0]
     via ::, Null0
L   FF00::/8 [0/0]
     via ::, Null0

R1#

Note that the link-local addresses are actually displayed only with the RIP-discovered routes and outgoing interface, as the RIP advertisement from a particular RIP neighbor was received on that particular interface, in this case, Fa0/0.

If I were to set up a static route using a link-local next hop address, it would look as follows:

R1(config)#ipv6 route 2001:1:1:ffff::/64 FE80::C201:65FF:FE02:0 
% Interface has to be specified for a link-local nexthop
R1(config)#ipv6 route 2001:1:1:ffff::/64 Fa0/0 FE80::C201:65FF:FE02:0 
R1(config)#

I need to specify the outgoing interface whenever I want to work with the link-local address.

Does this help? Please ask further!

Best regards,

Peter

sarahr202 · ‎08-27-2010

Hi Peter.

I really appreciate your help.

I am still digesting the your last post.

I have few questions.

A ipv6 node( router or host), generate link-local addresses. These addresses will use prefix FE8X,FE9A,FEBX where X could any combination of 4 bits. But I did not see any link-local address starting with those above mentioned prefix.

Lets focus on router# 1

router1 should have generated link-local address in its f0/0 but i did not see any in routing table

Router 1 has fastethernet 0/0 on 2001:1:1:123::/64

fastethernet0/0 has ipv6 address 2001:1:1:123 ::1

the routing table shows first the network on f0/0 by:

c 2001:1:1:123:: /64 via :: f0/0

then it shows the ip address configured on f0/0 (host route) as:

L 2001:1:1:123::1/128 via :: f0/0

is my observation correct?

=============================

Towards the end of routing table,

i found following entry.

L FE80::/10 (0/0) via :: null 0

L FF00 ::/8 (0/0) via :: null0

How did we get these entries?

In nut shell, where are the local-link addresses starting with FE8X,FE9X,FEAX,FEBX in routing table? others routers router2 and router 3 must have generated respective link-local addresses on their interfaces connected to switch. But Rip did not advertise those link-local addresses to router 1.

Router 1 learned two routes from rip advertisement received from router 2 and router 3.

only route learned from router 2 is shown below.

R 2001:1:FFF2::/64 (120/2) via FE80:: C201:65FE:FE02:0 f0/0

here we know router#2 has local-link address FE80::C201:65FE:FE02:0

But router 2 did not advertise its link-local address to router 1.

Thanks and have nice weekend.

Peter Paluch · ‎08-29-2010

Hello Sarah,

Sorry for replying lately. Let's go over your questions.

A ipv6 node( router or host),  generate link-local addresses.  These
addresses will use prefix FE8X,FE9A,FEBX    where  X  could any
combination of 4 bits.  But  I did not see any link-local address
starting with those above mentioned prefix.

Yes, they are not directly indicated in the routing table, nor should they be. Let's have the routing table displayed again here as I have configured the network anew and the MAC addresses are not entirely the same as the last time:

R1#show ipv6 route
IPv6 Routing Table - 8 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
       U - Per-user Static route
       I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
       O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
       ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
C   2001:1:1:123::/64 [0/0]
     via ::, FastEthernet0/0
L   2001:1:1:123::1/128 [0/0]
     via ::, FastEthernet0/0
C   2001:1:1:FFF1::/64 [0/0]
     via ::, Loopback0
L   2001:1:1:FFF1::1/128 [0/0]
     via ::, Loopback0
R   2001:1:1:FFF2::/64 [120/2]
     via FE80::C201:77FF:FE0B:0, FastEthernet0/0
R   2001:1:1:FFF3::/64 [120/2]
     via FE80::C202:77FF:FE0B:0, FastEthernet0/0
L   FE80::/10 [0/0]
     via ::, Null0
L   FF00::/8 [0/0]
     via ::, Null0
R1#

However, you can see link local addresses when you display the individual interface's IPv6 properties, for example, on R2:

R2#show ipv6 interface Fa0/0
FastEthernet0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::C201:77FF:FE0B:0
Global unicast address(es):
    2001:1:1:123::2, subnet is 2001:1:1:123::/64
Joined group address(es):
    FF02::1
    FF02::2
    FF02::9
    FF02::1:FF00:2
    FF02::1:FF0B:0
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds
ND advertised reachable time is 0 milliseconds
ND advertised retransmit interval is 0 milliseconds
ND router advertisements are sent every 200 seconds
ND router advertisements live for 1800 seconds
Hosts use stateless autoconfig for addresses.
R2#

Notice that the link-local address of R2's Fa0/0 is the next-hop address as visible on R1 for the network 2001:1:1:FFF2::/64.

Towards the end of routing table,
i found following entry.
L FE80::/10 (0/0) via :: null 0
L FF00 ::/8 (0/0) via :: null0
How did we get these   entries?

It's the router's way of saying "all received packet destined to a link-local address will not be routed further". It is placed there by IOS automatically. Either the packet is destined to the router itself, or it will be dropped and not forwarded. It is also the reason why there are no individual link-local addresses recorded in the routing table: they would be considered routable. This Null0 entry makes sure that rerouting attempts of packets destined to link-local addresses will fail.

others  routers  router2 and router 3  must  have generated respective 
link-local addresses  on their interfaces connected to switch.  But 
Rip  did not  advertise those link-local   addresses to router 1.

Correct - and we also don't want the RIPng to advertise them at all, otherwise they would spread through the entire RIPng-enable network.