A number of configuration issues must be addressed before implementing bridging on the IBM 2212 to interoperate with bridging on the IBM 6611.
This appendix provides an overview of these issues, and indicates which features of IBM 2212's bridge implementation are not interoperable with the IBM 6611's bridge implementation.
To help avoid building an incompatible network, the following bridge configuration issues should be considered when using the IBM 6611 and the IBM 2212 as the two end-bridges over PPP and frame-relay serial links.
For PPP, the IBM 2212 bridge supports different MAC types (Ethernet and token-ring) as described in RFC 1638, PPP Bridging Control Protocol For frame-relay, the IBM 2212 supports RFC 1490/2427 Multiprotocol Interconnect over Frame Relay.
Currently the IBM 6611 bridge supports Ethernet and token-ring MAC types over PPP and Frame Relay. However, the IBM 6611 bridge only supports token-ring MAC frames when the bridge port associated with PPP or frame-relay is configured as a source-routing port. This leads to certain restrictions in network topologies when the IBM 6611 and IBM 2212 are the two end-bridges over PPP or frame-relay.
RFC 1638, section 5.3, describes how a vendor can announce to the peer bridge the MAC type that is supported over PPP so that the peer does not send unsupported MAC type traffic over PPP. Currently, the IBM 2212 bridge does not drop non-Ethernet frames destined for the PPP network. Neither does it attempt to convert all the frames to Ethernet frames before sending them over PPP. This results in the IBM 6611 bridge receiving non-Ethernet frames over PPP and discarding them when there is a mismatch in the configuration.
You should keep the following in mind when configuring a 2212 and a 6611 in a bridged network:
For example, an Ethernet frame can contain 1500 bytes of data. When bridged over a WAN link, an additional 14 bytes of Ethernet MAC header are included in the bridged traffic bringing the packet size to 1514. This means that the negotiated PPP MRU must be at least 1514 to bridge the frame.
You should consider an MRU size that will be more than large enough to hold any bridged frame. Try using 2000 or 2048 as a starting value for the MRU.
the following examples of network topologies will not work. Possible alternate configurations are marked Alt. When considering WAN, LAN types can be extended to MAC types.
Example 1: Token-Ring (SR) - IBM 2212 (SR-TB) - PPP (TB) - IBM 6611 (TB) - Ethernet
Alt: Token Ring (SR) - IBM 2212 (SRB) - PPP (SR) - IBM 6611 (SR-TB) - Ethernet
Example 2: Token Ring (TB) - IBM 2212 (TB) - PPP (TB) - IBM 6611 (TB) - ETH/TKR
Alt: Token Ring (SR) - IBM 2212 (SRB) - PPP (SR) - IBM 6611 (SR-TB) - ETH
Alt: Token Ring (SR) - IBM 2212 (SRB) - PPP (SR) - IBM 6611 (SRB) - TKR
Alt: Token Ring (TB) - IBM 2212 (SR-TB) - PPP (SR) - IBM 6611 (SRB) - TKR
Alt: Token Ring (TB) - IBM 2212 (SR-TB) - PPP (SR) - 6611 (SR-TB) - ETH
The LAN frames generated by Boundary Access Node (BAN) and DLSw are source-routed token-ring frames. Based on the media type and the bridge configuration behavior of the associated outgoing bridge port, the IBM 2212 bridge translates or converts the source-routed token-ring frame in the following manner.