Configuring IP Multicast over Unidirectional Links
IP multicast requires bidirectional communication, yet some networks include broadcast satellite links, which are unidirectional. Unidirectional link routing (UDLR) provides three mechanisms for a router to emulate a bidirectional link to enable the routing of unicast and multicast packets over a physical unidirectional interface, such as a broadcast satellite link. The mechanisms are a UDLR tunnel, Internet Group Management Protocol (IGMP) UDLR, and IGMP proxy. This document describes a UDLR tunnel and IGMP UDLR. IGMP proxy is described in the " Customizing IGMP " module. The three mechanisms may be used independently or in combination.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for UDLR
Information About UDLR
UDLR Overview
Both unicast and multicast routing protocols forward data on interfaces from which they have received routing control information. This model requires a bidirectional link. However, some network links are unidirectional. For networks that are unidirectional (such as broadcast satellite links), a method of communication that allows for control information to operate in a unidirectional environment is necessary. (Note that IGMP is not a routing protocol.)
Specifically, in unicast routing, when a router receives an update message on an interface for a prefix, it forwards data for destinations that match that prefix out that same interface. This is the case in distance vector routing protocols. Similarly, in multicast routing, when a router receives a Join message for a multicast group on an interface, it forwards copies of data destined for that group out that same interface. Based on these principles, unicast and multicast routing protocols cannot be supported over UDLs without the use of UDLR. UDLR is designed to enable the operation of routing protocols over UDLs without changing the routing protocols themselves.
UDLR enables a router to emulate the behavior of a bidirectional link for IP operations over UDLs. UDLR has three complementary mechanisms for bidirectional link emulation, which are described in the following sections:
- UDLR Tunnel--A mechanism for routing unicast and multicast traffic.
- Internet Group Management Protocol (IGMP) UDLR--Mechanism for routing multicast traffic. This method scales well for many broadcast satellite links.
- IGMP Proxy--Mechanism for routing multicast traffic.
You can use each mechanism independently or in conjunction with the others. IGMP proxy is described in the " Customizing IGMP " module.
UDLR Tunnel
The UDLR tunnel mechanism enables IP and its associated unicast and multicast routing protocols to treat the unidirectional link (UDL) as being logically bidirectional. A packet that is destined on a receive-only interface is picked up by the UDLR tunnel mechanism and sent to an upstream router using a generic routing encapsulation (GRE) tunnel. The control traffic flows in the opposite direction of the user data flow. When the upstream router receives this packet, the UDLR tunnel mechanism makes it appear that the packet was received on a send-only interface on the UDL.
The purpose of the unidirectional GRE tunnel is to move control packets from a downstream node to an upstream node. The one-way tunnel is mapped to a one-way interface (that goes in the opposite direction). Mapping is performed at the link layer, so the one-way interface appears bidirectional. When the upstream node receives packets over the tunnel, it must make the upper-layer protocols act as if the packets were received on the send-capable UDL.
A UDLR tunnel supports the following functionality:
- Address Resolution Protocol (ARP) and Next Hop Resolution Protocol (NHRP) over a UDL
- Emulation of bidirectional links for all IP traffic (as opposed to only control-only broadcast/multicast traffic)
- Support for IP GRE multipoint at a receive-only tunnel
 Note |
A UDL router can have many routing peers (for example, routers interconnected via a broadcast satellite link). As with bidirectional links, the number of peer routers a router has must be kept relatively small to limit the volume of routing updates that must be processed. For multicast operation, we recommend using the IGMP UDLR mechanism when interconnecting more than 20 routers. |
IGMP UDLR
In addition to a UDLR tunnel, another mechanism that enables support of multicast routing protocols over UDLs is using IP multicast routing with IGMP, which accommodates UDLR. This mechanism scales well for many broadcast satellite links.
With IGMP UDLR, an upstream router sends periodic queries for members on the UDL. The queries include a unicast address of the router that is not the unicast address of the unidirectional interface. The downstream routers forward IGMP reports received from directly connected members (on interfaces configured to helper forward IGMP reports) to the upstream router. The upstream router adds the unidirectional interface to the (*, G) outgoing interface list, thereby enabling multicast packets to be forwarded down the UDL.
In a large enterprise network, it is not possible to be able to receive IP multicast traffic via satellite and forward the traffic throughout the network. This limitation exists because receiving hosts must be directly connected to the downstream router. However, you can use the IGMP proxy mechanism to overcome this limitation. Refer to the " Customizing IGMP " module for more information on this mechanism.
How to Route IP Multicast over Unidirectional Links
This section includes the following procedures. You can do either or both in your network.
Configuring a UDLR Tunnel
To configure a UDLR tunnel, perform the task in this section. The tunnel mode defaults to GRE. You need not assign an IP address to the tunnel (you need not use the ip address or ip unnumbered commands). You must configure the tunnel endpoint addresses.
