What are the trade-offs between different P2MP inter-AS tunnel types and modes?

1 Option 1: P2MP LSPs

One option is to use P2MP label switched paths (LSPs) that are established by a protocol such as RSVP-TE or LDP. These LSPs can carry multicast traffic from a single source to multiple destinations across AS boundaries, using label stacking and label switching. The advantage of this option is that it provides end-to-end QoS, traffic engineering, and protection mechanisms for the multicast traffic. The disadvantage is that it requires coordination and cooperation between the AS, and it may consume a lot of label and routing resources.

2 Option 2: P2MP GRE

Another option is to use point-to-point (P2P) LSPs between the AS border routers, and then encapsulate the multicast traffic in P2MP generic routing encapsulation (GRE) tunnels over the P2P LSPs. This option does not require P2MP LSPs, and it can leverage existing P2P LSPs that are used for unicast traffic. The advantage of this option is that it simplifies the inter-AS signaling and routing, and it reduces the label and routing overhead. The disadvantage is that it adds an extra layer of encapsulation and processing, and it may not support QoS, traffic engineering, and protection features for the multicast traffic.

3 Option 3: P2MP BGP

A third option is to use P2MP border gateway protocol (BGP) to distribute the multicast routing information and labels across the AS. This option does not require any other protocol or tunneling mechanism, and it can use the existing BGP infrastructure and policies. The advantage of this option is that it provides a unified and scalable way of managing the inter-AS multicast traffic, and it can support QoS, traffic engineering, and protection features. The disadvantage is that it may require some extensions and modifications to the BGP protocol and configuration, and it may introduce some complexity and security issues.

4 Mode 1: Option A

Each of the above options can be implemented in two different modes, depending on how the ASBRs (autonomous system boundary routers) are connected and labeled. The first mode, called option A, is when the ASBRs are connected by a P2P LSP, and each ASBR assigns a unique label to the P2MP tunnel or group. This mode is also known as the inter-AS tunnel mode. The advantage of this mode is that it isolates the P2MP traffic from the rest of the inter-AS traffic, and it allows each AS to control its own label space and routing policies. The disadvantage is that it requires label stacking and label swapping at each ASBR, which may increase the packet size and processing overhead.

5 Mode 2: Option B

The second mode, called option B, is when the ASBRs are connected by a P2MP LSP, and the same label is used for the P2MP tunnel or group across the AS. This mode is also known as the inter-AS label mode. The advantage of this mode is that it avoids label stacking and label swapping at the ASBRs, which may reduce the packet size and processing overhead. The disadvantage is that it requires coordination and cooperation between the AS to agree on a common label space and routing policies, which may limit the flexibility and autonomy of each AS.

6 How to choose

When deciding which option and mode is best for your network, there are many factors to consider, such as the network topology, traffic patterns, performance requirements, security policies, and operational preferences. Generally speaking, if you need end-to-end QoS, traffic engineering, and protection features for your multicast traffic, option 1 or option 3 may be a better choice than option 2. On the other hand, if you want to simplify the inter-AS signaling and routing and reduce label and routing overhead, option 2 or 3 may be preferable. Additionally, if you want to leverage existing P2P LSPs or BGP infrastructure or control your own label space and routing policies, then option 2 or 3 could be your best bet. Finally, if you want to avoid label stacking and label swapping at the ASBRs and reduce packet size and processing overhead, then option B could be a better fit than option A.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?