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Diffstat (limited to 'docs/gettingstarted/developers/fib20/multicast.rst')
| -rw-r--r-- | docs/gettingstarted/developers/fib20/multicast.rst | 40 |
1 files changed, 29 insertions, 11 deletions
diff --git a/docs/gettingstarted/developers/fib20/multicast.rst b/docs/gettingstarted/developers/fib20/multicast.rst index 15bcddfec46..7cffba81666 100644 --- a/docs/gettingstarted/developers/fib20/multicast.rst +++ b/docs/gettingstarted/developers/fib20/multicast.rst @@ -3,22 +3,40 @@ IP Multicast FIB ---------------- -Basics -^^^^^^ +The two principal differences between multicast and unicast forwarding +are: + +* there is no load-balancing among paths, there is only replication + across paths. +* multicast forwarding has an explicit reverse path forwarding (RPF) + check. It will only forward a packet if it arrives from a peer for + which it has been explicitly configured to accept. + +The other factor that influences the design of the mFIB is that the +match criteria (the prefix) is different. For multicast it is +necessary to be able to match on source and destination/group +addresses (termed an (S,G)) and only on a destination prefix (a (\*, +G/m)). This prefix is much bigger than a unicast prefix, and since +unicast scale is almost always greater than multicast scale, it is not +a good idea to have a single definition of a prefix. Therefore, +there is a fib_prefix_t (and hence a fib_entry_t) and an +mfib_prefix_t (and hence a mfib_entry_t). + +The fib_path_t and fib_path_list_t are reused. A path can represent +either a peer from which to accept packets or a peer to which to send +packets. A path-extension is added to the fib_path_t/mfib_entry_t to +describe the role the path plays. Logically the path-list is split +into two sets; an accepting set and a forwarding set. The forwarding set +contributes a replicate DPO for forwarding and the accepting set +contributes a list of interfaces (an mfib_itf_t) for the RPF check. An IP multicast FIB (mFIB) is a data-structure that holds entries that -represent a (S,G) or a (\*,G) multicast group. There is one IPv4 and +represent a (S,G) or a (\*,G/m) multicast group. There is one IPv4 and one IPv6 mFIB per IP table, i.e. each time the user calls 'ip[6] table add X' an mFIB is created. - -A path describes either where a packet is sent to or where a packet is -received from. mFIB entries maintain two sets of 'paths'; the -forwarding set and the accepting set. Each path in the forwarding set -will output a replica of a received packet. A received packet is only -accepted for forwarding if it ingresses on a path that matches in the -accepting set - this is the RPF check. - +Usage +^^^^^ To add an entry to the default mFIB for the group (1.1.1.1, 239.1.1.1) that will replicate packets to GigEthernet0/0/0 and GigEthernet0/0/1, do: |