Distance-vector Multicast Routing Protocol (DVMRP)

Distance-vector Multicast Routing Protocol

DVMRP consists of two major components:– a conventional distance-vector routing protocol

(like RIP)

– a protocol for determining how to forward multicast packets, based on the routing table

Multicast Forwarding

• A DVMRP router forwards a packet if– the packet arrived from the link used to reach the

source of the packet (Reverse path forwarding - RPF)

• similar (but not quite the same) to flooding each packet once

– if downstream links have not pruned the tree

Example Topologyg g

s

g

Phase 1: Flood Using Truncated Broadcast

g g

s

g

Phase 2: Pruneg g

s

prune (s,g)

prune (s,g)

g

graft (s,g)

graft (s,g)

Phase 3: Graftg g

s

g

g

report (g)

Phase 4: Steady Stateg g

s

g

g

Multicast Routing: MOSPF

Multicast OSPF (MOSPF)

• Add-on to OSPF (Open Shortest-Path First,a link-state, intra-domain routing protocol)

• Multicast-capable routers flag link state routing advertisements

• Each router indicates groups for which there are directly-connected members

MOSPF (Cont.)

• Link-state advertisements augmented with multicast group addresses to which local members have joined

• Link-state routing algorithm augmented to compute shortest-path distribution tree from any source to any set of destinations

S1

R1

R2

X

Y

Z

Link state: Each router floods link state advertisementMulticast: add membership information to “link state” Each router computes multicast tree for each active source, builds forwarding entry with outgoing interface list.

S1

R1

R2

X

Y

Z has network map, including membership at X and YZ computes shortest path tree from S1 to X and YZ builds multicast entry with one outgoing interfaceW, Q, R, each build multicast entries

Z

W

Q

R

R1

R2

X

Y

Z

W

Q

R

S1

Link state advertisement with new topology may requirere-computation of tree and forwarding entry

R1

R2

X

Y

Z

W

Q

R

S1

T

R3

Link state advertisement (T) with new membership (R3) may require incremental computation and addition of interfaceto outgoing interface list (Z)

Impact on Route Computation

• Can’t pre-compute all source multicast trees• Compute on demand when first packet from a

source S to a group G arrives• Forward packet onto outgoing interfaces that

correspond to local portion of the tree