You must configure both the upstream and downstream routers to meet the following conditions:
- On the upstream router, where the UDL can only send, you must configure the tunnel to receive. When packets are received over the tunnel, the upper-layer protocols treat the packet as though it is received over the unidirectional, send-only interface.
- On the downstream router, where the UDL can only receive, you must configure the tunnel to send. When packets are sent by upper-layer protocols over the interface, they will be redirected and sent over this GRE tunnel.
Before configuring UDLR tunnel, ensure that all routers on the UDL have the same subnet address. If all routers on the UDL cannot have the same subnet address, the upstream router must be configured with secondary addresses to match all the subnets that the downstream routers are attached to.
DETAILED STEPS
Configuring IGMP UDLR
To configure an IGMP UDL, you must configure both the upstream and downstream routers. You need not specify whether the direction is sending or receiving; IGMP learns the direction by the nature of the physical connection.
When the downstream router receives an IGMP report from a host, the router sends the report to the IGMP querier associated with the UDL interface identified in the ip igmp helper-address command.
- All routers on the UDL have the same subnet address. If all routers on the UDL cannot have the same subnet address, the upstream router must be configured with secondary addresses to match all the subnets that the downstream routers are attached to.
- Multicast receivers are directly connected to the downstream routers.
DETAILED STEPS
Configuration Examples for UDLR
UDLR Tunnel Example
The following example shows how to configure a UDLR tunnel. In the example, Router A (the upstream router) is configured with Open Shortest Path First (OSPF) and PIM. Serial interface 0 has send-only capability. Therefore, the UDLR tunnel is configured as receive only, and points to serial 0.
Router B (the downstream router) is configured with OSPF and PIM. Serial interface 1 has receive-only capability. Therefore, the UDLR tunnel is configured as send-only, and points to serial 1. The forwarding of ARP and NHRP is enabled. The figure below illustrates the example.
| Figure 1 | UDLR Tunnel Example |
Router A Configuration
ip multicast-routing ! ! Serial0/0/0 has send-only capability ! interface serial 0/0/0 encapsulation hdlc ip address 10.1.0.1 255.255.0.0 ip pim sparse-dense-mode ! ! Configure tunnel as receive-only UDLR tunnel. ! interface tunnel 0 tunnel source 10.20.0.1 tunnel destination 10.41.0.2 tunnel udlr receive-only serial 0/0/0 ! ! Configure OSPF. ! router ospf network 10.0.0.0 0.255.255.255 area 0
Router B Configuration
ip multicast-routing ! ! Serial1 has receive-only capability ! interface serial 1/0/0 encapsulation hdlc ip address 10.1.0.2 255.255.0.0 ip pim sparse-dense-mode ! ! Configure tunnel as send-only UDLR tunnel. ! interface tunnel 0 tunnel source 10.41.0.2 tunnel destination 10.20.0.1 tunnel udlr send-only serial 1/0/0 tunnel udlr address-resolution ! ! Configure OSPF. ! router ospf network 10.0.0.0 0.255.255.255 area 0
IGMP UDLR Example
The following example shows how to configure IGMP UDLR. In this example, uplink-rtr is the local upstream router and downlink-rtr is the downstream router.
Both routers are also connected to each other by a back channel connection. Both routers have two IP addresses: one on the UDL and one on the interface that leads to the back channel. The back channel is any return route and can have any number of routers.
Note |
Configuring PIM on the back channel interfaces on the uplink router and downlink router is optional. |
All routers on a UDL must have the same subnet address. If all routers on a UDL cannot have the same subnet address, the upstream router must be configured with secondary addresses to match all the subnets that the downstream routers are attached to.
| Figure 2 | IGMP Unidirectional Link Routing Example |
Uplink Router (uplink-rtr) Configuration
ip multicast-routing ! ! Interface that source is attached to ! interface gigabitethernet 0/0/0 description Typical IP multicast enabled interface ip address 12.0.0.1 255.0.0.0 ip pim sparse-dense-mode ! ! Back channel ! interface gigabitethernet 1/0/0 description Back channel which has connectivity to downlink-rtr ip address 11.0.0.1 255.0.0.0 ip pim sparse-dense-mode ! ! Unidirectional link ! interface serial 0/0/0 description Unidirectional to downlink-rtr ip address 10.0.0.1 255.0.0.0 ip pim sparse-dense-mode ip igmp unidirectional-link no keepalive
Downlink Router (downlink-rtr) Configuration
ip multicast-routing ! ! Interface that receiver is attached to, configure for IGMP reports to be ! helpered for the unidirectional interface. ! interface gigabitethernet 0/0/0 description Typical IP multicast-enabled interface ip address 14.0.0.2 255.0.0.0 ip pim sparse-dense-mode ip igmp helper-address udl serial 0/0/0 ! ! Back channel ! interface gigabitethernet 1/0/0 description Back channel that has connectivity to downlink-rtr ip address 13.0.0.2 255.0.0.0 ip pim sparse-dense-mode ! ! Unidirectional link ! interface serial 0/0/0 description Unidirectional to uplink-rtr ip address 10.0.0.2 255.0.0.0 ip pim sparse-dense-mode ip igmp unidirectional-link no keepalive
Integrated UDLR Tunnel IGMP UDLR and IGMP Proxy Example
The following example shows how to configure UDLR tunnels, IGMP UDLR, and IGMP proxy on both the upstream and downstream routers sharing a UDL.
