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Understanding Advanced Interworking and Local SwitchingThis final section covers two topics:
connect CommandAt this point, you know how to use the connect command in multiple contexts. You used it to create AToM and L2TPv3 pseudowire endpoints in Frame Relay DLCI attachment circuits to perform local switching and Frame Relay local switching. This section compares the different modes of this command using examples. You have used the connect command in three different contexts and created different configuration modes. Example 14-45 shows the connect command that performs Frame Relay pseudowire switching. Example 14-45. connect Command and Frame Relay Pseudowire Switching
SanFran(config)#connect fr-vlan Serial5/0 100 l2transport
SanFran(config-fr-pw-switching)#
When you use the connect command with the l2transport keyword, you are taken into config-fr-pw-switching configuration mode. Example 14-46 shows the connect command that performs local Frame Relay switching. Example 14-46. connect Command and Frame Relay Switching
SanFran(config)#connect fr_local_sw serial 7/0 70 serial 8/0 80
SanFran(config-fr-switching)#
When you use the connect command to cross connect two local Frame Relay DLCIs, you are taken into config-fr-switching configuration mode. Example 14-47 shows the connect command that performs local cross connection between two attachment circuits. Example 14-47. connect Command for Local Switching ConnectionsSanFran(config)#connect eth-eth ethernet 3/0 ethernet 4/0 SanFran(config-connection)# SanFran(config)#connect eth-fr ethernet 3/0 serial 7/0 100 interworking ip SanFran(config-connection)# SanFran(config)#connect eth-eth ethernet 3/0.1 ethernet 4/0.1 SanFran(config-connection)#exit SanFran(config)#connect atm_local ATM 4/0 0/40 ATM 3/0 0/40 SanFran(config-connection)# Example 14-47 shows various cases used throughout this chapter. In the case of any-to-any (not like-to-like) attachment circuits, the IW option is presented. These different configuration submodes also present different commands that are applicable to the specific function that is being performed. (For example, the local switching submodes have no xconnect command.) EncapsulationThis section introduces you to encapsulation details for some of the case studies presented in this chapter. To view the encapsulation details, use Subscriber Service Switch (SSS) exec commands. The connections are represented as attachment circuit session types to SSS. All the examples use the command show sss circuits, which provides the status, encapsulation length, and encapsulation hexadecimal dump for the circuits. The encapsulation that is presented as output of this command, also called rewrite, indicates data that is added to the packet. This section's goal is that you obtain a better understanding of the underlying processes and protocols in the case studies. The following examples are presented: Encapsulation 1: Ethernet-to-VLAN Local Switching Ethernet IWThis scenario presents the local switching Case Study 14-10in the SanFran PE router between Ethernet 2/0 and VLAN 27 in Ethernet 3/0.1 (see Example 14-48). Example 14-48. SSS Circuit Encapsulation for Ethernet-to-VLAN Local Switching Ethernet-IWSanFran#show sss circuits Current SSS Circuit Information: Total number of circuits 5 !Output omitted for brevity Common Circuit ID 0 Serial Num 5 Switch ID 18796512 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 0 Y AES 4 8100001B SanFran# Focusing on the encapsulation, you know that in Ethernet IW, Ethernet frames are sent over the Layer 2 circuit. Therefore, the Ethernet endpoint has no specific encapsulation, as denoted in the encapsulation length of 0. For the VLAN side, however, the encapsulation includes the 4-byte VLAN tag. In this example, the encapsulation is represented as 0x8100001B, from which you see the following:
Encapsulation 2: Frame Relay-to-VLAN IP IW Using AToMThis scenario presents the AToM IP IW Case Study 14-4 from both the SanFran and New York PE routers. The scenario starts from the SanFran PE router, in which the local cross-connection is Frame Relay-to-AToM (see Example 14-49). Example 14-49. SSS Circuit Encapsulation for Frame Relay IP-IW Using AToMSanFran#show sss circuits Current SSS Circuit Information: Total number of circuits 5 !Output omitted for brevity Common Circuit ID 0 Serial Num 3 Switch ID 18796912 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 4 184103CC Y AES 0 SanFran# The show sss circuits command in the SanFran PE shows the encapsulations that are local to this PE router; it does not show the remote VLAN encapsulation. In IP IW, only raw IP packets are sent over the pseudowire; therefore, the rewrite includes the complete Layer 2 encapsulation. You can see that the encapsulation for the Frame Relay endpoint is 4 bytes in length, is equal to 0x184103CC, and is made out of the following:
Because IP is received over the pseudowire, after you append this 4-byte encapsulation and set the values of FECN, BECN, and DE, you send the packet over the Frame Relay attachment circuit. Toward the AToM side, the encapsulation is always shown as NULL. The second part of this example presents the VLAN side of the AToM routed IW pseudowire from the New York PE, in which the local cross connection is VLAN to AToM (see Example 14-50). Example 14-50. SSS Circuit Encapsulation for VLAN IP-IW Using AToMNew York#show sss circuits Current SSS Circuit Information: Total number of circuits 4 [snip] Common Circuit ID 0 Serial Num 3 Switch ID 18796912 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 18 FFFFFFFF FFFF000C CF552408 81000002 0800 Y AES 0 !Output omitted for brevity New York# To reiterate, the show sss circuits command in the New York PE illustrates the encapsulations that are local to this PE router and does not show the remote Frame Relay encapsulation. In IP IW, only raw IP packets are transmitted over the pseudowire; therefore, the VLAN side should show a complete 18-byte VLAN rewrite. When you compare the VLAN encapsulation for both bridged and routed IW, you see the following:
You can see that the encapsulation length is 18 bytes and is composed of the following:
Prepending this encapsulation to an IP packet that is received over the pseudowire and setting CoS bits appropriately makes a valid frame to be sent out of the Ethernet 2/0.1 interface in the New York PE. Figure 14-16 shows the encapsulation added in both SanFran and New York PEs for this scenario. The fields in gray represent the rewrite that has been described verbally. Figure 14-16. Frame Relay DLCI-to-VLAN AToM Routed IW Encapsulation Details
Encapsulation 3: VLAN-to-Ethernet Bridged IW Using L2TPv3This scenario presents the L2TPv3 Ethernet IW Case Study 14-2 from the VLAN attachment circuit in the New York PE router. The local cross-connection in the New York router is VLAN-to-L2TPv3 (see Example 14-51). Example 14-51. SSS Circuit Encapsulation for VLAN Ethernet-IW Using L2TPv3New York#show sss circuits Current SSS Circuit Information: Total number of circuits 4 Common Circuit ID 0 Serial Num 2 Switch ID 18797112 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 4 81000002 Y AES 28 45000000 00000000 FF73A5F7 0A0000CB 0A0000C9 000058FB 00000000 New York# The local encapsulations to the New York PE router are VLAN toward the attachment circuit and L2TPv3 toward the PSN. The attachment circuit side is equivalent to previous encapsulation scenario 1. Because this is bridged IW and Ethernet frames are received over the pseudowire, no other encapsulation is needed. However, the PSN side that uses L2TPv3 is new and shows an encapsulation length of 28 bytes, consisting of the following:
Note You can also display the L2TPv3 encapsulation by using the show adjacency detail command in the pseudowire IP adjacency. An Ethernet frame is encapsulated. After you complete the empty fields, such as DSCP in the IP Header and sequencing information in the L2-Specific Sublayer, you can send the L2TPv3 packet toward the PSN. Encapsulation 4: Frame Relay-to-PPP IP-IW Using L2TPv3This scenario presents the L2TPv3 Ethernet IW Case Study 14-5 from the SanFran and New York PE routers. The discussion starts from the SanFran PE router, in which the local cross connection is Frame Relay to L2TPv3 (see Example 14-52). Example 14-52. SSS Circuit Encapsulation for Frame Relay IP-IW Using L2TPv3SanFran#show sss circuits Current SSS Circuit Information: Total number of circuits 5 !Output omitted for brevity Common Circuit ID 0 Serial Num 4 Switch ID 18796712 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 4 0CC103CC Y AES 24 45000000 00000000 FF73A5F7 0A0000C9 0A0000CB 0000269D SanFran# The Frame Relay attachment circuit and L2TPv3 encapsulation in SanFran PE are similar to the previous examples, where you can see the following for the Frame Relay attachment circuit:
The Frame Relay encapsulation is a full Frame Relay header that, when appended to the received raw IP datagram, creates a complete Frame Relay frame carrying IP. The L2TPv3 side shows an encapsulation length of 24 bytes, because sequencing is not enabled and no L2-Specific Sublayer exists:
The second part of this scenario presents the PPP side of the L2TPv3 routed IW pseudowire from the New York PE, in which the local cross-connection is PPP-to-L2TPv3 (see Example 14-53). Example 14-53. SSS Circuit Encapsulation for PPP IP-IW Using L2TPv3New York#show sss circuits Current SSS Circuit Information: Total number of circuits 4 !Output omitted for brevity Common Circuit ID 0 Serial Num 4 Switch ID 18796712 --------------------------------------------------------------------------- Status Encapsulation UP flg len dump Y AES 4 FF030021 Y AES 24 45000000 00000000 FF73A5F7 0A0000CB 0A0000C9 000058FA !Output omitted for brevity New York# You can see in the New York PE that the encapsulation length for the PPP side is 4 bytes, comprising a full PPP header, and contains the following:
Prepending this encapsulation to an IP packet that is received over the pseudowire creates a PPP frame to be sent out of the Serial 6/0 interface in the New York PE. The L2TPv3 side is equivalent to the analysis in the SanFran PE with mirror source and destination IP addresses and a different Session ID. The L2TPv3 encapsulation includes the following:
Figure 14-17 shows the encapsulations added in both the SanFran and New York PEs for this scenario in addition to the L2TPv3 encapsulation added to the carried PDU. The fields in gray represent the rewrite that has been described verbally. Figure 14-17. Frame Relay DLCI-to-PPP L2TPv3 Routed IW Encapsulation Details
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