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Using Protocol Threading Through a Bridged Network

This document discusses basic information about protocol threading through bridged networks. It includes the following sections:

About Threading

Threading is the process whereby the network protocol (IPX, DNA, IP, AppleTalk, and Apollo) of the Token Ring end station discovers a route over segments of an SRB Network.

Threading is no different from the SRB operation. It is how threading is implemented by the end station that is different. The following sections describe threading for IP, DNA, IPX, AppleTalk, and Apollo.

IP Threading with ARP

IP end stations use Address Resolution Protocol (ARP) REQUEST and REPLY packets to discover a RIF. Both IP end stations and the bridges participate in the route discovery and forwarding process. The following steps describe IP threading.

: 1. An IP end station maintains an ARP table and a RIF table. It uses the MAC address in the ARP table as a cross reference for the destination RIF in the RIF table. If a RIF does not exist for that specific MAC address, the end station transmits an ARP REQUEST packet with an ARE (All Routes Explore) or a STE (Spanning Tree Explore) onto the local segment.
: 2. All bridges on the local segment capture the ARP REQUEST packet and send it over their connected networks.
: 3. As the ARP REQUEST packet continues its search for the destination end station, each bridge that forwards it adds its own bridge number and segment number to the RIF in the packet. As the frame continues to pass through the bridged network, the RIF compiles a list of bridge and segment number pairs describing the path to the destination.
: 4. When the ARP REQUEST packet finally reaches its destination, it contains the exact sequence of bridge and segment numbers from source to destination.
: 5. When the destination end station receives the frame, it puts the MAC address and its RIF into its own ARP and RIF tables. If the destination end station receives any other ARP REQUEST packets from the same source, it drops that packet.
: 6. The destination end station then generates an ARP REPLY packet including the RIF and sends it back to the source end station.
: 7. The source end station receives the learned-route path. It puts the MAC address and its RIF into the ARP and RIF tables. The RIF is then attached to the data packet and forwarded onto the destination.
: 8. Aging of RIF entries is handled by the IP ARP refresh timer.

DNA Threading

Digital Network Architecture (DNA) end stations use ARE to discover a route. Both the DNA end stations and the bridges participate in the route discovery process and forwarding. The following steps describe the DNA threading process.

: 1. If there is no entry in the RIF table for the MAC address, an entry is created with the state NO_ROUTE. When this occurs the end station sends the data packet out with an STE attached. The STE is used for discovery without attempting to flood the network with ARE.
: 2. The end station then transmits an ARE in a loop-back frame to the destination MAC address.
: 3. All bridges on the local segment capture the STE and loop-back frame and send it over their connected networks.
: 4. As the packets continue their search for the destination end station, each bridge that forwards it adds its own bridge number and segment number to the RIF in the STE and the ARE. As the frame passes through the bridged network, the RIF compiles a list of bridge and segment number pairs describing the path to the destination.
: 5. When the STE and loop-back frame finally reaches the destination, it contains the exact sequence of bridge and segment numbers from the source to the destination.
: 6. When the destination end station receives the loop-back frame it puts the MAC address and the RIF of the source station into its own RIF table. If a RIF already exists for that entry, it either updates the RIF if that previous entry is an ST_ROUTE or it ignores the RIF. In any case the entry state is changed to HAVE_ROUTE.
: 7. The destination end station then sends the loop-back reply frame including the specific RIF back to the source end station.
: 8. The source end station receives the learned specific route path. It puts the RIF into the RIF table and the entry changes to HAVE_ROUTE.
: 9. Packets destined for a functional address are sent with an STE. DNA end stations can create an RIF entry using this STE frame. When this happens the state of the entry is changed to ST_ROUTE.

The DNA end stations contain an independent RIF timer. When this timer expires for a specific RIF entry, an ARE in a loop-back packet is sent out to that specific destination. When the loop-back frame returns, the RIF entry is updated. If the destination end station is on the same ring and the loop-back frame contains no RIF, the loop-back packet is returned with no RIF entry.