Upstream Configuration
ip multicast-routing ! interface Tunnel0 ip address 9.1.89.97 255.255.255.252 no ip directed-broadcast tunnel source 9.1.89.97 tunnel mode gre multipoint tunnel key 5 tunnel udlr receive-only GigabitEthernet2/3/0 ! interface GigabitEthernet2/0/0 no ip address shutdown ! ! user network interface GigabitEthernet2/1/0 ip address 9.1.89.1 255.255.255.240 no ip directed-broadcast ip pim dense-mode ip cgmp fair-queue 64 256 128 no cdp enable ip rsvp bandwidth 1000 100 ! interface GigabitEthernet2/2/0 ip address 9.1.95.1 255.255.255.240 no ip directed-broadcast ! ! physical send-only interface interface GigabitEthernet2/3/0 ip address 9.1.92.100 255.255.255.240 no ip directed-broadcast ip pim dense-mode ip nhrp network-id 5 ip nhrp server-only ip igmp unidirectional-link fair-queue 64 256 31 ip rsvp bandwidth 1000 100 ! router ospf 1 network 9.1.92.96 0.0.0.15 area 1 ! ip classless ip route 9.1.90.0 255.255.255.0 9.1.92.99
Downstream Configuration
ip multicast-routing ! interface Loopback0 ip address 9.1.90.161 255.255.255.252 ip pim sparse-mode ip igmp helper-address udl GigabitEthernet2/3/0 ip igmp proxy-service ! interface Tunnel0 ip address 9.1.90.97 255.255.255.252 ip access-group 120 out no ip directed-broadcast no ip mroute-cache tunnel source 9.1.90.97 tunnel destination 9.1.89.97 tunnel key 5 tunnel udlr send-only GigabitEthernet2/3/0 tunnel udlr address-resolution ! interface GigabitEthernet2/0/0 no ip address no ip directed-broadcast shutdown no cdp enable ! ! user network interface GigabitEthernet2/1/0 ip address 9.1.90.1 255.255.255.240 no ip directed-broadcast ip pim sparse-mode ip igmp mroute-proxy Loopback0 no cdp enable ! ! Backchannel interface GigabitEthernet2/2/0 ip address 9.1.95.3 255.255.255.240 no ip directed-broadcast no cdp enable ! ! physical receive-only interface interface GigabitEthernet2/3/0 ip address 9.1.92.99 255.255.255.240 no ip directed-broadcast ip pim sparse-mode ip igmp unidirectional-link no keepalive no cdp enable ! router ospf 1 network 9.1.90.0 0.0.0.255 area 1 network 9.1.92.96 0.0.0.15 area 1 ! ip classless ip route 0.0.0.0 0.0.0.0 9.1.95.1 ! set rpf to be the physical receive-only interface ip mroute 0.0.0.0 0.0.0.0 9.1.92.96 ip pim rp-address 9.1.90.1 ! ! permit ospf, ping and rsvp, deny others access-list 120 permit icmp any any access-list 120 permit 46 any any access-list 120 permit ospf any any
Additional References
Related Documents
| Related Topic |
Document Title |
|---|---|
| IP multicast commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples |
Cisco IOS IP Multicast Command Reference |
| Tunnel interfaces |
" Implementing Tunnels " module |
| IGMP and IGMP Proxy |
" Customizing IGMP " module |
MIBs
| MIB |
MIBs Link |
|---|---|
| None |
To locate and download MIBs for selected platforms, Cisco IOS XE releases, and feature sets, use Cisco MIB Locator found at the following URL: |
Technical Assistance
| Description |
Link |
|---|---|
| The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password. |
Feature Information for Configuring IP Multicast over Unidirectional Links
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
| Table 1 | Feature Information for Configuring IP Multicast over Unidirectional Links |
| Feature Name |
Releases |
Feature Configuration Information |
|---|---|---|
| UDLR Tunnel ARP and IGMP Proxy |
12.2(8)T |
This feature enables arp over a unidirectional link and overcomes the existing limitation of requiring downstream multicast receivers to be directly connected to the unidirectional link downstream router. |
| Uni-Directional Link Routing (UDLR) |
12.2(2)T 12.2(17d)SXB1 |
Unidirectional link routing is used to allow routing protocols to function in environments where routers are connected through unidirectional links. Unidirectional link routing enables layer 3 connectivity by tunneling routing information to the router on the upstream side of a unidirectional link. |
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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