Hello everyone,
Today, I'm going to share the discussion on IS-IS in the new HCIE R&S lab.
Topic
1. What are the possible reasons why RA cannot learn routes correctly after running IS-IS?
2. The value in the figure represents the cost value of the corresponding link: RouterA, RouterC, and RouterD support IPv4 and IPv6 dual protocol stacks, RouterB only supports IPv4 protocol and cannot forward IPv6 packets.
a) If RouterA does not support ISIS MT, what will happen?
b) If ISIS MT is enabled on RouterA, please explain how to calculate the topology after enabling MT? Please write the MT configuration command.
c) Please explain what is the difference between ST and MT?
Question1
1. There is route filtering.
2. IS-IS neighbor parameter negotiation does not match.
3. Network type does not match.
4. Interface authentication type and authentication password do not match, routing domain or area authentication does not match.
5. The neighbor establishment handshake mechanism does not match.
6. Invalid MTU.
7. Mismatched cost value types.
8. If the router maintains a Level-1 neighbor relationship, there may be an area ID mismatch
9. Multi-topology is not enabled.
Question2-a
If RouterA does not support IS-IS MT and only a single overall topology is considered when performing SPF calculation, the shortest path from RouterA to RouterC is RouterA->RouterB->RouterC, but because RouterB does not support IPv6, the IPv6 packet sent by RouterA will not be possible to reach RouterC through RouterB.
Question2-b
If IS-IS MT is enabled on RouterA, then RouterA will calculate the SPF according to different topologies. When RouterA needs to send IPv6 packets to RouterC, RouterA only considers IPv6 links to determine the forwarding path of the IPv6 packets. RouterA-> RouterD-> Routerc is selected as the shortest IPv6 path from RouterA to RoutercC, and IPv6 packets are forwarded correctly.
[Huawei-isis-1]ipv6 enable topology ipv6 # Enable IS-IS process multi-topology
[Huawei-GigabitEthernetX/Y/Z]isis ipv6 enable # Enable IPv6 IS-IS on the interface
Qusttion2-c
By default, IS-IS uses a single topology. When IS-IS is deployed in single topology, IS-IS maintains the same SPT for all protocols. Therefore, IPv4 and IPv6 topologies must be the same. Therefore, separate topologies are not applicable to a single topology. When a single topology is deployed, the IPv4 and IPv6 paths are the same during topology calculation. When IS-IS IPv6 traffic passes through a device that does not have IS-IS IPv6 enabled, the traffic is discarded. Multi-topology enables IS-IS to maintain multiple topologies in a routing domain. If a device supports MT, IS-IS can maintain separate topologies for IPv4 and IPv6. IS-IS IPv4 and IS-IS IPv6 costs cannot be configured on an interface in a single topology.
That is all I want to share with you!