Apollo Threading

Apollo end stations use STE frames to discover a route. Both the Apollo end stations and the bridges participate in the route discovery process and forwarding. The following steps describe the Apollo threading process.

: 1. If there is no entry in the RIF table for the MAC address the data packet is sent out with an STE. An entry is added to the RIF table designated as NO_ROUTE.
: 2. The end station then transmits another STE with XID for the destination MAC address.
: 3. All bridges on the local segment capture the STE and send it over their connected networks.
: 4. As the packets continue their search for the destination end station, each bridge that forwards it adds its own bridge number and segment number to the RIF in the STE. As the frame continues to pass through the bridged network, the RIF compiles a list of bridge and segment number pairs describing the path to the destination.
: 5. When the STEs finally reaches the destination, it contains the exact sequence of bridge and segment numbers from the source to the destination.
: 6. When the destination end station receives the STE with XID, it puts the MAC address and the RIF of the source station into its own RIF table. If a RIF already exists for that entry, it either updates the RIF if that previous entry is a ST_ROUTE or it ignores the RIF. In any case the entry state is changed to HAVE_ROUTE.
: 7. The destination end station then sends an XID reply frame including the specific RIF back to the source end station.
: 8. The source end station receives the learned specific route path. The RIF is entered into the RIF table and the entry is changed to HAVE_ROUTE.
: 9. Packets destined for a functional address are sent with an STE with no XID. Apollo end stations can create a RIF entry using this STE frame. When this happens the state of the entry is changed to ST_ROUTE.

The Apollo end stations contain an independent RIF timer. When this timer expires for a specific RIF entry, an STE with XID packet is sent out to that specific destination. When the XID reply frame returns the RIF entry is updated. If the destination end station is on the same ring, the loop-back packet is sent and returned with no RIF entry.

IPX Threading

IPX end stations check each packet they receive for a RIF. If the RIF does not exist in the table, they add the RIF to the table and designate that route as HAVE_ROUTE. If the RIF indicates that the packet came from an end station on the local ring, the route is designated as ON_RING.

If the end station needs to send out a packet and there is no entry in RIF table for the MAC address, the end station transmits the data as an STE.

When the RIF timer expires, the entry in the table is cleared and won't be reentered until another packet arrives containing a RIF for that entry.

AppleTalk Threading

AppleTalk end stations use ARP and XID packets to discover a route. Both the AppleTalk end stations and the bridges participate in the route discovery process and forwarding. The following steps describe the AppleTalk threading process.

: 1. If a RIF does not exist for a specific MAC address, the end station transmits an ARP REQUEST packet with an ARE (All Routes Explore) onto the local segment.
: 2. All bridges on the local segment capture the ARP REQUEST packet and send it over their connected networks.
: 3. As the ARP REQUEST packet continues its search for the destination end station, each bridge that forwards it adds its own bridge number and segment number to the RIF in the packet. As the frame continues to pass through the bridged network, the RIF compiles a list of bridge and segment number pairs describing the path to the destination.
: 4. When the destination end station receives the frame, it puts the MAC address and its RIF into its own ARP and RIF tables and the state of the entry is designated as HAVE_ROUTE. If the destination end station receives any other ARP REQUEST packets from the same source, that packet is dropped.
: 5. The destination end station then generates an ARP REPLY packet including the RIF and sends it back to the source end station with the direction bit in the RIF flipped.
: 6. The source end station receives the learned route path. The MAC address and its RIF are then entered into the ARP and RIF tables and the state is designated as HAVE_ROUTE. If the RIF indicates that the packet came from an end station on the local ring, the route is designated as ON_RING.
: 7. If the RIF timer expires, a XID is sent out with an ARE and the state is changed to DISCOVERING. If no XID reply is received, the entry is discarded.

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